Examiner Tips for O Level Physics

How to Use These Tips
These tips highlight some common mistakes made by students. They are collected under various subheadings to help you when you revise a particular topic.

General Advice
• There is no escaping it; thorough and careful revision is the best way to prepare for a physics examination.
• Make your revision productive by making it interesting and fun.
Make notes, revision cards or mind maps. Revision should
be an active process, i.e you should be ‘doing things’, not just
sitting and reading a book.
• Do not try to learn it all in one go! Take regular breaks and
review what you have learnt regularly.
• Learning equations is essential; put them on small pieces of paper and stick them on your mirror so you will see them every morning, then revise with a friend so you can test each other.
• Try explaining the physics of a topic to a friend – as if you were a teacher!
• Working through past paper questions is then the best way to complete your revision. This helps you to know the type and style of questions to expect in the examination.
• Try timed questions so you can learn to answer quickly.
• Make sure you get your answers checked so you know you are doing the right things!
Spelling
The spelling of technical terms is important, so make sure your writing is legible as well as spelt correctly. Some words are very similar, such as reflection and refraction. If the examiner cannot tell which one you have written, then you will lose the mark. Make a list of technical terms and definitions in each section of the syllabus, checking the spellings carefully.

General Tips
In O Level Physics examinations you have to be able to complete a
variety of tasks; always read the question carefully to make sure
you have understood what you are expected to do. In descriptive
answers,you should:
• check the number of marks available and make sure you give sufficient points.
• plan your answer first so that you don’t repeat yourself or contradict yourself.
• read your answer through carefully afterwards to check you have not missed out important words.
• use sketches and diagrams wherever you can to help your explanation.
• add labels when referring to a diagram, e.g. point X, so that you can refer to it easily in your explanation. This can save many words and much confusion. In numerical answers, you should:
• quote any formulae you are going to use.
• show clearly all the steps in your working.
• check the units are consistent, e.g. if the distance is given in km and the speed in m/s, then you must convert the km to m.
• be careful when you are converting minutes and seconds: 1 minute 30 seconds is not 1.3 minutes and 150 seconds is not 1.5 minutes. These are common mistakes, so always double check any conversion of units of time.
• state the answer clearly at the end.
• give your answer as a decimal to an appropriate number of significant figures unless specifically asked to give the answer as a fraction.
• check that you have given the unit on your final answer.
• look at your final answer and see if it is reasonable. If you have the cost of using an electrical appliance such as a kettle used for six minutes as more than a few cents, then check the powers of ten in your calculation. Plotting graphs can be tested in Papers 2, 3 or 4. When drawing graphs, you should:
• remember to label the axes with both quantity (e.g. distance or d) and unit (e.g. metres or m). Then write it as distance / metres or even just d / m.
• make sure the axes are the correct way round. You are usually told, for example, to plot distance on the x-axis, so make sure you know that x is the horizontal axis!
• make the scales go up in sensible amounts, i.e. not 0, 3, 6… or 0, 7, 14 … but 0, 5, 10 … or 0, 2, 4 ….
• make sure that the plotted points fill at least half the graph paper. (This means you cannot double the scale and still plot all the points on the graph.)
• check if you have been told to start the scales from the origin. If not, then think carefully about where to start the axes.
• use a sharp pencil to plot the points and draw the line.
• plot the points carefully. It is best to use small neat crosses. Every point will be checked by the marker, and you will lose the mark if any are wrongly plotted.
• draw either a straight line or a smooth curve. In physics we never join the dots!
• remember that a best fit line (curve or straight) should have some points both above and below the line. When taking readings from a graph, you should:
• draw a large triangle when measuring the gradient of a line. It must be at least half the length of the line. Examiner’s tip – draw a
triangle the full size of the graph!
• always use points on the line, not your plotted points, when calculating the gradient.
• draw a tangent to find the gradient of a curve. Make sure it is at the right place on the curve. Again, use a large triangle.
• make sure you read the scales correctly when reading a value from a graph. It may be that they are in mA rather than A or km rather than m. When describing the shape of a graph, remember that:
• directly proportional means a straight line through the origin. In this case, doubling one quantity will cause the other to double.
• if the straight line does not go through the origin, then it is just called a linear graph.
• if doubling one quantity causes the other to halve, then they are inversely proportional.
• if increasing one quantity causes the other to decrease, it is called an inverse relationship.

