What you'll learn

• How to test for hydrogen, oxygen, carbon dioxide, ammonia and chlorine.
• How flame tests identify some metal cations.
• How sodium hydroxide, silver nitrate, barium chloride and acids are used to identify ions.
• How to test for water, and how to check whether water is pure.

Big picture: what chemical tests do

A chemical test uses a reagent or a simple physical method to identify a substance from a clear observation, such as a flame colour, a gas relighting a splint, or a coloured precipitate.

A positive result is the observation that confirms the substance is present. For example, “limewater turns milky” is a positive result for carbon dioxide.

A precipitate is an insoluble solid that forms when two solutions react. In ion tests, precipitate colour is often the main clue.

Tests for common gases

Most gas tests involve either a splint, limewater, or damp litmus paper. Indicator paper must be damp because many gases need to dissolve in water before they affect the indicator.

The diagram summarises the five gas tests you need for this section.

Hydrogen, $H_2$

Test hydrogen using a burning splint. If hydrogen is present, it burns with a squeaky pop.

The reaction is:

$$2H_2(g)+O_2(g)\to 2H_{2}O(l)$$

Oxygen, $O_2$

Test oxygen using a glowing splint. If oxygen is present, the glowing splint relights because oxygen supports combustion.

Carbon dioxide, $C{O}_2$

Test carbon dioxide by bubbling it through limewater, which is aqueous calcium hydroxide, $Ca(OH{)}_2(aq)$. Carbon dioxide turns limewater milky because insoluble calcium carbonate forms.

$$C{O}_2(g)+Ca(OH{)}_2(aq)\to CaC{O}_3(s)+H_{2}O(l)$$

Ammonia, $N{H}_3$

Test ammonia using damp red litmus paper. Ammonia turns damp red litmus paper blue because ammonia is alkaline when dissolved in water.

Chlorine, $C{l}_2$

Test chlorine using damp blue litmus paper. Chlorine turns the paper red, then bleaches it white.

Chlorine is toxic, so in real practical work it must be handled carefully, usually in very small amounts and with good ventilation.

Flame tests for metal cations

A cation is a positively charged ion. Flame tests identify some metal cations because their compounds give characteristic flame colours.

To carry out a flame test:

1. Clean a nichrome or platinum wire loop by dipping it in dilute hydrochloric acid and heating it in a Bunsen flame until no flame colour is seen.
2. Dip the clean loop into the solid sample, or into a solution of the sample.
3. Place the loop in the hottest part of a blue Bunsen flame.
4. Observe and record the flame colour.

The flame-test colours you must know are:

• $L{i}^{+}$ gives a red flame.
• $N{a}^{+}$ gives a yellow flame.
• $K^{+}$ gives a lilac flame.
• $C{a}^{2+}$ gives an orange-red flame.
• $C{u}^{2+}$ gives a blue-green flame.

The chart below brings together the required cation and anion tests.

Cation tests using sodium hydroxide solution

Sodium hydroxide solution, $NaOH(aq)$, supplies hydroxide ions, $O{H}^{-}$. Some metal ions form insoluble metal hydroxide precipitates with hydroxide ions.

For the cations in this section:

• $N{H}_4^{+}$: warm with sodium hydroxide solution. Ammonia gas is produced, which turns damp red litmus paper blue.
• $C{u}^{2+}$: add sodium hydroxide solution. A blue precipitate forms.
• $F{e}^{2+}$: add sodium hydroxide solution. A green precipitate forms.
• $F{e}^{3+}$: add sodium hydroxide solution. A brown precipitate forms.

Useful ionic equations are:

$$\begin{array}{rl}N{H}_4^{+}(aq)+O{H}^{-}(aq)& \to N{H}_3(g)+H_{2}O(l)\\ C{u}^{2+}(aq)+2O{H}^{-}(aq)& \to Cu(OH{)}_2(s)\\ F{e}^{2+}(aq)+2O{H}^{-}(aq)& \to Fe(OH{)}_2(s)\\ F{e}^{3+}(aq)+3O{H}^{-}(aq)& \to Fe(OH{)}_3(s)\end{array}$$

Anion tests

An anion is a negatively charged ion. For anion tests, use a fresh portion of the sample for each test, because one reagent can interfere with another test.

Halide ions: $C{l}^{-}$, $B{r}^{-}$ and $I^{-}$

Halide ions are tested using acidified silver nitrate solution.

Method:

1. Acidify the sample with dilute nitric acid.
2. Add silver nitrate solution.
3. Observe the precipitate colour.

Positive results:

• $C{l}^{-}$ gives a white precipitate of silver chloride, $AgCl$.
• $B{r}^{-}$ gives a cream precipitate of silver bromide, $AgBr$.
• $I^{-}$ gives a yellow precipitate of silver iodide, $AgI$.

$$\begin{array}{rl}A{g}^{+}(aq)+C{l}^{-}(aq)& \to AgCl(s)\\ A{g}^{+}(aq)+B{r}^{-}(aq)& \to AgBr(s)\\ A{g}^{+}(aq)+I^{-}(aq)& \to AgI(s)\end{array}$$

Sulfate ions, $S{O}_4^{2-}$

Test sulfate ions using acidified barium chloride solution. A positive result is a white precipitate of barium sulfate, $BaS{O}_4$.

$$B{a}^{2+}(aq)+S{O}_4^{2-}(aq)\to BaS{O}_4(s)$$

The acid helps remove carbonate ions, which could otherwise also produce a white precipitate with barium ions.

Carbonate ions, $C{O}_3^{2-}$

Test carbonate ions by adding dilute hydrochloric acid. A positive result is effervescence — fizzing caused by carbon dioxide gas. Confirm the gas by bubbling it through limewater; it turns limewater milky.

$$C{O}_3^{2-}(aq)+2H^{+}(aq)\to C{O}_2(g)+H_{2}O(l)$$

Testing for water

Chemical test for the presence of water

To test for water, add the sample to anhydrous copper(II) sulfate.

If water is present, anhydrous copper(II) sulfate changes from white to blue as hydrated copper(II) sulfate forms.

$$CuS{O}_4(s)+5H_{2}O(l)\to CuS{O}_4\cdot 5H_{2}O(s)$$

This test shows that water is present. It does not prove that the water is pure.

Testing whether water is pure

A physical test does not make a new substance. To test whether water is pure, measure its boiling point or freezing point.

At normal atmospheric pressure:

• pure water boils at 100°C
• pure water freezes at 0°C

Impure water usually boils over a range of temperatures and often has a boiling point above 100°C.