Changes to Materials
Section: Chemistry | Syllabus: Cambridge Primary Science (0846)
Physical and Chemical Changes
- Substances can undergo different types of changes - some can be undone, while others create something entirely new
Reversible Physical Changes
- A physical change alters how a substance looks or feels, but does not change it into a new substance
- most physical changes are reversible
- Change of state: melting ice becomes water; freezing water becomes ice
- Liquid water ↔ Solid ice (heating melts ice, cooling freezes water)
- Dissolving: mixing salt into water
- this is different from melting - melting is a change of state caused by heat, while dissolving is a solid mixing into a liquid to form a solution
- Compressing a gas: squashing a gas into a smaller space (e.g. in a bicycle pump) - releasing it lets it expand back
Heating and cooling reverse the change between ice and water
Irreversible Chemical Changes
- Sometimes changes cannot be reversed - these are often chemical changes, where one substance turns into another
- Burning: when a match burns, the wood changes into a black substance called carbon
- Cooking: frying an egg or baking a cake changes the ingredients' structure forever
- Decaying: fruit rotting or decaying is also a chemical change - it cannot be turned back into fresh fruit
A burnt match cannot be changed back into a new match
Dissolving and Solutions
- Some substances can dissolve in liquids to form a solution
- Solute: the substance that dissolves (e.g. salt)
- Solvent: the substance in which the solute dissolves (e.g. water)
- Solution: the mixture of solute and solvent
- has a uniform appearance (looks the same throughout)
Solute particles spread evenly between solvent particles in a solution
Making Solids Dissolve Faster
The rate (speed) of dissolving can be increased by:
- Stirring: causes particles to spread out into the spaces between solvent particles more quickly
- Increasing temperature: heat adds energy to particles, causing them to move faster and spread out more easily
Dissolved Substances Change Boiling Point
- Dissolving a solute (like salt) in water raises its boiling point above 100°C
- the more solute that is dissolved, the higher the boiling point becomes
Boiling vs Evaporation
- Both processes involve a liquid changing into a gas, but they are different
| Feature | Evaporation | Boiling |
|---|---|---|
| Location | Happens only at the surface | Happens throughout the liquid |
| Temperature | Can occur at any temperature | Only happens at the boiling point |
| Energy | Particles on the surface gain enough energy to leave | All particles gain enough energy to become gas |
Evaporation happens at the surface only; boiling happens throughout the liquid
Condensation
- Condensation is the reverse of boiling and evaporation - it happens when a gas cools and turns back into a liquid
- e.g. water vapour in the air condenses into water droplets on a cold glass
Chemical Reactions
- When substances are mixed and form a new substance, it is a chemical reaction - the substances react together
- Reactants: the starting substances (e.g. vinegar + bicarbonate of soda)
- Products: the new substances formed (e.g. carbon dioxide gas)
Evidence of a Reaction
- Gas production: bubbles or fizzing can be seen
- if the gas escapes from an open container, the total mass of the mixture appears to decrease
- Colour change
- Temperature change: the mixture gets hotter or colder
- Smell change: a new smell can appear
Bubbles rising show that a gas has been produced in this reaction
Worked Example: Identifying Reactants
- Question: in the following reaction, name the reactants: Iron + Oxygen → Iron Oxide (Rust)
- Answer: Iron and Oxygen
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