Paper 1 Tips: Multiple Choice
When reading the question, you should:
• read the question carefully, e.g. if a question refers to a cooling liquid, then it will solidify, not boil. If you know you tend to jump to a quick conclusion, cover up the answers while you read the stem of the question.
• not rush through the questions. Some will be very quick to answer, others take more time.
• check whether a positive or negative answer is being asked for, i.e. does the question say “which of the following is or is not …?” For example, when looking at ray diagrams it is easy to just spot a correct diagram when you are asked for an incorrect diagram.
• underline or circle important information in the stem of the question.
• never leave a question unanswered; marks are not deducted for incorrect answers.
• try to eliminate some of the possible answers if you are not sure of the answer.
• write out your working to numerical questions clearly (on the question paper, near the question) so you can check it later.
• be aware of the topics which occur frequently, such as potential difference and potential dividers. The theory here just has to be learnt! When taking readings from a diagram, you should:
• check you are using the correct distance, e.g. in moments questions, remember you need to use the perpendicular distance from the force to the axis of rotation.
• draw on the diagram to help you understand what is happening,
e.g. in a travelling wave moving to the right, draw in the new
wave outline after a short time, or in deciding the direction
of the magnetic field at a point near a bar magnet, draw in the
shape of the field. Choosing the right response:
• When several answers seem correct, re-read the stem of
the question. You must choose the answer that is not only a
correct statement, but also answers the question, e.g. swapping
the live and neutral wires in a plug is a fault, but will not
cause the fuse to blow. The live wire touching the metal case
of a kettle is a fault which will cause the fuse to blow!
Choosing the right equation:
• Many equations are very similar, e.g. E = mc2 (energy equivalence of mass) and E = ½ m v2 (kinetic energy of a moving object) so make sure you know when to use each one.

Paper 2 Tips: Structured Questions
• Read the stem of the question to check which topic in physics is being tested. Then read all the parts of the question. It is often tempting to write too much in the first part of the question and then realise you have answered parts two and three as well.
• Only answer the question asked. Don’t be tempted to give more detail than is required. This wastes time and gains you no extra marks!
• If you are asked for two points (e.g. name two materials that are magnetic ….) then don’t give three. If you give three and the second is incorrect, you will only get one mark out of two.
• Your answer should fit the space available. If it doesn’t, you are writing too much! The number of lines given is a clue as to how much to write. Think about the size of your writing: if it is too big, it will not fit in the space; if it is too small, then the examiner will not be able to read it. Practice writing a size that is in between the extremes!
• If the question asks you to describe the movement of electrons, then not mentioning electrons and only referring to the movement of charged particles in the answer cannot gain full marks. Failure to give sufficient detail is a common cause of lost marks
• If describing the motion of molecules in a liquid then linking the temperature to the average kinetic energy of the molecules is important. Molecules of a gas exert a pressure on the walls of a container by colliding with the walls. To increase the pressure, they must collide at a greater rate, i.e. more frequently or with a higher speed. Take care to explain this clearly and without contradiction!
• Electrical circuits are common questions. Make sure you know where to put ammeters and voltmeters in a circuit. Then, if you need to vary the current, make sure you include a variable resistor or use a variable power supply.
• If the question asks you to”state and explain” you need to give a clear explanation. The amount of detail depends upon the number of marks for the question, e.g. if the direction of the current in a solenoid is reversed, then just saying that the magnetic field changes is not enough. This could mean increases or decreases in strength. You need to state that the field reverses or changes direction.
• Make sure you link your answer to the question, rather than just quote learnt facts, such as the penetration of radioactive radiation. Just stating what stops alpha, beta or gamma will not gain all the marks.
• If you are asked to draw forces on a diagram, be sure to draw them through the point where they act. Do not draw them floating in mid-air to the side of a diagram! Remember to label them. Make sure you add an arrow to show the direction, e.g. if the question asks for “the force exerted by the Sun on the Earth”, then since it is a force of attraction, the force arrow must go from the Earth towards the Sun.
• If you are asked to draw a forces diagram, make sure the diagram is large enough, and that all the forces are drawn with arrows and labelled.
• Where a question asks for a formula to be quoted, there will be one mark specifically for this. Even if you get the right answer, failure to quote the formula will lose you a mark. Some incorrect physics statements will lose a mark even if followed or accompanied by a correct statement. Examples of such statements are:
• Renewable energy sources can be used again and again. Please use the explanation that there is an infinite supply or it will not run out.
• Heat rises. Note that it is either hot air or hot liquids that rise, carrying the heat energy with them.
• Acceleration at a constant speed. This is a contradiction as if travelling at a constant speed, you cannot be accelerating! When describing a uniform acceleration, you can say constant acceleration or that it is accelerating at a constant rate.

