Tests for Gases, and for Water

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.44 — Tests for hydrogen, oxygen, carbon dioxide, ammonia and chlorine
• 2.49 — The test for water using anhydrous copper(II) sulfate
• 2.50 — A physical test to show whether a sample of water is pure

Specification section 2(h) Chemical tests — also taught in Titrations, Preparing a Soluble Salt, Preparing an Insoluble Salt, Flame Tests.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Five gases have to be identifiable, and each test is chosen because it gives an unambiguous result — something happens that nothing else on this specification would produce. A test that merely suggests an answer is no use for identification.

Statement 2.44 — the five gas tests

1. HYDROGEN — hold a LIT SPLINT at the mouth of the tube. It burns with a SQUEAKY POP.
2. OXYGEN — insert a GLOWING SPLINT. It RELIGHTS, because oxygen supports combustion far better than air does.
3. CARBON DIOXIDE — bubble the gas through LIMEWATER. It turns MILKY, or cloudy white.
4. AMMONIA — hold DAMP RED LITMUS paper in the gas. It turns BLUE, because ammonia is the only common alkaline gas.
5. CHLORINE — hold DAMP BLUE LITMUS paper in the gas. It turns red and is then BLEACHED WHITE.
6. Note the two litmus tests differ in the colour of paper used and in what happens, so they cannot be confused if both details are given.

The word DAMP in the last two tests is not decoration. Both gases must dissolve in the moisture on the paper before they can affect it, so dry litmus paper gives no result at all. Leaving 'damp' out of an answer is one of the commonest ways to lose a mark in this part of the specification.

Think of it like a set of keys that fit only one lock each

A key that opens several doors tells you very little about which door you are at. What makes a test useful for identification is that nothing else gives the same result — the squeaky pop is hydrogen and nothing else, limewater going milky is carbon dioxide and nothing else. This is also why the chlorine test has two stages, red then bleached. Turning damp litmus red would only tell you the gas is acidic, which several gases are; it is the bleaching afterwards that makes the identification certain. Two observations, one conclusion.

Now statement 2.49, the test for water. ANHYDROUS COPPER(II) SULFATE is white, and adding water turns it BLUE as it becomes the hydrated salt. That is the reverse of the reaction met when preparing hydrated copper sulfate, where heating drove the water off and turned blue crystals white.

Written out, anhydrous copper(II) sulfate + water → hydrated copper(II) sulfate, or CuSO₄ + 5H₂O → CuSO₄·5H₂O. White turns blue, and the reaction is reversible — heating drives the water off again and the blue fades back to white.

Now the distinction the specification is careful about, and it is the point of statement 2.50. The copper sulfate test shows that WATER IS PRESENT. It does not show that the liquid is PURE water, because a solution of salt in water would turn it blue just as readily. Those are two different questions and they need two different tests.

Statement 2.50 — the physical test for PURE water

1. Purity is tested by MELTING POINT and BOILING POINT, which are physical properties rather than chemical ones.
2. PURE water boils at EXACTLY 100 °C and freezes at EXACTLY 0 °C, at normal atmospheric pressure.
3. A dissolved impurity RAISES the boiling point and LOWERS the melting point.
4. An impure sample also boils and melts over a RANGE of temperature rather than at a fixed point.
5. So measure the boiling point: exactly 100 °C means pure; anything higher means something is dissolved in it.
6. This is a PHYSICAL test, and it is the only one of the two that can answer the purity question.

Two questions, two tests

A colourless liquid is thought to be water. (a) How would you show it contains water? (b) How would you show it is pure water? (c) Why will the first test not answer the second question?

1. (a) Add the liquid to white anhydrous copper(II) sulfate. Turning blue shows water is present.
2. (b) Measure its boiling point. Exactly 100 °C at normal pressure shows it is pure.
3. (c) The copper sulfate test responds to the PRESENCE of water and cannot distinguish pure water from a solution.
4. Salt water, sugar solution and dilute acid would all turn the copper sulfate blue, since all contain water.
5. So a positive copper sulfate result narrows the possibilities and does not settle purity, which is exactly why the specification asks for a separate physical test.

Answer: (a) Anhydrous copper sulfate turning blue. (b) Boiling point exactly 100 °C. (c) The first responds to water's presence, not its purity.

That distinction between a CHEMICAL test for presence and a PHYSICAL test for purity runs through the rest of this topic. Chemical tests tell you what is there; physical properties such as melting and boiling points tell you whether anything else is there as well.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Statement 2.44 — the five gas tests

The vocabulary of these tests

Anhydrous

Containing no water of crystallisation. Anhydrous copper(II) sulfate is WHITE.

Hydrated

Containing water of crystallisation. Hydrated copper(II) sulfate is BLUE.

Limewater

Calcium hydroxide solution. It turns milky with carbon dioxide, forming insoluble calcium carbonate.

Pure substance

A single substance with nothing else in it, melting and boiling at fixed temperatures rather than over a range.

