Wondering how to ace the GCE O Level Chemistry? Fret not! Tutopiya has some common mistakes and tips to help our secondary 4 Express and 5 Normal Academic GCE O Level Chemistry students! Continue to read to find out more!
General tips for the GCE O Level Chemistry paper
Black Ink
It is important to note that the examination scripts are now being marked ‘on-line’ by the examiners. As such, scripts have to be scanned and blue ink or light pencil marks do not show up very well.
Remember to use black ink or make dark pencil markings on your script!
Spotting keywords
Make sure to read the questions fully and pick out the keywords.
For example, if a question reads
‘Explain why the rate of a chemical reaction increases when the temperature increases’
To score this question, you have to use the idea of particle collisions in your answer.
Marks
Take careful note of how many marks are there for a question. If the question is 3 marks, you are expected to present 3 different points to answer the question.
For example
‘Explain how increasing the concentration of acid affects the rate of reaction between magnesium and hydrochloric acid’
The examiners are looking for:
the rate of reaction increases (1st mark)
because the more concentrated the acid, the closer together are the acid particles (2nd mark) the closer the particles, the more frequent are the collisions (3rd mark)
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 are expected to get the spelling correct.
Writing ammonium for ammonia or chlorine for chloride will not be given credit because this is a chemical mistake.
Ask yourself
Read over your answers and ask yourself if you have contradicted yourself – this generally refers to things written in the same sentence.
A common error is to write something like –
‘On adding ammonia a soluble blue precipitate is formed‘.
There is confusion here because if precipitate is not soluble.
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 that splitting it up into 2 sentences have altered the meaning.
Refer to this infographic for a quick overview.
GCE O Level Chemistry Paper 1
General Tips
If you are unsure of the answer to a multiple-choice question, don’t spend too long on it.
Put a star 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 left.
In a multiple-choice, question 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 answers ‘D’ in sequence, 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 r.t.p?’ Many Candidates
will focus on the numbers and ignore the word ‘volume’. Just a quick look at the figures
gives the incorrect answer 1:8 (using the molar gas volumes gives 1:1 as the correct
choice)
Theory tips
Gas
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 8 electrons in their outer shell. Remember that helium has 2!
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. If you are unsure which molecule is heavier, use your Periodic Table to calculate the relative molecular masses.
Metals
When given choices of why alloys are hard, it’s not the mass of the atoms which is
important but their size. Remember that metals have layers which slip over each other. A
different sized atom will distort the layers and stop them from slipping over each other. This
makes the alloy harder than pure metals.
Organic Chemistry
When given a choice about electrical conductivity of ionic structures remember that the
conduction is due to IONS moving (not electron). The ions can only move when the ionic
compound is molten or when dissolved in water.
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. It contains compounds dissolved from the rocks or carried in
the rain as well as the chemicals put in to purify it. It’s a mixture. (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 4 to 7 electrons in their outer shell. Remember that the number of
electrons in the outer shell is equal to the group number.
Remember that the valencies (combining powers) of the elements in Groups V to VII are
found by taking the group number away from 8. For example, the valency of oxygen in an
oxide is 8-6 = 2. (Oxygen is in Group VI)
GCE O Level Chemistry Paper 3 (Practical)
Observations
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.
Also, don’t write down things like ‘A gas is given off’.
This is not an observation. It is far better to write ‘The mixture effervesces (bubbles)’
The observation of effervescence is often missed out from practical observations. Look
for the bubbles!
Titration
In carrying out titrations you must repeat them until you get at least 2 consistent results or
results which are the closest to each other. You then tick these results only. 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.
Usage of words
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 observation as the presence of colour.
General
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 noticed that one was formed e.g. in the case of adding sodium hydroxide to
aluminium chloride solution. So add about five to ten drops of sodium hydroxide to about
1 cm3
of the solution to be tested. Then observe if a precipitate is formed. Only after this
has been done do you add excess sodium hydroxide.
When describing colours don’t use combinations e.g. bluish-green, yellowish-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 white or yellow
precipitates e.g. silver bromide, silver iodide, lead iodide. You can do this by calling them
creamy yellow, light yellow, deep yellow etc. But do not write green, as so many of you do
for the colour of a silver chloride precipitate when the colour is clearly white.
GCE O Level Chemistry Paper 4 (Alternative to Practical) Tips
Graphs
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
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.
Knowing your theory well
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 a method that is
expected.
Make sure that you know the difference between the tests for oxygen and hydrogen –
these are frequently muddled.
Hydrogen – Lighted splint – Pops (Hylight Pops).
Oxygen – Glowing splint – Relights (ogre)
Specifications
If you are asked to state a test for hydrogen does not just write down ‘Use the pop test’. This
description is too vague – it describes the result and not the test. It is far better to write
‘Insert a lighted splint into the test tube’.
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.
About Tutopiya
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