7.1Chemical Reactions
Chemical Reactions — Cambridge Checkpoint Science 0893 Revision Notes & Topical Questions
5 min read 7 parts Lesson 1 of 1 1 quiz
Lesson summary
7.1 Chemical Reactions — CHECKPOINT (CHEMISTRY) notes: Recognise chemical reactions, write word equations and explain conservation of mass and energy changes.Chemical reaction = a change in which reactants combine or break apart to make new substances called productsReactants = t…
- Chemical property
- Apparent changes
- General equations
- Exothermic
- Endothermic
- Precipitate
Section 1 of 7Chemical and physical properties
- Chemical reaction = a change in which reactants combine or break apart to make new substances called products
- Reactants = the substances that react
- Products = the new substances made
- Chemical property
- describes how a substance reacts, e.g. magnesium burns in air
- Examples
- reactions happen when plants grow and decay and when food is digested
Section 2 of 7Burning
- Burning
- a chemical reaction in which a substance reacts with oxygen
- The new substances formed, such as ash, are oxides.
- Carbon
- carbon + oxygen → carbon dioxide
- Magnesium
- magnesium + oxygen → magnesium oxide, a white powder
- Magnesium and oxygen are the reactants and magnesium oxide is the product.
- Magnesium burns very brightly, so do not look directly at the flame.
Section 3 of 7Clues that a reaction has happened
| Clue | Example |
|---|---|
| A gas is given off | Magnesium and hydrochloric acid give hydrogen bubbles |
| A reactant disappears | Magnesium ribbon is used up in acid |
| Colour change | Black copper oxide and sulfuric acid give blue copper sulfate |
| Heat is produced | Zinc with hydrochloric acid warms the tube |
| pH changes | Neutralisation of an alkali by an acid |
| A precipitate forms | Silver nitrate and calcium chloride give solid silver chloride |
Section 4 of 7Tests for gases
| Gas | Test | Result |
|---|---|---|
| Hydrogen | Lighted splint at the mouth of the tube | Squeaky pop |
| Carbon dioxide | Bubble through limewater | Limewater turns cloudy |
| Oxygen | Glowing splint in the tube | Splint relights |
Section 5 of 7Conservation of mass
- Atoms
- are not created or destroyed in a reaction; they are rearranged
- Mass
- the total mass of the products equals the total mass of the reactants
- Apparent changes
- a burning candle seems to lose mass because a gas escapes, and rusting iron gains mass because oxygen joins it
Section 6 of 7Word equations
- Format
- reactants on the left, products on the right, an arrow between
- Example
- magnesium + oxygen → magnesium oxide
- General equations
- 1metal + acid2salt + hydrogen; acid + alkali3salt + water
Section 7 of 7Energy changes
- Exothermic
- a reaction that releases heat, e.g. burning and neutralisation
- Endothermic
- a reaction that takes in heat, so the mixture feels cold
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