Unit 8: Chemical reactions
Detecting chemical reactions — HIA International - Grade 6 Revision Notes & Topical Questions
5 min read 10 parts Lesson 13 of 13 3 quizzes
Lesson summary
8.4 Detecting chemical reactions — HIA INTERNATIONAL notes: Recognise the clues that show a chemical reaction has happened and test for hydrogen, oxygen and carbon dioxide.Chemical reaction — new products form from the reactants, and the change is usually hard to reverse.Physical…
- Physical change
- Word equation
- Conservation of mass
- Precipitate
- Silver nitrate + calcium chloride
- Reason for cloudiness
Part 1 of 8Chemical reactions and physical changes
Chemical reaction
- Chemical reaction new products form from the reactants, and the change is usually hard to reverse.
Physical change
- Physical change no new substance forms, for example melting, dissolving and boiling, and it is usually easy to reverse.
Word equation
- Word equation reactants then products, for example magnesium + hydrochloric acid gives magnesium chloride + hydrogen.
Conservation of mass
- Conservation of mass atoms are rearranged, so the total mass of products equals that of reactants unless a gas escapes from an open container.
Part 2 of 8Clues that a chemical reaction has happened
| Clue | What you observe | Example |
|---|---|---|
| A gas is given off | Bubbles or fizzing | Magnesium with hydrochloric acid gives hydrogen |
| A reactant disappears | A solid is used up | Magnesium ribbon vanishes in acid |
| A colour change | A new colour appears | Black copper oxide with sulfuric acid gives blue copper sulfate |
| Heat is produced | The container gets hot | Zinc with hydrochloric acid, or potassium with water |
| A precipitate forms | A solid appears in a liquid | Silver nitrate with calcium chloride gives silver chloride |
| A change in pH | The pH moves to 7 | Neutralising an alkali with an acid |
| Light or sound | A flash or a bang | Fireworks |
Part 3 of 8Precipitates
1Precipitate
Precipitate an insoluble solid formed when two solutions react or when a gas reacts with a solution.
2Silver nitrate + calcium chloride
- Silver nitrate + calcium chloride gives a precipitate of silver chloride and calcium nitrate.
3Limewater
- Limewater a solution of calcium hydroxide that turns cloudy when carbon dioxide is bubbled through it.
4Reason for cloudiness
- Reason for cloudiness a precipitate of calcium carbonate forms, and calcium hydroxide + carbon dioxide gives calcium carbonate + water.
Part 4 of 8Temperature changes
Exothermic reaction
- Exothermic reaction gives out heat, so the temperature of the surroundings rises, as in burning and neutralisation.
Endothermic reaction
- Endothermic reaction takes in heat, so the temperature falls.
Temperature change
- Temperature change measured with a thermometer as final temperature minus starting temperature.
Part 5 of 8Tests for gases
| Gas | Test | Positive result | Source of the gas |
|---|---|---|---|
| Hydrogen | Lit splint at the mouth of the tube | Squeaky pop | Magnesium or zinc with hydrochloric acid |
| Oxygen | Glowing splint in the tube | Splint relights | Hydrogen peroxide with apple or manganese dioxide |
| Carbon dioxide | Bubble the gas through limewater | Limewater turns cloudy | Baking powder with vinegar, or calcium carbonate with hydrochloric acid |
Part 6 of 8Testing for carbon dioxide
1
Half-fill a test tube with limewater.
2
Put a small amount of calcium carbonate in another test tube and add hydrochloric acid.
3
Quickly fit a bung and delivery tube.
4
Make sure the delivery tube reaches into the limewater.
5
Watch the limewater turn cloudy.
Part 7 of 8Testing for oxygen
1
Half-fill a test tube with hydrogen peroxide.
2
Add a spatula of manganese dioxide.
3
Place a glowing splint into the neck of the test tube.
4
The splint relights if the gas is oxygen.
Part 8 of 8Safety in reaction experiments
Protection
- Protection wear safety glasses and carry out a risk assessment first.
Heating
- Heating never boil the blue copper sulfate solution, because harmful fumes are given off.
Potassium with water
- Potassium with water only a teacher may demonstrate it.
Key terms
| Term | Meaning |
|---|---|
| Chemical reaction | A change that forms new substances. |
| Precipitate | An insoluble solid formed in a reaction. |
| Limewater | A solution of calcium hydroxide used to test for carbon dioxide. |
| Exothermic | A reaction that gives out heat. |
| Endothermic | A reaction that takes in heat. |
| Glowing splint | A splint with a glowing end, used to test for oxygen. |
| Reactant | A starting substance in a reaction. |
| Product | A new substance formed in a reaction. |
Avoid these mistakes
1
Thinking bubbles always mean a chemical reaction — boiling also makes bubbles and is a physical change.
2
Saying a colour change alone proves a reaction — all clues should be considered.
3
Testing a gas without a specific test — each gas needs its own test.
4
Saying mass is lost when a gas escapes — the mass has left as a gas.
Marked practiceQuizzes for this lesson
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