GCSE EDEXCEL - SALTS

Making salts from acids and insoluble reactants

A soluble salt can be prepared by reacting an acid with a suitable insoluble reactant including:

• a metal
• a metal oxide
• a carbonate

The insoluble reactant chosen depends upon the particular salt required. For example, copper does not react with dilute acids, so copper salts are made using copper oxide or copper carbonate, not copper metal. On the other hand, sodium is too reactive to be used safely - again the metal is not used to make sodium salts.

As the reaction between metals and acids produces flammable hydrogen, chemists usually make salts by reacting a metal oxide or a metal carbonate with an acid.

Naming salts

The name of a salt has two parts. The first part comes from the metal, metal oxide or metal carbonate. The second part comes from the acid:

• hydrochloric acid produces chloride salts
• nitric acid produces nitrate salts
• sulfuric acid produces sulfate salts

The table shows some examples of the salts produced by different combinations of insoluble reactants and acids.

	Hydrochloric acid	Sulfuric acid	Nitric acid
Copper oxide	Copper chloride	Copper sulfate	Copper nitrate
Aluminium hydroxide	Aluminium chloride	Aluminium sulfate	Aluminium nitrate
Zinc carbonate	Zinc chloride	Zinc sulfate	Zinc nitrate

Making a salt

To make a soluble salt from an acid and an insoluble reactant:

1. Add some dilute hydrochloric acid to a beaker.
2. Add powdered insoluble reactant to some acid in a beaker, one spatula at a time, stirring to mix. The mixture will effervesce. Continue adding powder until some unreacted powder is left over - it is in excess.
3. Filter the mixture in the beaker to remove the excess powder.
4. Allow the water in the solution to evaporate (by heating and/or leaving for a few days) to obtain pure dry crystals of the salt.

Notes on each step

• To make sure all of the acid has reacted, add the excess of the solid reactant.
• Filtering removes the unreacted insoluble reactant from the salt solution.
• As the acid is all used up and the insoluble reactant has been removed, this only leaves the salt and water. Therefore evaporating the water leaves the pure salt.

Core practical

Making copper sulfate crystals

There are a number of ways that you could make copper sulfate crystals in Chemistry. This is an outline of the required steps to undertake one of these methods.

Aims

To investigate the preparation of pure, dry hydrated copper sulfate crystals starting from copper oxide.

Method

1. Place some sulfuric acid in a conical flask and warm it in a water bath.
2. Add a spatula of copper oxide powder to the acid and stir with a glass rod.
3. Continue adding copper oxide powder until it is in excess.
4. Filter the mixture to remove the excess copper oxide.
5. Pour the filtrate (the copper sulfate solution) into an evaporating basin.
6. Heat the copper sulfate solution to evaporate half of the water.
7. Pour the solution into a watch glass and leave to allow all of the water to evaporate.

Results

Record the appearance of the copper sulfate crystals, including their colour and shape.

Analysis

Hydrated copper sulfate crystals should be blue and regularly shaped. Describe how your crystals compare to this description. Suggest an explanation for any differences.

Copper sulfate crystals

Evaluation

Question

Explain why the sulfuric acid is warmed in a water bath in step 1.

Warm acid will react faster than cold acid, helping to make sure that all the acid reacts with the copper oxide.

Hazards, risks and precautions

It is important in this practical activity to use appropriate apparatus and methods. This includes the safe use and careful handling of substances and equipment.

Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:

Hazard	Possible harm	Precaution
Sulfuric acid	Concentrated acid is corrosive and damages skin and clothes	Use dilute sulfuric acid
Boiling water bath	Skin burns	Ensure the boiling water bath is stable
Hot copper sulfate solution spitting out during crystallisation	Damage to eyes and skin	Wear eye protection and avoid standing over the hot apparatus

Making salts from acids and alkalis

A soluble salt can be prepared by reacting an acid with a soluble reactant. This is usually a dilute solution of an alkali such as sodium hydroxide or ammonia. The main steps are:

1. Carry out a titration. This is to determine the volumes of acid and alkali that must be mixed to obtain a solution containing only salt and water.
2. Mix the acid and alkali in the correct proportions, as determined in step 1.
3. Allow the water in the solution to evaporate (by heating and/or leaving for a few days) to obtain pure dry crystals of the salt.

Carrying out a titration

Apparatus

• A pipette to accurately measure the volume of a reactant before transferring it to a conical flask.
• A burette to add small, measured volumes of one reactant to the other reactant.
• A suitable indicator.

A pipette filler is needed to use a pipette safely

Method

This is an outline method for carrying out a titration in which an acid is added to an alkali.

1. Use the pipette and pipette filler to add a measured volume of alkali to a clean conical flask.
2. Add a few drops of indicator and put the conical flask on a white tile.
3. Fill the burette with acid and note the starting volume.
4. Slowly add the acid from the burette to the alkali in the conical flask, swirling to mix.
5. Stop adding the acid when the end-point is reached (when the indicator first permanently changes colour). Note the final volume reading.
6. Repeat steps 1 to 5 until you get concordant titres. More accurate results are obtained if acid is added drop by drop near to the end-point.

Results

Record the results in a suitable table. The one here also shows some sample readings.

