Chemical tests for water

Definition

Anhydrous substance

A substance that contains no water of crystallisation.

Definition

Hydrated substance

A substance whose crystals contain water chemically combined within them, called water of crystallisation.

Many colourless liquids look exactly like water, so you cannot tell whether a liquid is water, or contains water, just by looking at it.
A chemical test for water uses an anhydrous salt that changes colour when it reacts with water and becomes hydrated.
Two anhydrous salts are used for this test: anhydrous cobalt(II) chloride and anhydrous copper(II) sulfate.
Anhydrous cobalt(II) chloride, $CoCl2\mathrm{CoCl_{2}}$ , is blue.
When water is added, it turns pink, because it combines with the water to form hydrated cobalt(II) chloride, $CoCl2⋅6H2O\mathrm{CoCl_{2} \cdot 6H_{2}O}$ .
In the laboratory this test is usually carried out with cobalt(II) chloride paper, which is filter paper that has been soaked in cobalt(II) chloride solution and dried in an oven until it is blue.
Anhydrous copper(II) sulfate, $CuSO4\mathrm{CuSO_{4}}$ , is a white powder.
When water is added, it turns blue, because it combines with the water to form hydrated copper(II) sulfate, $CuSO4⋅5H2O\mathrm{CuSO_{4} \cdot 5H_{2}O}$ .
The reaction between anhydrous copper(II) sulfate and water is exothermic, so the powder also becomes warm as it turns blue.
Both colour changes are the reverse of the changes seen when the hydrated salts are heated, so used test paper or powder can be heated to dry it and then used again.
A positive result shows only that water is present in the liquid.
Any liquid that contains water gives the same colour change, including salt solutions, dilute acids and fruit juice.
These tests therefore can never show that a liquid is pure water.

Practical

Testing liquids for the presence of water

Aim: to find out which of several colourless liquids contain water, using anhydrous copper(II) sulfate and cobalt(II) chloride paper.
Apparatus: white spotting tile, watch glasses, spatula, a clean dropping pipette for each liquid, forceps, anhydrous copper(II) sulfate stored in a sealed container, blue cobalt(II) chloride paper stored in a desiccator, the liquids to be tested (for example distilled water, sodium chloride solution and dry hexane), gloves and eye protection.
Method, anhydrous copper(II) sulfate:
- Check that the anhydrous copper(II) sulfate is completely white before you start, because blue specks show that it has already absorbed water vapour from the air.
- Place a small spatula of the white powder in each dimple of a clean, dry spotting tile.
- Using a separate clean dropping pipette for each liquid, add two or three drops of each liquid to a different dimple.
- Record the colour of the powder in each dimple straight away, before it has time to absorb water vapour from the air.
Method, cobalt(II) chloride paper:
- Remove a strip of cobalt(II) chloride paper from the desiccator with forceps, because moisture from your fingers would turn the paper pink.
- Check that the strip is blue all over.
- Place a few drops of each liquid on a separate watch glass.
- Dip the end of a fresh strip into each liquid and compare the colour of the wet end with the colour of the dry end.
Results: any liquid that contains water turns anhydrous copper(II) sulfate from white to blue and turns cobalt(II) chloride paper from blue to pink, while a liquid that contains no water, such as dry hexane, leaves both colours unchanged.
Watch out: reagents left open to the air absorb water vapour and change colour on their own, which gives a false positive result.
Safety: wear eye protection and gloves, because cobalt(II) chloride is toxic and copper(II) sulfate is harmful and irritates the eyes, keep hexane away from flames because it is highly flammable, and wash your hands at the end.

Testing the purity of water

Definition

Pure substance

A single substance, either one element or one compound, that has no other substances mixed with it.

Definition

Melting point

The temperature at which a solid changes into a liquid.

Definition

Boiling point

The temperature at which a liquid boils and changes into a gas throughout the liquid.

