Testing for Water

1Is it water?

A colourless liquid is spilled on the laboratory bench. It could be water, but it could also be ethanol, dilute acid or salt solution, all of which look exactly the same. You must never taste or smell an unknown liquid to find out. Chemists use two kinds of test instead: a chemical test shows whether water is present, and a physical test shows whether the liquid is pure water.

2Experiment A: making the test powder

Blue copper(II) sulfate crystals contain water locked inside the crystal, called water of crystallisation. Put two spatulas of the blue crystals in a dry test tube and heat them gently as in Figure 2.1. Watch the colour of the solid and the inside of the tube near the mouth.

Figure 2.1: Driving the water out of blue copper(II) sulfate crystals.

What you see: the blue crystals crumble into a white powder, and drops of a colourless liquid collect near the mouth of the tube. The heat has driven off the water of crystallisation:

$$\mathrm{C}\mathrm{u}\mathrm{S}{\mathrm{O}}_4\cdot 5{\mathrm{H}}_2\mathrm{O}(\mathrm{s})\to \mathrm{C}\mathrm{u}\mathrm{S}{\mathrm{O}}_4(\mathrm{s})+5{\mathrm{H}}_2\mathrm{O}(\mathrm{g})\backslash mathrm\{CuSO\_4\backslash cdot\; 5H\_2O(s)\; \backslash rightarrow\; CuSO\_4(s)\; +\; 5H\_2O(g)\}$$

The white powder is anhydrous copper(II) sulfate. Let it cool and keep it in a stoppered bottle, because it slowly absorbs water from the air.

3Experiment B: the test for water

Put a little anhydrous copper(II) sulfate on a watch glass and add two or three drops of the liquid to be tested (Figure 2.2). If the liquid contains water, the white powder turns blue and becomes warm. The reaction is the reverse of Experiment A:

$$\mathrm{C}\mathrm{u}\mathrm{S}{\mathrm{O}}_4(\mathrm{s})+5{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\to \mathrm{C}\mathrm{u}\mathrm{S}{\mathrm{O}}_4\cdot 5{\mathrm{H}}_2\mathrm{O}(\mathrm{s})\backslash mathrm\{CuSO\_4(s)\; +\; 5H\_2O(l)\; \backslash rightarrow\; CuSO\_4\backslash cdot\; 5H\_2O(s)\}$$

Figure 2.2: The anhydrous copper(II) sulfate test for water.

4Cobalt(II) chloride paper

Filter paper soaked in cobalt(II) chloride solution and then dried in an oven is blue. Touch it with a liquid that contains water and it turns pink (Figure 2.3), because the blue anhydrous salt takes up water to form pink hydrated cobalt(II) chloride:

$$\mathrm{C}\mathrm{o}\mathrm{C}{\mathrm{l}}_2(\mathrm{s})+6{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\to \mathrm{C}\mathrm{o}\mathrm{C}{\mathrm{l}}_2\cdot 6{\mathrm{H}}_2\mathrm{O}(\mathrm{s})\backslash mathrm\{CoCl\_2(s)\; +\; 6H\_2O(l)\; \backslash rightarrow\; CoCl\_2\backslash cdot\; 6H\_2O(s)\}$$

The paper must be completely dry before you use it, so keep it in a closed bottle or a desiccator. Paper that has already turned pink can be dried in a warm oven until it is blue again.

Figure 2.3: Cobalt(II) chloride paper turns from blue to pink with water.

Table 2.1: Chemical tests for water

5Present is not the same as pure

Both tests give the same result with tap water, sea water, tea, milk or dilute acid, because each of them contains water. They cannot tell you whether the liquid is pure water. For that you need the physical test from Lesson 1: pure water boils at exactly $100{}^{\circ }\mathrm{C}100\backslash ,^\backslash circ\backslash mathrm\{C\}$ at sea level (about $95{}^{\circ }\mathrm{C}95\backslash ,^\backslash circ\backslash mathrm\{C\}$ in Kigali) and freezes at $0{}^{\circ }\mathrm{C}0\backslash ,^\backslash circ\backslash mathrm\{C\}$.

Table 2.2: Chemical and physical tests compared

6Water tests in daily life

The small sachets of silica gel packed with new shoes, medicines and electronic goods absorb damp air, which matters in Rwanda's rainy seasons. Some contain blue beads coloured with cobalt(II) chloride, which turn pink when the sachet is full of water and needs drying or replacing. Because cobalt(II) chloride is harmful, many newer sachets use an orange indicator that changes colour instead. The same idea is used on humidity cards packed with electronics.