Paper 3 Tips: Practical Test
• You will have three short experiments (20 minutes each) and one longer experiment (1 hour).
• Read the instructions carefully. Make absolutely sure you know exactly what you are asked to do each time.
• You do not have any time to waste, so you need to be sure you are doing the right thing first time.
• Write down all your readings clearly in the answer booklet.
• When asked to take a single reading, make sure you include the unit.
• Do not write anything you are not asked for – you are not expected to write an account of the experiment.
• If you are asked to “use your results” to explain something, then quote them, not just use the theory you know!
• Think about the experiment as you do it – you are often asked for
sources of error or difficulties you met while doing the experiment.
Make sure you give sufficient detail, e.g. don’t just say "to avoid
parallax error” but say how this is avoided. This can be done
by drawing a suitable diagram showing the position of the observer
relative to the scale.
• Significant figures are important in the practical papers. Do not quote too many – or too few! Just right is important.
• If you are reading a measuring instrument, give all the values on the scale, e.g. on a hundredth of a second stopwatch, write 9.24 s (but not 09:24 s).
• Many marks are lost by giving too few significant figures. This usually occurs when reading a scale where the value is on a major mark, e.g. 6 V. If the scale measures to 0.1 V, then the reading is 6.0 V, and you must include the point zero!
• In calculated values, you should give the same number of significant figures as in the values used, e.g. the average of 27.95, 26.54 and 27.36 is 27.28333333 and should be given as 27.28.
• Make sure you understand the technical terms used in the question, e.g. extension means the increase in length of a spring when a load is added.
• When measuring vertical heights, a setsquare should always be used to ensure the ruler is vertical. The setsquare should be shown correctly positioned in the diagram. When you have completed an experiment, go back over your answers and:
• check that you have answered all the parts of the question. Read the instructions again. You may be asked to draw a diagram after a calculation and this can easily be missed.
• check that you have read scales to the correct power of ten, e.g. when reading an ammeter should it be 0.012A, 0.12A or 1.2A?
• check that you have the correct number of significant figures.
• check that you have added a unit to all your measurements and any calculated values, and then check that it is the correct unit! In the Section B question, you will be asked to take a set of readings to plot a graph. When recording your readings in a table:
• Write both the quantity and unit in the heading. Note that the quantity means current, not "reading on the ammeter”. Don’t write the unit after every reading in the table. This just clutters up the table and makes it difficult to see the values clearly; a heading should say current / ampere or just I / A.
• You do not need a column labelled ”reading number” which just
goes 1, 2, 3 etc. If you are given a table outline in which to record your results, this will use one of them and you will not have enough columns for your results.
• Make sure you have taken sufficient readings, e.g. if you are asked to measure the temperature of a cooling liquid for five minutes, then a reading every minute gives you too few readings. Every 30 seconds is acceptable.
• Make sure you record the temperature for the full time.
• Don’t forget to note the initial temperature when you start the stopwatch.
• If using a liquid in glass thermometer, you should be able to estimate within a degree, e.g. to 0.5oC or even 0.25oC.

Paper 4 Tips: Alternative to Practical
• This paper asks you questions about how you would perform practicals in the laboratory at school. When you observe your teacher demonstrating experiments, you should:
• watch closely how the apparatus is set up.
• think about any problems with the apparatus that occur during the experiment.
• think about any sources of error in taking the readings.
When you do practical work at school, you should:
• handle the apparatus carefully.
• think about how the apparatus is set up.
• ask your teacher for help if you are not sure.
• think about how you take down the readings in a clear table – never just write numbers on a page, as you may well forget what they were later!
• think about the number of significant figures in your readings.
• Significant figures are important in the practical papers. Do not quote too many – or too few! Just right is important.
• If you are reading a measuring instrument, give all the values on the scale, e.g. on a hundredth of a second stopwatch, write 9.24 s (but not 09:24 s).
• Many marks are lost by giving too few significant figures. This usually occurs when reading a scale where the value is on a major mark, e.g. 6 V. If the scale measures to 0.1 V, then the reading is 6.0 V, and you must include the point zero!
• In calculated values, you should give the same number of significant figures as in the values used, e.g. the average of 27.95, 26.54 and 27.36 is 27.28333333 and should be given as 27.28.
• Be careful when calculating values for a table. These are usually straightforward and do not involve complex calculations!
• When answering questions about sources of error in an experiment, just writing “more accurate” is usually not enough and more detail
is required, e.g. when choosing the correct size measuring
cylinder to use in order to measure the volume of some marbles, the
measuring cylinder must be large enough to hold all the marbles!
Sometimes the answers appear too obvious,but they are good practical points.
• Make sure you can explain the difference between the source of
error and what you could do to reduce it, e.g. in transferring a
hot object from one place to another: the source of error is the
heat it loses during the transfer and you could reduce this error
by reducing the distance it has to be moved.
• When substituting values into a given equation, always check your
calculation twice. Check that you have included all the quantities,
e.g. if the equation contains an l for length, do not confuse it
with a 1. Then check that the units are consistent, and do not
change them if you don’t need to!
• When measuring a time or a length be careful with the phrase
“how long” as it can mean either. Make sure there is no confusion
here by using the words “a longer time” or “a shorter time”.
• If a question asks for the effect of changing something such as ”the length of the wings” then make sure your answer shows a comparison, e.g. “the longer the wings, the longer the time to fall”.
• Technical terms such as calibration need to be understood. This means, "to put a scale on a measuring instrument”. Although you may have met this only with reference to thermometers, it applies to any measuring instrument. If a question involves familiar equipment used in a novel way, e.g. circuits or ray diagrams:
• take time to look at the equipment used.
• do not assume that it is the same as an experiment you have done or seen before.
• follow round the circuit or the rays of light to be sure you understand what is happening. You should then be able to work out how to answer the question.