Pure water contains water molecules and nothing else.
Every pure substance has a fixed melting point and a fixed boiling point, so these temperatures can be used to identify it and to check its purity.
At normal atmospheric pressure, pure water melts at exactly $∘C0\,^{\circ}\text{C}$ and boils at exactly $∘C100\,^{\circ}\text{C}$ .
Pure water melts and boils at a sharp temperature, which means the temperature stays constant for the whole time that the change of state is happening.
Dissolved impurities change these values in a predictable way.
1. An impurity lowers the melting point, so impure water melts and freezes below $∘C0\,^{\circ}\text{C}$ .
2. An impurity raises the boiling point, so impure water boils above $∘C100\,^{\circ}\text{C}$ .
3. An impure sample changes state over a range of temperatures instead of at one sharp temperature.
Seawater contains dissolved salts, so it freezes at about $∘C-2\,^{\circ}\text{C}$ and boils at slightly above $∘C100\,^{\circ}\text{C}$ .
To test whether a water sample is pure, measure its melting point or its boiling point and compare the value with $∘C0\,^{\circ}\text{C}$ or $∘C100\,^{\circ}\text{C}$ .
The sample is pure water only if it melts at exactly $∘C0\,^{\circ}\text{C}$ or boils at exactly $∘C100\,^{\circ}\text{C}$ .
Any other value, or a change of state spread over a range of temperatures, shows that the sample contains impurities.
The colour tests in the previous section cannot do this job, because impure water still contains water and still gives a positive result.

Practical

Measuring the boiling point of a water sample

Aim: to find out whether a water sample is pure by measuring its boiling point and comparing it with the boiling point of distilled water.
Apparatus: boiling tube, two clamps and a stand, thermometer reading from $∘C-10\,^{\circ}\text{C}$ to $∘C110\,^{\circ}\text{C}$ , anti-bumping granules, $cm310\,\text{cm}^{3}$ measuring cylinder, Bunsen burner, heat-resistant mat, the water sample, distilled water and eye protection.
Method:
- Measure $cm310\,\text{cm}^{3}$ of the water sample into a boiling tube and add two or three anti-bumping granules, which make the water boil smoothly instead of in sudden violent bursts.
- Clamp the boiling tube above the heat-resistant mat with its mouth pointing away from everyone.
- Hold the thermometer in a second clamp so that its bulb is in the water but does not touch the glass, because the glass is hotter than the water.
- Heat the boiling tube gently with a Bunsen burner until the water boils steadily.
- Read the thermometer at eye level every $30$ seconds while the water boils, and stop when three readings in a row are the same.
- Repeat the experiment with $cm310\,\text{cm}^{3}$ of distilled water in a clean boiling tube, using the same thermometer.
Results: distilled water boils at a steady $100 ∘C100\,^{\circ}\text{C}$ $100^{\circ} C$ , while a sample that contains dissolved impurities boils above $100 ∘C100\,^{\circ}\text{C}$ $100^{\circ} C$ .
- The boiling point of an impure sample also creeps upwards as boiling continues, because the dissolved impurities become more concentrated as the water boils away.
Watch out: the boiling point of water changes slightly with atmospheric pressure, so testing distilled water in the same apparatus on the same day gives a fair comparison.
Safety: wear eye protection, never heat a sealed tube, and let the boiling tube cool before you touch it, because the hot glass and steam cause burns.

Example

Judging the purity of water samples

Sample A melts at $∘C0\,^{\circ}\text{C}$ and boils at $∘C100\,^{\circ}\text{C}$ , so it is pure water.
Sample B melts gradually between $∘C-3\,^{\circ}\text{C}$ and $∘C-1\,^{\circ}\text{C}$ , so it contains impurities, because its melting point is lowered and spread over a range.
Sample C boils at $∘C101.5\,^{\circ}\text{C}$ , so it contains impurities, because its boiling point is raised above $∘C100\,^{\circ}\text{C}$ .
Sample D turns cobalt(II) chloride paper from blue to pink and boils at $∘C102\,^{\circ}\text{C}$ , so it contains water but it is not pure water.

Distilled water in practical chemistry

Definition

Distilled water

Water that has been purified by distillation, so it contains far fewer dissolved chemical impurities than tap water.

Tap water is safe to drink, but it is not pure.
It contains dissolved ions picked up from the rocks and soil the water has passed through, such as calcium, magnesium, sodium, chloride and sulfate ions.
It also contains small amounts of chlorine compounds left over from the treatment of the water supply.
Distilled water is made by boiling water and condensing the steam, which leaves the dissolved salts behind.
Distilled water is used in practical chemistry rather than tap water because it contains fewer chemical impurities.
The impurities in tap water could react with the chemicals in an experiment, form unexpected precipitates or give misleading test results.
For example, tap water contains chloride ions, so a solution made up with tap water gives a white precipitate with acidified silver nitrate even when the substance being tested contains no chloride.
Dissolved ions from tap water would also contaminate the crystals of a salt being prepared, so the product would be less pure.
Using distilled water makes sure that the results of an experiment are caused by the substances being investigated and not by impurities in the water.

