Chemical Reactions: Reactants, Products and Evidence

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1. Key words

2. What is a chemical reaction?

In a physical change (melting, dissolving, boiling) the substance stays the same substance. In a chemical reaction, the starting substances react — their particles join up in a new way — and new substances are made. The new substances have different properties from the old ones.

• Reactants = the substances at the start (the ones that are used up).
• Products = the new substances made at the end.
• Most chemical reactions are irreversible: you cannot easily get the reactants back. A burnt match cannot become a match again; a cooked egg cannot be uncooked.

3. Writing a word equation

We show a reaction with a word equation. The reactants go on the left, the products on the right, and the arrow means "react to make". A plus sign (+) separates substances. Never use an equals sign.

Oxygen from the air is a reactant in burning and rusting, even though you cannot see it. That is why a candle under a jar goes out, and why a nail kept in dry air with no oxygen and no water does not rust.

4. Evidence of a chemical reaction

We cannot see particles, so we look for clues. At Stage 6 you must know three kinds of evidence:

Temperature change: put a thermometer in the reactants, read it before mixing, then read it again after the reaction. Temperature change = higher reading − lower reading. Say whether it went up (got hotter) or down (got colder) — both are evidence.

5. Physical change or chemical reaction?

A candle is a good example of both. The wax near the flame melts (physical, reversible: it sets again when it cools). The wax vapour in the flame burns (chemical: wax + oxygen → carbon dioxide + water), giving out heat and light.

6. Chemical reactions in everyday life

• Cooking: frying an egg, toasting bread and grilling meat all make new substances (the colour change shows this).
• Baking: baking powder reacts in the hot, wet cake mixture and makes carbon dioxide gas; the bubbles make the cake rise.
• Burning fuels: natural gas in a cooker, petrol in a car and wood on a fire react with oxygen and give out heat.
• Rusting: iron on bridges, cars and railings reacts with oxygen and water. Painting or oiling iron keeps the water and oxygen away.
• Food going bad: milk going sour and fruit going brown are slow reactions.

7. Gases have mass too

When a reaction makes a gas in an open flask, the gas escapes into the air and the balance reading goes down. The gas has mass, and it has left the flask. If a balloon (or a stopper) traps the gas, the reading stays the same, because nothing has left. Mass lost = starting mass − final mass.

8. Worked examples

Worked example 1 — Look at the word equation: magnesium + oxygen → magnesium oxide. (a) Name the two reactants. [1] (b) Name the product. [1] (c) Why is this a chemical reaction? [1]

1. (a) The reactants are before the arrow: magnesium and oxygen.
2. (b) The product is after the arrow: magnesium oxide.
3. (c) A new substance (magnesium oxide) is made.
4. Common mistake: writing "magnesium oxide" as a reactant because it contains the word magnesium. Look at which side of the arrow it is on.

Worked example 2 — Yara adds iron filings to blue copper sulfate solution. After a few minutes the solution turns pale green, a brown-red solid forms on the iron, and the thermometer reading rises from 21 °C to 26 °C. Give two pieces of evidence that a chemical reaction happened. [2 marks]

1. The colour changed (blue → pale green, and a red-brown solid appeared) (1).
2. The temperature rose by 5 °C (26 − 21 = 5) (1).
3. Exam tip: describe what you see ("the solution turned from blue to green"), not just "it changed".

Worked example 3 — Before Ziad mixes baking soda and vinegar the thermometer reads 21 °C. After mixing it reads 16 °C. (a) Calculate the temperature change. [1] (b) Is this evidence of a chemical reaction? Explain. [1]

1. (a) 21 − 16 = 5 °C (a fall of 5 °C — the mixture got colder).
2. (b) Yes — a change in temperature is evidence of a reaction; it can be a rise or a fall. (The fizzing, a gas being produced, is more evidence.)
3. Common mistake: thinking only "getting hotter" counts.

Worked example 4 — A flask of vinegar and chalk has a mass of 210.0 g at the start. When the fizzing stops, the mass is 207.8 g. The flask was open. (a) Calculate the mass lost. [1] (b) Explain why the mass went down. [1]

1. (a) 210.0 − 207.8 = 2.2 g.
2. (b) A gas (carbon dioxide) was produced and escaped into the air from the open flask; the gas has mass, so the flask lost mass.
3. Common mistake: "the mass was destroyed" or "gas has no mass". Gases do have mass.

Worked example 5 — Ahmed heats water in a kettle. It bubbles and gets hot. He says: "This is a chemical reaction because there are bubbles and a temperature change." Is he right? Explain. [2 marks]

1. No, he is not right (1).
2. No new substance is made — the bubbles are water vapour, which is still water; it can condense back into liquid water. Boiling is a physical (reversible) change (1).

Practice

All pages in this course

Start here

• Checkpoint 6 Science: The Test and How to Prepare
• Thinking and Working Scientifically: Investigations, Variables, Tables and Graphs

Biology

• The Heart and Circulatory System
• The Lungs and Respiratory System
• Human Reproductive System and Puberty
• Diseases, Hygiene and the Body's Defences
• Food Chains, Food Webs, Energy and Toxins

Chemistry

• Properties of Substances: Melting and Boiling Points, Gases, Conductors
• Reversible Changes and Dissolving
• Boiling and Evaporation
• Chemical Reactions: Reactants, Products and Evidence

Physics

• Mass, Weight and Gravity
• Force Diagrams and Forces on Moving Objects
• Floating and Sinking
• Light: Reflection and Refraction
• Electric Circuits, Symbols, Series and Parallel

Earth and space

• Rocks and Fossils
• Types of Soil
• The Rock Cycle
• The Solar System and the Phases of the Moon

Practice tests

• Checkpoint 6 Science — Practice Test 1
• Checkpoint 6 Science — Practice Test 2
• Checkpoint 6 Science — Practice Test 3