Putting it together
The whole chapter began with one question: has a new substance formed? Everything after that was a matter of carrying the answer into equations, into metals and into gases.
| Section | The central idea | What to remember |
|---|---|---|
| 16. Changes | physical or chemical? | whether a new substance formed |
| 16.1 Reactions | reactants → products | there are four types |
| 16.2 Equations | writing in symbols | balance with coefficients |
| 16.3 Metals | air, water, acids, salts | reactivity varies from metal to metal |
| 16.4 The series | K down to Au | it settles storage, corrosion and extraction |
| 16.5 Gases | H2, O2, CO2 | the properties fix the collection method |
The three gases together
| Gas | Combustion | Collection | Test |
|---|---|---|---|
| H2 | combustible | downward displacement of water / air | "pop" with a lighted splint |
| O2 | supports combustion | downward displacement of water | relights a glowing splint |
| CO2 | neither | downward displacement of water / upward of air | turns limewater milky |
Mixed exercise (1)
The problem
Write balanced chemical equations for the reactions described below.
(i) Sodium reacts with dilute hydrochloric acid to produce sodium chloride and hydrogen gas.
(ii) Lead nitrate and dilute sulphuric acid react to form lead sulphate and hydrogen nitrate.
(iii) Calcium reacts with water to give calcium hydroxide and hydrogen gas.
(iv) Sodium carbonate and dilute hydrochloric acid react to produce sodium chloride, carbon dioxide gas and water.
The solution
2Na + 2HCl → 2NaCl + H2
Pb(NO3)2 + H2SO4 → PbSO4 + 2HNO3
Ca + 2H2O → Ca(OH)2 + H2
Na2CO3 + 2HCl → 2NaCl + CO2 + H2O
The reaction between aluminium and dilute hydrochloric acid (2Al + 6HCl → 2AlCl3 + 3H2) was balanced step by step in section 16.2.
Mixed exercise (2)
The problem
By Fe2O3 + 3CO → 2Fe + 3CO2, when 100 kg of haematite is reduced, calculate (a) the mass of iron produced, (b) the mass of carbon monoxide consumed and (c) the mass of carbon dioxide added to the environment. (Fe = 56, C = 12, O = 16)
The solution
Fe2O3 = 160, 2Fe = 112, 3CO = 84, 3CO2 = 132
(a) iron = 100 × 112 ÷ 160 = 70 kg
(b) carbon monoxide = 100 × 84 ÷ 160 = 52.5 kg
(c) carbon dioxide = 100 × 132 ÷ 160 = 82.5 kg
The mass balances: in, 100 + 52.5 = 152.5 kg; out, 70 + 82.5 = 152.5 kg.
Mixed exercise (3)
The problem
Which gas is used in each of the following? (i) forming carbonic acid (ii) giving oxygen to critically ill patients (iii) making dry ice (iv) as a raw material for producing ammonia (v) making margarine from vegetable oils (vi) supporting respiration while diving under water (vii) supporting the burning of firewood
The solution
(i) carbon dioxide (ii) oxygen (iii) carbon dioxide
(iv) hydrogen (v) hydrogen (vi) oxygen (vii) oxygen
Glossary
| සක්රියතා ශ්රේණිය | Activity series |
| නිස්සාරණය | Extraction |
| සනීභවනය | Condensation |
| සංයෝජනය | Combination |
| වියෝජනය | Decomposition |
| විස්ථාපනය | Displacement |
| ශීඝ්රතාව | Rate |
| ප්රතිවර්ත්ය ප්රතික්රියාව | Reversible reactions |
| ධාරා ඌෂ්මකය | Thermal kiln |
Chapter summary
- Changes in matter are of two kinds: physical changes and chemical changes.
- Processes in which the composition of the substances present does not change and only their physical state does are called physical changes.
- Changes in which new substances arise from the substances present are called chemical changes.
- Chemical reactions are of four types: combination, decomposition, single displacement and double displacement.
- A chemical change can be shown by a balanced chemical equation, and a great deal of information can be obtained from it.
- The activity series has been built up by comparing the reactions various metals give with air, water, dilute acids and salt solutions.
- Iron, being a metal of moderate reactivity in the middle of the series, is extracted by reduction. The blast furnace is used for this, with haematite, limestone and coke as the raw materials.
- Gold occurs uncombined in the natural environment because its reactivity is very low.
- In the laboratory, different methods are used to collect and to identify gases according to the properties of the gas.