About the Examiner
Marianne Devereux has been an examiner for GCSE Physics (5054) for many years. At present she is Principal Examiner for Paper 4, the Alternative to Practical, and also works as an examiner on Paper 2. She has recently retired from teaching as Head of Physics at an independent girls’ boarding school.

Examiner Tips for O Level Chemistry

Examiner Tips for O Level Chemistry (5070)
How to Use These Tips
These tips highlight some common mistakes made by students.
They are collected under various subheadings to help you when you revise a particular topic.
General Advice
• It is very important that you understand the words or phrases used by examiners to prompt you to answer questions in a particular way. These terms are described in the syllabus in the section Glossary of Terms. You may sometimes lose marks because you do not understand what to write in response to the words “explain”, ”describe”, “suggest” etc. If you are unsure, ask your teacher to explain what each of these terms means; for example, the term ”explain” means that you have to refer to some idea or theory and write in detail, so if you are asked ”Explain why rate of reaction increases as temperature increases”, it means that you have to use the idea of particle collisions in your answer.
• Make sure that you read the question fully, picking out the key words. For example, in a question such as ”Give a use of copper that depends on the ability of ions to move past each other”, it is common to find incorrect answers referring to properties rather than uses. Therefore the answer “malleable” is incorrect but the answer “for water pipes” is correct. It is useful to underline the key words in a question as you read it through.
• Take careful note of how many marks there are for a question. If there are three marks, you will need to think of three different points that you can write down to answer the question, e.g. in the question ”Explain the effect of increasing the concentration of acid on rate of reaction”, the examiners are looking for:
• The more concentrated the acid, the closer together the acid particles (1st mark)
• The colder the particles the more frequent the collisions (2nd mark) so increasing the rate of reaction (3rd mark)
• Make sure that you keep referring back to the beginning of the question or main subsection for vital points of information. This is especially important with calculations. In Paper 4, you may sometimes need information which is several pages back.
• Don’t be afraid to write down or choose the answer “no reaction” if you feel that nothing should happen. The lack of reaction still gives valuable information. This is most commonly seen in questions such as “Write down the products of the reactions, if any, between (i) chlorine and potassium bromide (ii) iodine and potassium chloride”. The phrase “if any” keys you into the idea that one of these pairs may not react.
Spelling
The correct spelling of chemical names is not always essential as long as they cannot be mistaken for other chemicals. However, in simple questions where you are asked to select the names of chemicals from a list, you must get the spelling correct. Writing “ammonium” for “ammonia” or “chlorine” for “chloride” will not be given credit because this is a chemical mistake.
General Tips
• Work out exactly what the question is asking. Many mistakes are made by not reading the question correctly. If the question says “give two observations apart from temperature change”, then obviously you shouldn’t write “temperature change” as one of your two answers, but many students do!
• Read over your answers and ask yourself “Have I contradicted myself?” This generally refers to things written in the same sentence. A common error is to write something such as “On adding ammonia a blue insoluble precipitate dissolves”. If something is insoluble it doesn’t dissolve! The correct answer to a question about adding excess ammonia to copper ions would be “On adding ammonia a light blue precipitate is formed. The precipitate dissolves in excess ammonia solution”. Notice how splitting the answer into two sentences has altered the meaning.
• Significant figures are not the same as decimal places. For example, 123.08 is to two decimal places but to five significant figures. Zeros before any definite numbers do not count as significant figures, so 0.000045 is to two significant figures.
• When doing calculations, your final answers to each section should be to the correct number of significant figures. Generally, it should be to the same number of significant figures as the data. You may get penalised if you write your answers to an excess number of significant figures, e.g. 1.257487 instead of 1.26.
• Always show your working – even if your answer is wrong, you may get some marks for your method. It is not sensible to work everything out on your calculator then just put down the answer because if you make one slip you risk getting no marks for that question.
• Make sure that you know your syllabus statements and definitions exactly, e.g. the use of naphtha as a feedstock for the chemical industry. The Principal Examiner has to use the syllabus as a basis for the exam questions.
• Look out for the phrase ”what you would observe”. This means that you must write down what you see, hear or feel (e.g. test tube gets hot). It is a very common error to write something like “a gas is given off” or “copper is deposited”. These are not observations.
• Don’t get caught out by the phrase ”describe what you would see”. Some students put down observations about pops (sound or heat given off).
• When drawings diagrams:
i) make sure they are large enough to fill the space given on the paper and always label them.
ii) make sure that you draw apparatus for gas measurement without any places for the gas to escape. Don’t draw gas syringes with the plunger much smaller than the syringe barrel. This is a common error.
• Stick to the number of examples requested by the examiners. For example, if a question asks for two examples of a transition element, do not write three down. If one of the three is incorrect, you will lose a mark. If a question asks for a single use for a substance, stick to one – if you write long lists, the examiner will think that you are ‘playing safe’ and you won’t get the mark. Take for example the question ”State two properties of transition elements that are not shown by other metals (2 marks)”. The answer “they are good catalysts, they form ions with different charges, they are shiny” will only gain one mark because, although the first two answers are correct, the last one is incorrect (2 √ + 1 x = 1 mark).
• In chemistry, the examiners want you to draw graphs of reaction rates by joining up the points with a curve of best fit. If you draw lines with a ruler from point to point, you will not get the mark.