Exam technique

Answering questions on testing water

Name the test reagent in full, including the word anhydrous and the Roman numeral, such as anhydrous copper(II) sulfate.
Give both the starting colour and the final colour: blue to pink for cobalt(II) chloride and white to blue for copper(II) sulfate.
For a purity test, quote the exact value for pure water, either a melting point of $∘C0\,^{\circ}\text{C}$ or a boiling point of $∘C100\,^{\circ}\text{C}$ .
When you explain why distilled water is used, write that it contains fewer chemical impurities than tap water, and add that these impurities could interfere with the results.

Common Mistake

Do not say that a colour change with cobalt(II) chloride or copper(II) sulfate shows that water is pure, because it shows only that water is present.
Do not swap the colours, because cobalt(II) chloride changes from blue to pink, while copper(II) sulfate changes from white to blue.
Do not say that impurities raise the melting point of water, because they lower the melting point and raise the boiling point.
Do not describe distilled water as clean or safe, because the point that earns credit is that it contains fewer chemical impurities.

Self review

What colour change shows that water is present when it is added to anhydrous cobalt(II) chloride?
What colour change shows that water is present when it is added to anhydrous copper(II) sulfate?
State the melting point and the boiling point of pure water.
How does a dissolved impurity change the melting point and the boiling point of water?
Why is distilled water used in practical chemistry rather than tap water?

Chemical tests for water

Definition

Anhydrous substance

A substance that contains no water of crystallisation.

Definition

Hydrated substance

A substance whose crystals contain water chemically combined within them, called water of crystallisation.

Many colourless liquids look exactly like water, so you cannot tell whether a liquid is water, or contains water, just by looking at it.
A chemical test for water uses an anhydrous salt that changes colour when it reacts with water and becomes hydrated.
Two anhydrous salts are used for this test: anhydrous cobalt(II) chloride and anhydrous copper(II) sulfate.
Anhydrous cobalt(II) chloride, $CoCl2\mathrm{CoCl_{2}}$ , is blue.
When water is added, it turns pink, because it combines with the water to form hydrated cobalt(II) chloride, $CoCl2⋅6H2O\mathrm{CoCl_{2} \cdot 6H_{2}O}$ .
In the laboratory this test is usually carried out with cobalt(II) chloride paper, which is filter paper that has been soaked in cobalt(II) chloride solution and dried in an oven until it is blue.
Anhydrous copper(II) sulfate, $CuSO4\mathrm{CuSO_{4}}$ , is a white powder.
When water is added, it turns blue, because it combines with the water to form hydrated copper(II) sulfate, $CuSO4⋅5H2O\mathrm{CuSO_{4} \cdot 5H_{2}O}$ .
The reaction between anhydrous copper(II) sulfate and water is exothermic, so the powder also becomes warm as it turns blue.
Both colour changes are the reverse of the changes seen when the hydrated salts are heated, so used test paper or powder can be heated to dry it and then used again.
A positive result shows only that water is present in the liquid.
Any liquid that contains water gives the same colour change, including salt solutions, dilute acids and fruit juice.
These tests therefore can never show that a liquid is pure water.

Practical

Testing liquids for the presence of water

Aim: to find out which of several colourless liquids contain water, using anhydrous copper(II) sulfate and cobalt(II) chloride paper.
Apparatus: white spotting tile, watch glasses, spatula, a clean dropping pipette for each liquid, forceps, anhydrous copper(II) sulfate stored in a sealed container, blue cobalt(II) chloride paper stored in a desiccator, the liquids to be tested (for example distilled water, sodium chloride solution and dry hexane), gloves and eye protection.
Method, anhydrous copper(II) sulfate:
- Check that the anhydrous copper(II) sulfate is completely white before you start, because blue specks show that it has already absorbed water vapour from the air.
- Place a small spatula of the white powder in each dimple of a clean, dry spotting tile.
- Using a separate clean dropping pipette for each liquid, add two or three drops of each liquid to a different dimple.
- Record the colour of the powder in each dimple straight away, before it has time to absorb water vapour from the air.
Method, cobalt(II) chloride paper:
- Remove a strip of cobalt(II) chloride paper from the desiccator with forceps, because moisture from your fingers would turn the paper pink.
- Check that the strip is blue all over.
- Place a few drops of each liquid on a separate watch glass.
- Dip the end of a fresh strip into each liquid and compare the colour of the wet end with the colour of the dry end.
Results: any liquid that contains water turns anhydrous copper(II) sulfate from white to blue and turns cobalt(II) chloride paper from blue to pink, while a liquid that contains no water, such as dry hexane, leaves both colours unchanged.
Watch out: reagents left open to the air absorb water vapour and change colour on their own, which gives a false positive result.
Safety: wear eye protection and gloves, because cobalt(II) chloride is toxic and copper(II) sulfate is harmful and irritates the eyes, keep hexane away from flames because it is highly flammable, and wash your hands at the end.

