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Auralith Learning

Year 8 · Science · Chemical reactions · Lesson 1

Chemical reactions and word equations

Chemical and physical changes, signs of a reaction, writing word equations, conservation of mass, and exothermic and endothermic reactions.

Whiteboard video lesson: Chemical reactions and word equations

26-minute video lesson

Teaching, worked examples and 20 practice questions solved step by step.

Watch free for a month

What your child will learn

  • Tell chemical and physical changes apart
  • Identify reactants and products
  • Write word equations
  • Use conservation of mass in calculations
  • Identify exothermic and endothermic reactions

The method

  1. In a chemical reaction new substances are made; in a physical change they are not.
  2. Word equations: reactants → products.
  3. Mass is conserved: mass of reactants = mass of products.
  4. Exothermic reactions give out heat; endothermic reactions take it in.

Key words: chemical reaction, physical change, reactant, product, word equation, conservation of mass, exothermic, endothermic.

Worked examples

The first three practice questions from the lesson, with every step.

Question 1

Give four signs that a chemical reaction is happening.

  1. Step 1: Fizzing (a gas is given off); a colour change
  2. Step 2: A temperature change; light, or a smell

Answer: e.g. fizzing, colour change, temperature change, light

Question 2

Is burning paper a chemical or a physical change? Explain.

  1. Step 1: New substances: ash, smoke, carbon dioxide
  2. Step 2: Can't be reversed → chemical

Answer: Chemical: new substances are made and it can't be reversed

Question 3

Is dissolving sugar in water a chemical or a physical change?

  1. Step 1: The sugar is still sugar
  2. Step 2: Evaporate the water to get sugar back → physical

Answer: Physical