CHEMISTRY · Stage 7 · Chapter 4: Chemical Reactions 1
Chemistry 4.6 Chapter 4 Review: Chemical Reactions 1
5 min read 4 parts Lesson 6 of 6
Lesson summary
4.6 Chapter 4 Review: Chemical Reactions 1 — Alhussan International Academy — Chemistry (Stages 7–9) notes: Revise reactants and products, energy changes, conservation of atoms, precipitation, corrosion and practical observations from Chapter 4.Chemical reaction — it makes new su…
- Mass calculations
- Apparent mass changes
- Precipitation reaction
- Predicting
- Conditions
- Prevention
Part 1 of 4Conservation of atoms
1Principle
— atoms are rearranged, not created or destroyed, so mass is conserved in a closed system and equations must balance.
2Mass calculations2 points
— the total mass of reactants equals the total mass of products.
- If 56 g of iron reacts with sulfur to make 88 g of iron sulfide, the sulfur is 88 − 56 = 32 g.
- If 32 g of sulfur reacts with oxygen to make 64 g of sulfur dioxide, the oxygen is 64 − 32 = 32 g.
3Balancing1 point
— put numbers in front of formulae and never change subscripts, for example 2Mg + O2 → 2MgO.
- Check that every formula is correct before adjusting the numbers.
4Apparent mass changes2 points
— they are due to gases gained or lost.
- Burning magnesium appears to gain mass because oxygen is added.
- Heating a carbonate in an open container appears to lose mass because carbon dioxide escapes.
Part 2 of 4Precipitation
- Precipitation reactionmixing two solutions makes an insoluble solid, for example silver nitrate + sodium chloride → silver chloride (white) + sodium nitrate.
- Predictingnitrates and sodium salts are soluble, and most carbonates and hydroxides are insoluble.
- Filtratethe liquid that passes through the filter, which is different from the precipitate.
Part 3 of 4Rusting
- Conditions — rusting needs both oxygen and water.
- A nail left for a week in water and oxygen forms brown flakes of rust on the surface.
- Prevention — barriers (paint, oil, plastic), galvanising, sacrificial protection or alloying.
| Topic | Remember |
|---|---|
| Rusting | Iron + oxygen + water |
| Precipitate | Insoluble solid from solutions |
| Protection from rust | Barrier, galvanising, sacrificial, alloy |
Part 4 of 4Practical skills
- Observations and conclusionsrecord observations first, and keep conclusions separate.
- Reliabilitycontrol variables and repeat measurements.
- Narrow questionsa narrow, testable question such as the mass change of magnesium heated in air is better than a broad one about all metals.
- Gas testshydrogen gives a squeaky pop with a lighted splint, carbon dioxide turns limewater cloudy and oxygen relights a glowing splint.
- Explanationslink results to particles, because atoms are rearranged.
- Evaluationgive both an advantage and a disadvantage and a justified choice.
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