Testing the purity of water

Definition

Pure substance

A single substance, either one element or one compound, that has no other substances mixed with it.

Definition

Melting point

The temperature at which a solid changes into a liquid.

Definition

Boiling point

The temperature at which a liquid boils and changes into a gas throughout the liquid.

Pure water contains water molecules and nothing else.
Every pure substance has a fixed melting point and a fixed boiling point, so these temperatures can be used to identify it and to check its purity.
At normal atmospheric pressure, pure water melts at exactly $∘C0\,^{\circ}\text{C}$ and boils at exactly $∘C100\,^{\circ}\text{C}$ .
Pure water melts and boils at a sharp temperature, which means the temperature stays constant for the whole time that the change of state is happening.
Dissolved impurities change these values in a predictable way.
1. An impurity lowers the melting point, so impure water melts and freezes below $∘C0\,^{\circ}\text{C}$ .
2. An impurity raises the boiling point, so impure water boils above $∘C100\,^{\circ}\text{C}$ .
3. An impure sample changes state over a range of temperatures instead of at one sharp temperature.
Seawater contains dissolved salts, so it freezes at about $∘C-2\,^{\circ}\text{C}$ and boils at slightly above $∘C100\,^{\circ}\text{C}$ .
To test whether a water sample is pure, measure its melting point or its boiling point and compare the value with $∘C0\,^{\circ}\text{C}$ or $∘C100\,^{\circ}\text{C}$ .
The sample is pure water only if it melts at exactly $∘C0\,^{\circ}\text{C}$ or boils at exactly $∘C100\,^{\circ}\text{C}$ .
Any other value, or a change of state spread over a range of temperatures, shows that the sample contains impurities.
The colour tests in the previous section cannot do this job, because impure water still contains water and still gives a positive result.

Practical

Measuring the boiling point of a water sample

Aim: to find out whether a water sample is pure by measuring its boiling point and comparing it with the boiling point of distilled water.
Apparatus: boiling tube, two clamps and a stand, thermometer reading from $∘C-10\,^{\circ}\text{C}$ to $∘C110\,^{\circ}\text{C}$ , anti-bumping granules, $cm310\,\text{cm}^{3}$ measuring cylinder, Bunsen burner, heat-resistant mat, the water sample, distilled water and eye protection.
Method:
- Measure $cm310\,\text{cm}^{3}$ of the water sample into a boiling tube and add two or three anti-bumping granules, which make the water boil smoothly instead of in sudden violent bursts.
- Clamp the boiling tube above the heat-resistant mat with its mouth pointing away from everyone.
- Hold the thermometer in a second clamp so that its bulb is in the water but does not touch the glass, because the glass is hotter than the water.
- Heat the boiling tube gently with a Bunsen burner until the water boils steadily.
- Read the thermometer at eye level every $30$ seconds while the water boils, and stop when three readings in a row are the same.
- Repeat the experiment with $cm310\,\text{cm}^{3}$ of distilled water in a clean boiling tube, using the same thermometer.
Results: distilled water boils at a steady $100 ∘C100\,^{\circ}\text{C}$ $100^{\circ} C$ , while a sample that contains dissolved impurities boils above $100 ∘C100\,^{\circ}\text{C}$ $100^{\circ} C$ .
- The boiling point of an impure sample also creeps upwards as boiling continues, because the dissolved impurities become more concentrated as the water boils away.
Watch out: the boiling point of water changes slightly with atmospheric pressure, so testing distilled water in the same apparatus on the same day gives a fair comparison.
Safety: wear eye protection, never heat a sealed tube, and let the boiling tube cool before you touch it, because the hot glass and steam cause burns.

