Physical and chemical changes
- In a physical change, no new substance is made and the change is usually easy to reverse (melting, dissolving).
- In a chemical change, new substances are made and it is usually hard to reverse (burning, rusting, cooking).
- Signs of a chemical reaction: a colour change, fizzing (a gas is made), a temperature change, a solid forming in a liquid (a precipitate), light, or a smell.
Conservation of mass and equations
- Atoms aren't created or destroyed in a reaction, only rearranged. So the total mass of the reactants equals the total mass of the products.
- If a gas escapes from an open container, the mass seems to go down; if a substance gains oxygen from the air, the mass goes up.
- A word equation shows reactants → products: magnesium + oxygen → magnesium oxide.
- A balanced symbol equation has the same number of each type of atom on both sides: 2Mg + O₂ → 2MgO. To balance an equation, change the big numbers in front of the formulae, never the small numbers inside them.
Oxidation and reduction
- Oxidation is gaining oxygen, as in burning (combustion) and rusting. Reduction is losing oxygen.
- Magnesium burns with a bright white flame to make magnesium oxide, a white powder. Its mass increases because it gains oxygen.
- Burning a fuel: fuel + oxygen → carbon dioxide + water.
- Carbon can remove oxygen from some metal oxides: iron oxide + carbon → iron + carbon dioxide. The iron oxide is reduced.
Acids with metals, and testing for gases
- Acid + metal → salt + hydrogen. For example, magnesium + hydrochloric acid → magnesium chloride + hydrogen.
- Hydrogen: a lit splint makes a squeaky pop.
- Oxygen: it relights a glowing splint.
- Carbon dioxide: it turns limewater milky.
Key terms
- Physical change
- A change where no new substance is made.
- Chemical change
- A change where new substances are made.
- Reactant
- A substance that reacts at the start of a reaction.
- Product
- A substance made in a reaction.
- Conservation of mass
- The total mass stays the same in a reaction.
- Word equation
- An equation that shows the names of the reactants and products.
- Balanced equation
- A symbol equation with the same number of each atom on both sides.
- Oxidation
- Gaining oxygen.
- Reduction
- Losing oxygen.
- Combustion
- Burning a substance in oxygen.
- Precipitate
- A solid that forms when two solutions react.