Paper 1 Tips: Multiple Choice
• If you are unsure of the answer to a multiple choice question, don’t spend too long on it. Put a cross by it and return to it later.
• Within a single multiple choice question, use a pencil to cross out the statements which are clearly incorrect, then choose between those that are left.
• Don’t be swayed by one of the choices just because it has got a longer (or shorter) statement than the others.
• Don’t make any assumptions about the order of responses – just because there have been two ‘D’ answers in a row, it does not mean that the next answer cannot be ‘D’.
• Take care to read the whole question word by word. For example, in the question ”What is the ratio of the volumes of 2g of H2 and 16g of methane, CH4, at RTP?” many students will focus on the numbers and ignore the word ”volume”, resulting in the incorrect answer 1:8, rather than using volume of a moles of gas and giving 1:1 as the correct choice.
• When given a choice of picking out a noble gas from a group of electronic structures, don’t jump to the conclusion that noble gases always have eight electrons in their outer shell. Remember that helium has two!
• When given a choice about electrical conductivity of ionic structures remember that the conduction is due to ions moving (not electrons) and they can only move in liquid or when dissolved in water. This confusion often arises because metals conduct when solids but metals are NOT ionic structures.
• When given choices of why alloys are hard, it is not the mass of the atoms which is important but their size. Remember that metals have layers which slip over each other. A bigger atom will stop the layers slipping and so make it hard.
• When given choices about the rate of diffusion of gases, remember that the rate of diffusion depends on the mass of the molecules. Heavier molecules (lower relative molecular mass) move and diffuse slower than lighter molecules. Use your Periodic Table to calculate the relative molecular masses if you are unsure which molecule is heavier.
• If you are given a choice of tap water and several other substances as examples from which to select a pure compound, it’s not going to be tap water. It is a common error to think that tap water is pure but it contains the compounds from the rocks and those carried in the rain as well as the chemicals put in to purify it. Therefore tap water is a mixture. (And don’t be fooled by the adverts of the mineral water companies which say ‘pure mineral water’!)
• If you are given choices of electronic structures of atoms to select to make a compound of type XY2, first check the type of compound that the examiner wants, e.g. ionic or covalent.
If it is ionic, then you can choose an atom with one or two electrons in its outer shell and combine it with a non-metal atom. If it is covalent, look for the structures of two non-metal atoms, i.e. those with four to seven electrons in their outer shell. Remember that the number of electrons in the outer shell is equal to the group number.