Example

Judging the purity of water samples

Sample A melts at $∘C0\,^{\circ}\text{C}$ and boils at $∘C100\,^{\circ}\text{C}$ , so it is pure water.
Sample B melts gradually between $∘C-3\,^{\circ}\text{C}$ and $∘C-1\,^{\circ}\text{C}$ , so it contains impurities, because its melting point is lowered and spread over a range.
Sample C boils at $∘C101.5\,^{\circ}\text{C}$ , so it contains impurities, because its boiling point is raised above $∘C100\,^{\circ}\text{C}$ .
Sample D turns cobalt(II) chloride paper from blue to pink and boils at $∘C102\,^{\circ}\text{C}$ , so it contains water but it is not pure water.

Distilled water in practical chemistry

Definition

Distilled water

Water that has been purified by distillation, so it contains far fewer dissolved chemical impurities than tap water.

Tap water is safe to drink, but it is not pure.
It contains dissolved ions picked up from the rocks and soil the water has passed through, such as calcium, magnesium, sodium, chloride and sulfate ions.
It also contains small amounts of chlorine compounds left over from the treatment of the water supply.
Distilled water is made by boiling water and condensing the steam, which leaves the dissolved salts behind.
Distilled water is used in practical chemistry rather than tap water because it contains fewer chemical impurities.
The impurities in tap water could react with the chemicals in an experiment, form unexpected precipitates or give misleading test results.
For example, tap water contains chloride ions, so a solution made up with tap water gives a white precipitate with acidified silver nitrate even when the substance being tested contains no chloride.
Dissolved ions from tap water would also contaminate the crystals of a salt being prepared, so the product would be less pure.
Using distilled water makes sure that the results of an experiment are caused by the substances being investigated and not by impurities in the water.

Exam technique

Answering questions on testing water

Name the test reagent in full, including the word anhydrous and the Roman numeral, such as anhydrous copper(II) sulfate.
Give both the starting colour and the final colour: blue to pink for cobalt(II) chloride and white to blue for copper(II) sulfate.
For a purity test, quote the exact value for pure water, either a melting point of $∘C0\,^{\circ}\text{C}$ or a boiling point of $∘C100\,^{\circ}\text{C}$ .
When you explain why distilled water is used, write that it contains fewer chemical impurities than tap water, and add that these impurities could interfere with the results.

Common Mistake

Do not say that a colour change with cobalt(II) chloride or copper(II) sulfate shows that water is pure, because it shows only that water is present.
Do not swap the colours, because cobalt(II) chloride changes from blue to pink, while copper(II) sulfate changes from white to blue.
Do not say that impurities raise the melting point of water, because they lower the melting point and raise the boiling point.
Do not describe distilled water as clean or safe, because the point that earns credit is that it contains fewer chemical impurities.

Self review

What colour change shows that water is present when it is added to anhydrous cobalt(II) chloride?
What colour change shows that water is present when it is added to anhydrous copper(II) sulfate?
State the melting point and the boiling point of pure water.
How does a dissolved impurity change the melting point and the boiling point of water?
Why is distilled water used in practical chemistry rather than tap water?

10.1.1 Testing for water and its purity

Chemical tests for water

Testing liquids for the presence of water

Testing the purity of water

Measuring the boiling point of a water sample

Judging the purity of water samples

Distilled water in practical chemistry

Answering questions on testing water

Review 10.1.1 Testing for water and its purity by teaching Genie

10.1.1 Testing for water and its purity

Chemical tests for water

Testing liquids for the presence of water

Testing the purity of water

Measuring the boiling point of a water sample

Judging the purity of water samples

Distilled water in practical chemistry

Answering questions on testing water

Review 10.1.1 Testing for water and its purity by teaching Genie

10.1.1 Testing for water and its purity

Chemical tests for water

Testing liquids for the presence of water

Testing the purity of water

Measuring the boiling point of a water sample

Judging the purity of water samples

Distilled water in practical chemistry

Answering questions on testing water

Review 10.1.1 Testing for water and its purity by teaching Genie

10.1.1 Testing for water and its purity Revision Guide

10.1.1 Testing for water and its purity

Chemical tests for water

Testing liquids for the presence of water

Testing the purity of water

Measuring the boiling point of a water sample

Judging the purity of water samples

Distilled water in practical chemistry

Answering questions on testing water

Review 10.1.1 Testing for water and its purity by teaching Genie

10.1.1 Testing for water and its purity Revision Guide