Revise Chemistry Paper 2 Key Concepts

Chemistry Paper 2 tests understanding of units 6 – 10 of the GCSE. Revisit key concepts within these units and brush up your knowledge in preparation for the exam.

Unit 6: The Rate and Extent of Chemical Change


The rate of a chemical reaction will be increased if there are more frequent successful collisions between reactant particles.

Factors Affecting the Rate of a Chemical Reaction

  • Concentration and Pressure – If the number of reactant particles in a given space is doubled, there will be more frequent successful collisions between reactant particles, therefore, increasing the rate of reaction
  • Catalyst – A catalyst is a substance that speeds up a chemical reaction without getting used up itself. Catalysts are able to offer an alternative pathway at a lower activation energy. When a catalyst is used in a chemical reaction, the frequency of collisions is unchanged. More particles are able to react. The particles have energy greater than that of the activation energy. Consequently, there is in an increase in the rate successful of collisions.
  • Surface area – Large lumps of a solid have a small surface area to volume ratio. If the solid is broken up into smaller lumps or crushed into a powder, this will increase the surface area to volume ratio. A larger area of the solid is now exposed to other reactant particles, therefore, increasing the frequency of successful collisions thus increasing the rate of reaction
  • Temperature – When the temperature of the reaction mixture is increased, the reactant particles gain kinetic energy and move much more quickly. This results in more frequent successful collisions between the reactant particles, therefore, increasing the rate of the reaction.

Reversible Reactions

A reversible reaction is one in which the reactants form products. The products are then able to react together to reform the reactants.

For example:

A reacts with B to form C and D.

C and D are able to react to form A and B.

The equation would be as follows (where the double arrow symbol represents a reversible reaction is taking place):

  • A + B ⇌ C + D

The forward reaction goes to the left and the backwards reaction goes to the right. For example, if the forward reaction is exothermic then the backward reaction will be endothermic. The amount of energy that is transferred is the same for both the forward and reverse reaction.

Get all the facts in our Chemistry Unit 6 knowledge organiser.

Unit 7: Organic Chemistry

oil rig

Crude Oil

Hydrocarbons are compounds that are made up of the elements hydrogen and carbon only.

Crude oil is a non-renewable resource, a fossil fuel. Crude oil is made up of a mixture of compounds, most of which are long- and short-chain hydrocarbons.

Most of the compounds in crude oil are hydrocarbons called alkanes. The alkanes form a homologous series. This is a family of hydrocarbons that all share the same general formula and have chemical properties that are similar.

Alkanes are held together by single bonds.

Alkanes are saturated hydrocarbons. This means that all their bonds are taken up and they cannot bond to any more atoms.

Alkanes have similar chemical properties but have different physical properties due to differences in chain length. The longer the chain, the higher the boiling point of the hydrocarbon.

The first four alkanes are: methane, ethane, propane and butane.

A mnemonic to help you remember the order of the alkanes: mice eat paper bags.

Test for Alkanes

Bromine, when added to an alkane, will remain brown/orange. Alkanes are saturated hydrocarbons, they have no double bonds which could be broken to accept the bromine molecule and so remains orange.

Bromine, when added to an alkene, will change from brown/orange to colourless. This is because alkenes are unsaturated hydrocarbons. The double bond breaks and the bromine molecule is accepted.

Get all the facts in our Chemistry Unit 7 knowledge organiser. 

Unit 8: Chemical Analysis

Abstract mixture

Pure Substances

Pure substances, in chemistry, only contain one type of element or one type of compound. For example, pure water will just contain water (a compound).


Formulations are mixtures of compounds or substances that do not react together. They do produce a useful product with desirable characteristics or properties to suit a particular function.

There are examples of formulations all around us such as medicines, cleaning products, deodorants, hair colouring, cosmetics and sun cream.

Identification of the Common Gases

The Test for Hydrogen – Place a burning splint at the opening of a test tube. If hydrogen gas is present, it will burn rapidly with a squeaky-pop sound.

The Test for Oxygen – Place a glowing splint inside a test tube. The splint will relight in the presence of oxygen.

The Test for Carbon DioxideCalcium hydroxide (lime water) is used to test for the presence of carbon dioxide. When carbon dioxide is bubbled through or shaken with limewater, the limewater turns cloudy.

The Test for ChlorineDamp litmus paper is used to test for chlorine gas. The litmus paper becomes bleached and turns white.

Get all the facts in our Chemistry Unit 8 knowledge organiser. 

Unit 9: Chemistry of the Atmosphere

Earth surface

What is the Difference Between Climate Change and Global Warming?

Since the Earth was formed over 4.6 billion years ago, its climate has constantly been changing with several ice ages followed by warmer temperatures. Changes in the Sun’s energy reaching the Earth and volcanic eruptions were responsible until about 200 years ago.

Global warming is different to climate change and is used to explain how the earth’s climate has warmed up over the past 200 years. Scientists believe that the warming of the climate is due to the activities of humans.

Carbon Footprint

The carbon footprint is the total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service or event.

An individual’s carbon footprint is a calculation of all the activities that that person has taken part in throughout the year.

These activities might involve flying abroad or travelling by bus or rail. Each of which might be powered by petrol or diesel. Heating a home in winter by using a gas-powered boiler and using electricity to power lights and electronic devices. Food also has a carbon footprint, for example, beef and rice produces huge amounts of methane when farmed.

Get all the facts in our Chemistry Unit 9 knowledge organiser. 

Unit 10: Using Resources

Sea water


Potable water is water that is safe to drink. Potable water is not pure, dissolved impurities remain in the water. Pure water is odourless, tasteless and colourless compared to rainfall from streams and wells, which harbour chemicals such as acid.

Desalination of Sea Water

If fresh water supplies are limited sea water can undergo a process called desalination. This process requires large amounts of energy but can be done by distillation or the use of membranes such as reverse osmosis.

Distillation of Sea Water

Distillation involves heating the sea water until it reaches boiling point. Once the water is boiling it will begin to evaporate, the steam rises up where it cools and condenses in a condensing tube. The salt is left behind. The downside to this process is the energy cost of boiling the water and cooling down the steam sufficiently in the condensing tube. Not all of the water evaporates leaving behind salty waste water that can be difficult to sustainably dispose of without harming aquatic organisms.


There are both advantages and disadvantages to recycling materials.


  • Fewer resources such as mines and quarries are needed to remove raw finite materials from the ground, for example, copper.
  • Crude oil, the raw material used in the production of plastics does not need to be extracted. This, in turn, avoids high energy cost processes such as fractional distillation and cracking. If more items are recycled, less would end up in landfill sites.
  • The amount of greenhouse gases would reduce as the energy cost of recycling is a lot less than making a new product.


  • Recycling items require collection and transport of the goods to the organisation. This involves using staff, vehicles and the use of fuel.
  • Some materials such as metals can be difficult to sort, the amount of sorting is dependent on the purity of the materials or metals and the level of purity required for the final product. For example copper used in electrical appliances must have a high purity, to achieve this the copper needs to be processed and then melted down again to make copper wiring.

Steel used in the construction industry does not require such high purity. Often scrap iron is added to the furnace when steel is made, this avoids the need for so much iron ore and therefore reduces the cost of making steel.

Get all the facts in our Chemistry Unit 10 knowledge organiser. 

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