Run	End vol	Start vol	Titre
Rough	26.85 cm3	1.00 cm3	25.85 cm3
1	24.60 cm3	0.00 cm3	24.60 cm3 ✔
2	25.90 cm3	0.60 cm3	24.30 cm3
3	24.00 cm3	0.80 cm3	24.70 cm3 ✔

Readings should be recorded to two decimal places, ending in 0 or 5 (where the liquid level is between two graduations on the burette). The titre is the volume added (the difference between the end and start readings).

Analysis

Tick (✔) at least two concordant titres. These are titres within 0.20 cm³ (or sometimes 0.10 cm³) of each other.

Worked example

Calculate the mean titre.

Worked example answer

Ignoring the rough run, and run 2 (because it is not concordant):

Mean titre = 

= 24.65 cm³

Titration must be used to obtain a solution of a salt and water only, when using an acid and an alkali. This is because there is no insoluble excess reactant that could be removed by filtration.

Solubility rules

Solubility

A substance's solubility is a measure of the maximum mass that will dissolve in a given volume of solvent, at a particular temperature. Substances that are very soluble have high solubilities. Substances that are insoluble or sparingly soluble (almost none dissolves) have low solubilities.

Solubility in water

The table summarises whether common ionic compounds are soluble or insoluble in water.

Soluble	Insoluble
All common sodium, potassium and ammonium salts
All nitrates
Most common chlorides	Silver chloride, lead chloride
Most common sulfates	Lead sulfate, barium sulfate, calcium sulfate
Sodium carbonate, potassium carbonate, ammonium carbonate	Most common carbonates
Sodium hydroxide, potassium hydroxide, ammonium hydroxide	Most common hydroxides

The top two rows explain why so many salt solutions used in the laboratory are sodium or potassium compounds or nitrates.

Precipitates

A precipitate is an insoluble product that forms when two solutions are mixed and react together. The reaction that produces a precipitate is called a precipitation reaction.

For example, a precipitate of lead iodide forms when potassium iodide solution and lead nitrate solution are mixed:

potassium iodide + lead nitrate → potassium nitrate + lead iodide

2KI(aq) + Pb(NO₃)₂(aq) → 2KNO₃(aq) + PbI₂(s)

Notice how important state symbols are in the balanced equations for precipitation reactions. Without the (s) for solid, it would not be obvious that PbI₂ (lead iodide) was the precipitate.

A yellow precipitate of lead iodide

Predicting precipitates

The formation of a precipitate, when two solutions are mixed, can be predicted using the solubility rules in the table:

• if the products meet the 'soluble' rules in the left hand column, no precipitate will form
• if at least one product meets the 'insoluble' rule in the right hand column, a precipitate will form

Worked example

Silver nitrate solution is mixed with sodium chloride solution. Predict whether a precipitate forms and name any precipitate.

Worked example answer

A precipitate of silver chloride will form.

Question

Potassium sulfate solution is mixed with barium chloride solution. Predict whether a precipitate forms, and name any precipitate.

A precipitate of barium sulfate will form.

Making insoluble salts

An insoluble salt can be prepared by reacting two suitable solutions together to form a precipitate.

Determining suitable solutions

All nitrates and all sodium salts are soluble. This means a given precipitate XY can be produced by mixing together solutions of:

• X nitrate
• sodium Y

For example, to prepare a precipitate of calcium carbonate:

• X = calcium and Y = carbonate
• mix calcium nitrate solution and sodium carbonate solution together

calcium nitrate + sodium carbonate → sodium nitrate + calcium carbonate

Ca(NO₃)₂(aq) + Na₂CO₃(aq) → 2NaNO₃(aq) + CaCO₃(s)

It also works if potassium carbonate solution or ammonium carbonate solution is used instead of sodium carbonate solution. Remember that all common potassium and ammonium salts are soluble.

Question

Name two suitable solutions that, when mixed together, will produce a precipitate of silver chloride.

Silver nitrate solution and sodium chloride solution (or potassium chloride solution, or ammonium chloride solution).

An outline method

This is an outline method for making a pure, dry sample of an insoluble salt.

1. Mix together two suitable solutions (see above).
2. Use filtration to separate the precipitate as a residue from the solution.
3. Wash the precipitate th distilled water while it is in the filter funnel.
4. Leave the washed precipitate aside or in a warm oven to dry.

Reasons for each step

• Filtration separates insoluble substances from liquids and solutions.
• The precipitate is insoluble in water, so any remaining contaminating solution can be removed by washing it with distilled water.
  
  
  
  
  
  
  

Rough

26.85 cm³

1.00 cm³

25.85 cm³

1

24.60 cm³

0.00 cm³

24.60 cm³ ✔

2

25.90 cm³

0.60 cm³

24.30 cm³

3

24.00 cm³

0.80 cm³

24.70 cm³ ✔

All common sodium, potassium and ammonium salts

All nitrates

Most common chlorides

Silver chloride, lead chloride

Most common sulfates

Lead sulfate, barium sulfate, calcium sulfate

Sodium carbonate, potassium carbonate, ammonium carbonate

Most common carbonates

Sodium hydroxide, potassium hydroxide, ammonium hydroxide

Most common hydroxides

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