IGCSE Chemistry Paper 6 – Common Errors
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What is the purpose of ice or cold water?
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To cool down the gas so that it condenses and turns into a liquid.
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When the gas collecting tube is upside down, give a property of this gas.
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It is less dense than air.
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Why is a pencil used in drawing the origin line in chromatography?
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If pen was used, it would dissolve, giving colours and so the experiment wouldn't be accurate.
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When using ethanol, give a better apparatus arrangement, and why?
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Cover the apparatus with a lid, because ethanol is volatile.
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In the tests and observation tables, when in the first row, they tell you that upon heating condensation occurred. There will be a question asking what this shows about the solid.
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It is hydrated.
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When copper is used in a test and the answer is worth four marks, the best answer is
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light blue precipitate which is soluble in excess to form dark blue solution.
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Why is this experiment done in a fume cupboard?
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It releases harmful gases that are poisonous. It is toxic.
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Which result appears to be inaccurate?
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It is the point not appearing on the drawn graph, you read its x-axis and write it with a reason indicating that it doesn't occur in the graph.
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Why should the solid be crushed?
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It increases the surface area for a faster rate of reaction.
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Why is the experiment done in a well-ventilated room?
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To prevent the burning of the substance.
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Explain the term decant.
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Filter/pour the liquid leaving the solid alone.
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Why is concentrated sulphuric acid not used to dry ammonia?
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Because it will react with the base ammonia, which is a neutralisation reaction.
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Why should samples be taken from different parts of a field?
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To get more accurate results.
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Suggest why it is important to know the pH of soil.
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To see which is the best place for growing, and what kind of base to use for neutralising it.
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What is necessary for rusting?
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Water (humidity) and oxygen (air).
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Suggest why in an experiment for rusting the water level increases.
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Oxygen is used up, and water is used to take its place.
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For electrolysis, state general observations.
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The bulb will light, a metal is formed on the cathode, fizz of gases produced.
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Suggest a suitable material for the electrodes.
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Graphite, carbon, steel.
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When copper oxide is reacted with hydrogen, what is the colour change?
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Black to brown, because copper oxide is reduced to copper.
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How can you distinguish between water and ethanol?
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Use cobalt chloride paper, it turns from pink to blue with water, but there will be no change with ethanol.
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How can you distinguish between sulphuric acid and aqueous sodium sulphate?
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There are there tests. You can use a metal carbonate in which carbon dioxide will be produced with sulphuric acid but there will be no change with sodium sulphate. You can add a metal, in which hydrogen is produced with sulphuric acid, but no change in sodium sulphate. Finally you could use an indicator like litmus paper, it will change to red with sulphuric acid but there will be no change with sodium sulphate.
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How can you distinguish between hydrochloric acid and nitric acid?
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Add silver nitrate, in which white ppt will be formed with hydrochloric acid, but there will be no reaction with nitric acid.
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What is the purpose of mineral wool?
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To absorb and hold the liquid.
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When there is a delivery tube involved in a question, what precaution should be taken in the experiment when the heat is removed?
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Remove the delivery tube from water to prevent suck-back.
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In rate of reactions, always include the words:
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'collisions between particles'
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How can you distinguish between alkanes and alkenes?
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Use bromine water, in which the alkene will decolourise to colourless, but nothing happens with an alkane.
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How can you distinguish between chlorine and sodium chloride?
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Add litmus paper, it will bleach the chlorine, but nothing happens to it with sodium chloride.
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How can you distinguish between copper sulphate and copper carbonate?
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Acidify with hydrochloric acid, add barium chloride, there will be a white ppt with sulphate, but no white ppt with carbonate. OR just add hydrochloric acid, in which nothing happens with sulphate, but a fizz or effervescence of carbon dioxide will occur with carbonate.
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When a measuring cylinder is used, how could you get more accurate results?
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A burette could be used as it is more accurate.
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What happens to the volume of reagent if it is more concentrated?
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The volume of reagent decreases.
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What is the difference between a strong acid/base and a weak acid/base?
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A strong acid lies between pH values of 0 and 2, and a weak one lies between 3 and 6. 7 is neutral. A weak base lies between 8 and 11, while a strong one lies between 12 and 14.
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What is a concentrated acid?
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An acid that contains a large number of H+ ions. Vice versa with dilute acid.
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What is a concentrated base?
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A base that contains a large number of OH- hydroxide ions. Vice versa with dilute base.
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What is a strong acid?
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An acid that completely ionises giving H+ ions in solution. Vice versa with a weak one.
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What is a strong base?
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A base that completely ionises giving OH- ions in solution. Vice versa with a weak one.
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What does concentrated mean?
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A solution that contains a large number of solute molecules or little amount of water is involved.
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How can you make crystals?
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1. Heat to the point of crystallisation. 2. Leave to cool gradually. 3. Filter, dry and collect the crystals.
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How can you detect the point of crystallisation?
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Place a stirring rod in the solution and see the formation of the first crystals on it.
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What do you use to crush?
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Pestle and mortar.
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Why is grass ground with ethanol?
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Grass is ground with ethanol rather than water because chlorophyll is more soluble in ethanol.
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What is the colour of rusty iron filings?
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Brown (orange and red are also acceptable)
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What is the effect on using pure oxygen on rusting?
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Rusting is faster when pure oxygen is used.
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How can you speed up drying?
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By using a fan or by increasing temperature, or by using a hairdryer if you have one. NOT by using a catalyst.
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What is the purpose of a Liebig condenser?
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To change steam to water.
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How do you check the purity of a collected solvent?
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By testing its melting or boiling points.
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What is a locating agent?
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A chromatogram needs to be sprayed with a locating agent if amino acids are investigated because they are colourless.
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What is the effect of salty water on rusting?
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If water contained salt, it would have no effect on rusting, however if a bigger substance is being rusted, it will be slower.
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Colour change of hydrated copper sulphate upon heating
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Copper sulphate will turn from blue to white.
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Saturated
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No more solute can be dissolved in a solvent AT A CONSTANT TEMPERATURE.
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What is the purpose of excess reactants?
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To make sure all the other reactant will be used.
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Drying of crystals
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Sometimes, crystals are dried using filter paper instead of heating to prevent the compete loss of water from crystals, and to prevent crystals from breaking.
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How could you know which reactant is in excess?
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At the end of the reaction, the excess reactant will be visible.
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What does excess mean?
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More than what you need
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Safety precautions
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When lead bromide is used, you need to use a fume cupboard or use goggles, lab coat, gloves because it is toxic.
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Fractional distillation
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The separation of two or more different solutions with different boiling points.
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Physical test for water
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Heat, it will boil at 100 degrees Celsius, or heat ice and it will melt at 0 degrees Celsius.
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Chemical test for water
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Add blue cobalt chloride paper, it turns pink. OR add anhydrous copper sulphate, it turns blue.
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Water baths
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Fire will be produced if alcohol is touched with a lighted splint, so water baths should be used when heating alcohol. Lagging or cloth can be used to control temperature for accurate results.
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Excess reactant
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Unreacted reactant which has not been used
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Collisions
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As reactants are heated, the particles gain energy, move faster, and their kinetic energy increases therefore there will be more collisions and the rate of reaction increases.