Statement 2.50 — testing whether water is pure

1. Heat the sample and measure the temperature at which it boils.
2. Pure water boils at exactly 100 °C at normal atmospheric pressure.
3. A dissolved impurity raises the boiling point above 100 °C.
4. Alternatively cool the sample: pure water freezes at exactly 0 °C, and an impurity lowers this.
5. An impure sample also boils or melts over a RANGE rather than at a fixed temperature.

Testing for water against testing for PURE water

A. ANHYDROUS COPPER(II) SULFATE turning blue shows water is PRESENT. Salt solution would also turn it blue.

B. BOILING POINT of exactly 100 °C shows the water is PURE. This is a physical test, and only it answers the purity question.

Model answer [4 marks]

Describe the tests you would use to identify (a) carbon dioxide and (b) chlorine. [4]

(a) Bubble the gas through limewater. If the gas is carbon dioxide the limewater turns milky, because insoluble calcium carbonate is formed. (b) Hold a piece of damp blue litmus paper in the gas. If the gas is chlorine the paper first turns red, because chlorine forms an acidic solution, and is then bleached white. The litmus must be damp so that the gas can dissolve in the moisture before it acts on the paper.

Model answer [4 marks]

A student adds a liquid to white anhydrous copper(II) sulfate and it turns blue. They conclude the liquid is pure water. Explain whether they are correct, and describe a test that would settle it. [4]

They are not correct. The copper(II) sulfate test shows only that water is present, and any aqueous solution such as salt water or dilute acid would also turn it blue, since all of them contain water. To show that the liquid is pure water a physical test is needed: measure its boiling point, which should be exactly 100 °C at normal atmospheric pressure. If the liquid contains any dissolved impurity the boiling point will be higher than 100 °C and the liquid will boil over a range of temperatures rather than at a fixed value.

Not this: Anhydrous copper(II) sulfate turning blue proves that a liquid is pure water.

This: It proves only that WATER IS PRESENT. Salt solution, sugar solution and dilute acid would all turn it blue. Purity is a separate question, answered by a PHYSICAL test — the boiling point.

Mark-losing trap. The litmus must be DAMP for both the ammonia and chlorine tests. Dry paper gives no result.

Mark-losing trap. Ammonia uses damp RED litmus (turns blue); chlorine uses damp BLUE litmus (turns red then bleaches).

Mark-losing trap. Copper sulfate tests for WATER; boiling point tests for PURE water. Two questions, two tests.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [2 marks] — What is the test for hydrogen gas, and what is the positive result?
2. Grade 7 · Explain [2 marks] — Explain why the litmus paper must be damp when testing for chlorine.
3. Grade 8 · Explain [3 marks] — A liquid turns white anhydrous copper(II) sulfate blue. Explain what this shows and what it does not show.
4. Grade 9 · Describe [6 marks] — Select every statement that belongs in a full-mark description of how to distinguish between samples of hydrogen, oxygen, carbon dioxide and chlorine.
5. 9+ · Analyse [6 marks] — A student tests a colourless liquid with anhydrous copper(II) sulfate and it turns blue. They then boil it and find it boils at 102 °C. They conclude the liquid is not water at all. Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one who found the gases these tests identify, and the one whose test result was the evidence for an element. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Joseph Priestley — the one who found the gases these tests identify

Several of the gases on this page were first prepared and characterised by Priestley, who isolated around a dozen including oxygen, ammonia and hydrogen chloride. His method was essentially to make a gas and then see what it did — to a candle, to a mouse, to a plant, to himself. That is exactly the logic of a gas test, arrived at before there was any theory to guide it. He is the right person to ask how you begin to describe something invisible, and how he decided which properties were worth recording.

• “How do you study a gas you have only just made?”
• “What did you do to a new gas to find out about it?”
• “How did you collect gases that dissolve in water?”
• “Which of your gases surprised you most?”
• “How did you decide two samples were the same gas?”

Humphry Davy — the one whose test result was the evidence for an element

The chlorine test on this page is a property Davy studied closely, and its bleaching action was one of the things that made the gas commercially important long before anyone understood it. More importantly, his investigation of chlorine is the case where a careful set of tests led to a conclusion nobody wanted: that the gas contained no oxygen and was an element, against Lavoisier's authority. He is the right figure for the question of when a test result should be trusted over a theory.

• “What did you learn from the way chlorine bleaches things?”
• “How did you test whether chlorine contained oxygen?”
• “When should a test result outweigh an accepted theory?”
• “What made you confident enough to rename the gas?”
• “How did you handle a gas that damages the lungs?”

Carry on through the course

• Previous lesson: Preparing an Insoluble Salt
• Next lesson: Flame Tests
• Group 1 — the Alkali Metals — Metals soft enough to cut with a knife, which set fire to water — and get worse as you go down.
• Group 7 — the Halogens — A green gas, a brown liquid and a grey solid — and a trend that runs the opposite way to Group 1.
• The Gases in the Air — Four gases, one of which is doing almost all the chemistry — and how to measure it without ever touching it.
• Combustion and Carbonates — Three things burning in oxygen, one carbonate breaking apart, and the 0.04% gas that changes the climate.
• The Reactivity Series — Putting eleven metals in order by watching what each one will and will not do.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification