GCSE AQA Chemistry Combined Science Paper 2 (Topics C6-C10) Mindmaps

Measuring Reaction Rates

You can easily measure how fast chemical reactions happen by tracking changes over time. Reaction rate simply means how quickly reactants turn into products, and there are three main ways to measure this.

For reactions that produce a precipitate (solid particles), like mixing sodium thiosulfate with hydrochloric acid, you time how long it takes for a black cross underneath your flask to disappear. Higher concentrations make reactions go faster - it's that simple!

When reactions produce gas (like magnesium with HCl), you can measure the volume of gas released using a syringe, or track the mass lost on a balance as gas escapes. The steeper your graph, the faster your reaction.

Remember: Temperature, concentration, and pressure all affect reaction rates - increase any of these and your reaction speeds up!

Crude Oil and Hydrocarbons

Crude oil is basically ancient plankton that's been cooking underground for millions of years - and it's the source of most modern transport fuels. It's a mixture of hydrocarbons (compounds containing only hydrogen and carbon atoms).

Alkanes are the simplest hydrocarbons with the formula CnH2n+2. Think methane (CH4) or ethane (C2H6) - they only have single bonds between atoms. Shorter hydrocarbon chains are more useful because they're more flammable and easier to vaporise.

Fractional distillation separates crude oil into useful products like petrol, diesel, and kerosene. Cracking breaks long, less useful hydrocarbon chains into shorter, more valuable ones using high temperatures and catalysts.

When hydrocarbons burn completely, they produce carbon dioxide and water. Incomplete combustion (not enough oxygen) produces dangerous carbon monoxide instead - that's why proper ventilation matters!

Key point: Shorter hydrocarbon chains = lower boiling points, more flammable, and more useful as fuels.

Chromatography and Chemical Tests

Chromatography is your go-to method for separating and identifying mixtures - it's like giving different substances a race up paper! You place a drop of your mixture on chromatography paper, then let a solvent travel up and carry the substances at different speeds.

Calculate Rf values using: distance travelled by substance ÷ distance travelled by solvent. Pure substances produce single spots, whilst mixtures create multiple spots. This helps you identify unknown substances by comparing Rf values.

Gas tests are dead useful for identification. Hydrogen burns with a squeaky pop, oxygen relights a glowing splint, carbon dioxide turns limewater cloudy, and chlorine bleaches damp indicator paper white.

Understanding whether something is a pure substance, compound, or mixture is crucial. Elements contain one type of atom, compounds have different elements chemically joined, and mixtures contain substances that aren't chemically bonded.

Pro tip: Always use pencil to draw your starting line in chromatography - ink would interfere with your results!

Earth's Atmosphere and Climate Change

Earth's atmosphere has changed dramatically over billions of years, and human activity is now the main driver of change. Originally, our atmosphere was mostly carbon dioxide with no oxygen - quite different from today's 78% nitrogen and 21% oxygen!

Photosynthesis by early plants and algae produced the oxygen we breathe today, whilst also removing carbon dioxide from the atmosphere. This allowed complex life to evolve and created the atmosphere we depend on.

The greenhouse effect occurs when gases like carbon dioxide, methane, and water vapour absorb heat radiation from Earth's surface. More greenhouse gases from burning fossil fuels and deforestation means more heat gets trapped, causing global warming.

Climate change consequences include melting ice caps, rising sea levels, and more extreme weather. Your carbon footprint measures the greenhouse gases released throughout your lifetime - reducing this through renewable energy and efficient processes helps tackle the problem.

Reality check: While the science is clear, reducing emissions requires global cooperation and changes to how we live and work.

Water Treatment and Earth's Resources

Making potable water (safe drinking water) involves multiple treatment stages that remove dangers and improve taste. Water gets screened, filtered through sand and gravel beds, then disinfected with chlorine or UV light to kill harmful microbes.

Sewage treatment uses both physical and biological processes. Screening removes large objects, sedimentation settles heavy particles, and anaerobic digestion by bacteria breaks down organic matter. The treated water can then be safely returned to rivers.

Finite resources like fossil fuels and metal ores will eventually run out, making recycling and sustainability increasingly important. Life cycle assessments (LCA) help evaluate the environmental impact of products from creation to disposal.

Alternative water sources include desalination (removing salt from seawater using reverse osmosis) and better water conservation. These technologies require significant energy but become essential as freshwater becomes scarcer.

Think ahead: Sustainable resource use isn't just environmental - it's about ensuring future generations have what they need to thrive.