CP - Acids and Alkalis


CP - Preparing a soluble salt

You can make asoluble salt by reacting an acid that contains one of the ions you want in the salt with an insoluble base that contains the other ion you need.


1. Start by heating the acid in a water bath - this speeds up the reaction.

2. Then add the base to the acid - the base and acid will react to produce a soluble salt (and water). You will know when the base is in excess and all the acid has been neutralised because the excess solid will just sink to the bottom of the flask.

3. Filter off the excess solid to get a solution containing only the salt and water.

4. Heat the solution gently, using a Bunsen burner, to slowly evaporate off some of the water. Leave the solution to cool and allow the salt to crystallise. Filter off the solid salt and leave it to dry.

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CP - Testing for anions

Anions are negative ions.

1. Test for halide ions using silver nitrate solution.
To test for halide ions add some dilute nitric acid followed by a few drops of silver nitrate solution.

Chloride Cl- - white precipitate.
Bromide Br- - cream precipitate.
Iodide I- - yellow precipitate.

2. Test for carbonates using dilute acid.
If there are carbonate ions present, the mixture will fizz as the reaction produces carbon dioxide gas.

3. Test for surface ions using barium chloride solution.
First add some dilute hydrochloric acid to the test sample - this stops any precipitation reactions not involving surface ions from taking place. Then add some barium chloride solution. If there are surface ions in the solution, a white precipitate of barium surface will form.

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CP - Testing for Cations

Cations are positive ions.

Method... (precipitation test)
1. Using a pipette, fill a test tube to a depth of about 2cm with one of the known substances.
2. Using a different pipette, add a few drops of sodium hydroxide solution to the tube. Shake gently.
3. Observe and record the colour of the precipitate.
4. Repeat with the other known solutions.
Aluminium Al3+ - white precipitate and dissolves in sodium hydroxide.
Calcium Ca2+ - white precipitate.
Copper Cu2+ - blue precipitate.
Iron (II) Fe2+ - green precipitate.
Iron (III) Fe3+ - orange precipitate.

You can also test for metal ions using flames tests.
Lithium Li+ - red flame.
Sodium Na+ - orange flame.
Potassium K+ - lilac flame.
Calcium Ca2+ - orange/red flame.
Copper Cu+ - green flame.

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CP - Neutralisation reactions

Investigating the neutralisation reaction between calcium oxide (a base) and dilute hydrochloric acid.

1. Start by measuring out a set volume of dilute hydrochloric acid into a conical flask.
2. Measure out a fixed mass of calcium oxide using a mass balance.
3. Add the calcium oxide to the hydrochloric acid.
4. Wait for the base to completely react, then record the pH of the solution.
5. Repeat steps 2 to 4 until all the acid has reacted.

When you first start adding the base, the pH changes slowly, but there’s a sudden change as you approach the point where the solution is neutral.

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Acid - Alkali Titrations

1. Measure out a set amount of acid into a conical flask using a pipette. Add a few drops of indicator.
2. Slowly add alkali to the acid, usinga burette until you reach the end point - when the acids been exactly neutralised and the indicator changes colour.
3. Then carry out the reaction using the exact volumes of alkali and acid but wth no indicator, so the salt won’t be contaminated. The solution that remains is only salt and water.
4. Slowly evaporate off the water and leave the solution to crystallise. Filter off the solis, dry it and you’re left with a pure, dry salt.

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