Paper 2 Tips: Theory
• Reading the question thoroughly and noting the number of marks available is important. In response to a question such as ”Use you knowledge of the structure of metals to explain how they conduct electricity”, many students just write down that “metals have a sea of electrons”, thinking that this answers everything. What the examiners are looking for is (1) the idea of positive ions in a sea of electrons and (2) the fact that the electrons move.
• If you are asked to compare things in the question, your answer must make the comparison obvious. In response to the question “How do different proportions of carbon affect the properties of steel?”, the answer “Mild steel has a low % of carbon” will not receive any marks because nothing has been said about steel with a higher % of carbon. An answer such as “the higher the % of carbon the more brittle the steel’ gets the marks because this is a comparison.
• Properties of transition elements often cause problems. Remember that transition elements themselves are NOT coloured, it is their compounds that are coloured.
• If you see the words “what observations are made?” remember that this means what you see, hear or feel and NOT, for example, “gas given off”.
• Make sure that you know the use of the various substances stated in the syllabus. Go through your syllabus and make a note of these. For example, the uses of the naphtha fraction from petroleum distillation are not well known.
• If you are asked to describe the meaning of a term which has two words in it, you must make sure that the description has included the meaning of both words. So to get full
marks on the question ”What is the meaning of the term saturated hydrocarbon?”, you have to define (1) saturated as well as (2) hydrocarbon.
• Always check back in the question to see the wording in the stem of the question – you will not get any marks for putting what is in the stem into different words. For example, if the
stem includes the words ”Explain why the reaction rate increases as the concentration of acid increases”, you will not get any marks for putting this in another way, e.g. ”The reaction speeds up because the concentration of acid increases”.
• Be on the lookout for questions involving processes, e.g. ”What advantages are there in using hydrogen as a fuel?” The section underlined is a process. The answer ”Hydrogen is
not polluting” is therefore not correct because there is no indication that it is being used as a fuel. A correct answer would be “It forms no pollutants when burnt”.
• Watch out for the words if any, e.g. “State the products in the following reactions, if any”. Sometimes the examiner wants to test your understanding of why a reaction does NOT occur.
• If you are asked to draw electronic structures of atoms or ions, it means full electronic structures showing all shells, not just the outer shell.
• Explaining the properties of ionic structures is always a problem area. Never mention atoms, molecules, covalent bonds or sharing electrons even if you mention ions as well.
These are banned words! In answer to the question ”Why does sodium chloride conduct electricity when molten?”, the answer ‘Because the ions and the electrons can move’ gets no marks because the incorrect ”electrons” negates the mark for ”ions”.
• A common error involving the conductivity of molten ionic compounds is to state that the electrons are responsible for conduction. Remember that it is the ions which move when
ionic compounds conduct electricity.
• It is a common mistake to believe that energy is needed to form bonds. In answering questions about bond making and bond breaking, think about a model of a compound – to break the bonds, you have to physically pull them apart. In other words you’re putting in energy. To form bonds, it must be the opposite, i.e. energy is given out on bond formation.
• Look out for the term “explain”. This indicates that you have to write in detail about why something happens. If, for example, you are asked to explain why the reaction between copper(I) chloride and chlorine is a redox reaction, you must write about redox in terms of either electrons or oxidation number changes.
• Make use of all the information given in a question, including graphs. For example, if a graph of % yield of ammonia against temperature for various pressures is given, it is there for a purpose. If you use the information given you are more likely to get marks than if you try to remember figures from a book.
• Never write “ammonium hydroxide” as the product when ammonia dissolves in water – ammonium hydroxide does not exist! (Even though you may see it still on bottles in the lab and even in some books). The correct term is aqueous ammonia. In addition, make sure that you know the difference between ammonia, NH3 and ammonium, NH4+. The latter is an ion present in ammonium salts.
• The number of carbon atoms in formulae of carboxylic acids often cause problems. Make sure that you include the carbon of the -CO2H group when you name the acid. CH3CH2CH2CH2CO2H has five carbon atoms so is the 5th member of this homologous series, pentanoic acid.
• Take care with the formulae of the metal salts of the carboxylic acids if the metal is from group 2. Remember that group 2 metals form 2+ ions and so they need two carboxylate
ions to balance, e.g. the formula for magnesium ethanoate is (CH3CO2- )2 Mg2+.
• When doing mole calculations if given an equation such as:
Mg + 2CH3CO2H → (CH3CO2 )2 Mg + H2 Ignore the 2 in the equation when calculating the molar mass of ethanoic acid, i.e. the molar
mass of ethanoic acid is 60, not 120. Remember though that when calculating reacting masses, the 2 needs to be taken into account because two moles of ethanoic acid react with only one mole of magnesium.
• Try to be as accurate as possible in all your answers. In response to a question such as “Why is the reaction between ethanoic acid and magnesium much slower than the reaction
between hydrochloric acid and magnesium?”, it is too vague just to say that ethanoic acid is a weaker acid. Some reference to the differences in hydrogen ion concentration is needed.
• When there are questions with large unspecified mark allocations, e.g. seven marks, you need to plan your answers out carefully and underline or list on the question paper all the
points that need to be written about. It is very common to miss parts out in extended questions like this. If you have written a list of the points you need to deal with, you can cross these out one by one as you complete them, e.g. equations, gas tests, colour changes etc.
• Answers to questions on “chemistry and the environment” are often answered in too vague a manner. The word ”pollution” is too vague to be given credit as an answer to any question. In answering a question such as “What is a disadvantage of the use of nylon for fishing nets?”, “pollution” would not score a mark, nor would vague statements such as ”dangerous to sea life”. “Non-biodegradable” would score the mark because it is much
more exact.
• In doing calculations, always check that the relative molecular masses are correct. Incorrect addition or extraction of the relative atomic masses is often a reason for failure in
a calculation. In addition, double check that you have used atomic masses and not atomic numbers. If you are unsure, use the key in the Periodic Table at the bottom left to check which number is which.
• Look out for the word each, e.g. in the question “Explain the purpose of adding each of calcium hydroxide and ammonium sulphate to soil”. You will lose marks if you do not make
it clear exactly which compound you are writing about. To make sure that there is no ambiguity, start each sentence with the name of the compound you are referring to, e.g.calcium hydroxide is added/ammonium sulphate is added.
• Avoid missing out connecting processes. For example if you are asked about how ammonium sulphate helps soil fertility, the answer ”ammonium sulphate forms nitrate ions” is not good enough because it suggests that the ammonium sulphate contains nitrate ions. A better answer would include the connecting process, in this example “ammonium sulphate reacts to form nitrate ions”.
• Know the difference between –ides and –ates. Compounds ending with –ide contain only two types of atoms, e.g. magnesium oxide and potassium chloride. Compounds ending with –ate contain three or more types of atom, one of which is usually oxygen, e.g. sodium sulphate and potassium nitrate. The ions of -ides are simple, e.g. sulphide, S2-, whereas – ates have compound ions, e.g. carbonate CO32- .
• When writing oxidation numbers, remember that the + or – sign should be included. The oxidation number is not written like the charge on the ion. For example, Cu2+ is a copper(II) ion. The oxidation number of copper in this ion is +2 (NOT 2+).
• Make sure that you know the solubility rules. These become important when writing state symbols in equations. If you know that carbonates of group 2 metals are insoluble in water, then you know to write CaCO3(s) in an equation rather guessing.
• Know the difference between (aq) and (l)! The state symbol (aq) refers to a substance dissolved in water. The state symbol (l) refers to a substance as a pure liquid, e.g. Br2(l), H2O(l).
• Remember that when writing equations for the reactions between aqueous solutions of halide and aqueous halogens, the state symbols are all (aq). This is because the halogens are dissolved in water in the first place.
• When writing the electronic structures of ions don’t show the charge inside the nucleus. The charge should go on the top righthand corner of the ion, outside the square brackets
(which are put round the ion to show that the charge is spread out all over the ion).
• When given graphs which read back in time from the present day be careful to remember to read the graphs in a forward direction if you are asked about the order of a sequence of events, e.g. how carbon dioxide concentration has changed over the last 2000 million years.
• Always read the scales on graphs very carefully, especially when very large numbers are involved. For example, if the graph has the figures 1000, 2000, and 3000 with “millions of years” underneath, it is all too easy to miss the word “millions” when answering questions.
• Remember that if you want to separate a particular gas from the air, you can’t just heat the air up. You have to make it liquid first by lowering the temperature so that all the gases
liquefy. Then you raise the temperature gradually and collect the gases as they evaporate off one by one. Although the actual process is more complex, this is all that you have to know for your examination.
• With questions which require extended answers, especially in part B of the paper, do not write too much. There is often a danger that you will contradict yourself.
• When asked to draw diagrams, make sure that you have included all the pieces of apparatus necessary. Go over each point in the question carefully to check. It is quite common, for example, to leave out the test tubes to collect gases when asked to draw the
apparatus for electrolysis and test the gaseous products.

