GoalDescribe the reaction of an acid with a carbonate, recognise a precipitation reaction, and use these ideas for hard water.
New words
precipitate · an insoluble solid that forms when two solutions reacthard water · water that contains calcium and magnesium saltscarbonate · a compound that contains the CO₃²⁻ ion, such as sodium carbonate
Explanation
When an acid meets a carbonate the result is a salt, water and carbon dioxide gas: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. First the acid forms carbonic acid, which is unstable and breaks into water and carbon dioxide, so we see bubbles. At home, baking soda and vinegar in an open cup fizz for the same reason. The test of the gas with limewater is a laboratory test: we only read about it. A precipitation reaction is another double replacement: two clear salt solutions are mixed and an insoluble solid, the precipitate, appears. AgNO₃ + NaCl → NaNO₃ + AgCl↓, where the white solid AgCl is the precipitate, not the liquid. Hard water has calcium and magnesium salts, which leave deposits in pipes and kettles. A treatment plant adds sodium carbonate, so the calcium and magnesium ions form insoluble carbonates that can be removed. Silverware turns dull because a black layer of silver sulphide forms. Aluminium is more reactive than silver, so it can take the sulphur away and leave the silver bright again.
acid + carbonate→salt + carbonic acid→water + carbon dioxide gas
Worked examples
CaCO3 + 2HCl→CaCl2 + H2O + CO2
Which gas bubbles off in CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂? Carbon dioxide, so a carbonate (chalk, marble, a shell) fizzes with acid. The other products, calcium chloride and water, stay in the liquid.
Ahmed mixes two clear solutions and a white cloudy solid forms at the bottom. The solid is a precipitate. The clear liquid above it still has dissolved salts. So the precipitate is the solid, not the whole mixture.
A kettle in a hard-water area gets a white crust. The calcium and magnesium salts left behind form the crust. Treatment plants add sodium carbonate so that these ions form insoluble carbonates, which are removed, and the water becomes softer.
Class activity
Fizz and observe, with a grown-up. Put a spoonful of baking soda in an open cup and pour in a little vinegar. Describe what you see. Do not use a closed bottle. Then write the idea as a word equation: acid + carbonate → salt + water + carbon dioxide. Add a second activity: sort cards (dull silver spoon, kettle crust, fizzing cup, white solid in a clear liquid) under the headings hard water, tarnish, acid with carbonate, precipitate.
Practice
1
Name the three products when sodium carbonate reacts with hydrochloric acid.
Sodium chloride, water and carbon dioxide.
2
Why does a mixture of baking soda and vinegar fizz?
The acid reacts with the carbonate and carbon dioxide gas escapes.
3
In AgNO₃ + NaCl → NaNO₃ + AgCl↓, which substance is the precipitate?
AgCl, the white solid (marked by the downward arrow).
4
Why does adding sodium carbonate soften hard water?
Calcium and magnesium ions form insoluble carbonates that can be removed.