Paper 3 Tips: Practical
• If you are asked to heat up a substance with sodium hydroxide and aluminium, don’t assume that ammonia is the gas that is given off. It could be hydrogen. Get your answer from your observations not from theory.
• The observation of effervescence is often missed out from practical observations. Look for the bubbles!
• In carrying out titrations you must repeat them until you get at least two consistent results which you can tick. Examiners often find that only one result is ticked – make sure that two
are ticked.
• In titrations you must only average the consistent results that you have ticked, not all the titration results.
• When describing solutions do not use the word ”clear” when you mean colourless. In chemistry, clear just means you can see through it – it is the opposite of cloudy.
• The word precipitate is often used incorrectly. You can only use it about a solid formed when two solutions are mixed.
• When making observations about a solution don’t forget that ”colourless solution” may also gain a mark. Lack of colour is just as important an observation as presence of colour.
• Take care when adding a solution of sodium hydroxide to test for ions. If you add a large volume of sodium hydroxide too quickly, you may get the precipitate re-dissolving without you ever noticing that one was formed, e.g. in the case of adding sodium hydroxide to aluminium chloride solution.
• When describing colours don’t use combinations e.g. blue-green or yellow-red, unless absolutely necessary for distinction and certainly don’t use contrasting colours, e.g. greenish brown.
• When observing colour changes, make sure that you observe all the colour changes, not just the first and last. For example, when adding silver nitrate to sodium thiosulphate, the colour changes are white → yellow → red → black.
• You must be able to distinguish between the different shades of yellow precipitates, e.g. silver bromide, silver iodide and lead iodide. You can do this by calling them creamy yellow, light yellow, deep yellow etc, but do not write green (a common mistake) when the colour is clearly yellow.

Paper 4 Tips: Alternative to Practical
• When plotting graphs, you should be able to accurately plot to within one-half a small square and the lines should go through the 0-0 point which should also be plotted if there is data for it. However do NOT draw a line through the 0-0 point if it is clear that the trend shows that the line is unlikely to go through this point.
• Practice extrapolating graphs. The extrapolated curve must follow the pattern of the line or curve that is already there. If it is levelling off gradually, the extrapolated curve must
continue this levelling off.
• In the practical papers marks are not usually given for suggesting that you can separate a solid from a solution by decanting off the solution. Filtration is the method that is expected.
• Make sure that you know the difference between the tests for oxygen and hydrogen–these are frequently confused. Useful mnemonic devices for remembering them are:
o Hydrogen – Lighted splint – Pops (Hylight Pops)
o Oxygen – Glowing splint – Relights (ogre)
• Practical examiners do not generally like the term ”burns with a pop” for the result for the hydrogen test. Just ”pops” or “explodes” is fine. This is because the explosion puts the
flame out, so it doesn’t burn.
• In describing colour changes which you expect to see when a metal such as zinc reacts with a solution of copper sulphate, don’t write that the solution goes white. This is a common error. You should state that the blue-coloured solution loses its colour.
• When given several diagrams of thermometers showing the temperatures as an exothermic reaction proceeds and asked to plot a graph from the data, make sure that you always calculate the temperature change by taking away the initial thermometer reading from each of the other readings. A common mistake is to take the 3rd from the 4th, the 4th from the 5th etc.

About the Examiner
Dr. Roger Norris has been an examiner for many years. He is currently a Principal Examiner for Cambridge International O Level Chemistry as well as IGCSE Chemistry. He is also involved in developing and examining OCR Chemistry courses and examinations in Britain. As well as undertaking several years of research he has 27 years of teaching experience in Chemistry (as well as in Biology and Physics). He has contributed to a number of publications ranging from General Science for 11-13 year olds to advanced level Chemistry courses.

Halt the Salt

When it comes to salt, the National Institute of Health and the
American Heart Association recommend no more than 2,300
milligrams daily (that's one teaspoon of salt). However, most
Americans consume between 4,000 to 6,000 milligrams on a daily basis!

What Is Salt?
Salt is comprised of two minerals, sodium and chloride, but sodium
is the specific mineral of concern (and sodium is what you'll find
on your food labels). Some sodium is essential. In fact, sodium
helps to maintain proper fluid balance in and out of cells, regulate
blood pressure, and transmit nerve impulses. Sodium occurs naturally
in some foods, but most of the sodium we consume is from processed
and packaged products. That's because, sodium not only affects
flavor, but can change texture, control the speed of fermentation,
stabilize volume, and promote color enhancement.

Just to name 2 health consequences
There is a strong link between sodium and high blood pressure in
people who are salt sensitive. Salt attracts water -- salt pulls
water into the blood vessels & this extra vol creates added pressure.

Increased dietary sodium is known to trigger urinary calcium loss.
With high levels of sodium intake,the body compensates by
increasing urinary excretion.Because sodium & calcium excretion
occur together, higher levels of urinary sodium result in increased
calcium excretion with possible adverse effects on bone health.

So ask mummy to cook with less salt.

Paper2_Math_D_Jun2005

Pls see my comment.

Q1c(ii) You should have scored full marks for this Q. You have
derived the equation but you forgot to express the values
of x=+5,-5

Q3b Technique : Remember the value of the interior angles of
(a) 4 sided shape eg rectangle is 360 deg (b) 5 sided
pentagon is 540 deg (c)6 sided hexagon is 720 deg.
I hope you know how to explain the above. A rectangle
contains 2 triangles, thus their interior angles is equal
to 180x2 = 360 deg.
Note the Q said that the hexagon has rotational symmetry
of order 3. It just mean that each of the 3 mini hexagon
inside the big hexagon are symmetry.
Given one of the interior angle is 105 deg, thus 2 other
interior angles of the other 2 mini hexagon are also 105
deg. Similarly there are also 3 angles with z deg.
Thus (3*105)+3z=720; z=(720-315)/3=135 deg
This is a technique you must remember.

Q3c Pls check whether there are formulae for similar triangle.

Q4c You must first know what is the path? A straight line or
an arc of a circle? In this case it is an arc of a circle
with angle ACA’=125 deg.
Thus 125/360 * 2pi(r) = (125/360)*2*22/7*20=43.6 cm

Q7d Vol of the outer cylinder–Vol of inner cylinder = 500 cm3
Let radius of inner cylinder = r cm
Radius of outer cylinder = r + 1.5 cm

Q8b(i) Your answer -0.50 is quite near -0.44. Did you round up?

Q9 Pls check whether there are formulae for such triangles
which are not right angle triangles

Q11(i) Pls check again.
Q11(iv) Pls check again. You are quite ok with this chapter.
Maybe you careless here.