Properties of Acids and Alkalis

1What do all acids have in common?

Hydrochloric acid in a laboratory bottle and the citric acid in a lemon look nothing alike, yet they share a set of properties because both release hydrogen ions in water. In this lesson you will test some of those properties, then compare them with the properties of alkalis. You will study these reactions in more depth in S2.

Experiment. Wearing goggles, add a 2 cm piece of magnesium ribbon to 3 cm depth of dilute hydrochloric acid in a test tube (Figure 2.1 A). Hold a lighted splint at the mouth of the tube. Then drop two marble chips (calcium carbonate) into dilute hydrochloric acid in a boiling tube and bubble the gas through limewater (Figure 2.1 B).

Figure 2.1: Testing the gases given off when acids react with a metal (A) and with a carbonate (B).

2Results

In A the magnesium fizzes and slowly disappears, the tube becomes warm, and the gas burns with a squeaky "pop": it is hydrogen. In B the marble fizzes and the limewater turns milky: the gas is carbon dioxide. Most acids behave in the same two ways.

$$\text{acid}+\text{metal}\to \text{salt}+\text{hydrogen}\backslash text\{acid\}\; +\; \backslash text\{metal\}\; \backslash rightarrow\; \backslash text\{salt\}\; +\; \backslash text\{hydrogen\}$$

$$\text{acid}+\text{carbonate}\to \text{salt}+\text{water}+\text{carbon dioxide}\backslash text\{acid\}\; +\; \backslash text\{carbonate\}\; \backslash rightarrow\; \backslash text\{salt\}\; +\; \backslash text\{water\}\; +\; \backslash text\{carbon\; dioxide\}$$

3Neutralisation

Add alkali to an acid, a little at a time, and the acid's properties disappear; so do the alkali's. The two have neutralised each other:

$$\text{acid}+\text{base}\to \text{salt}+\text{water}\backslash text\{acid\}\; +\; \backslash text\{base\}\; \backslash rightarrow\; \backslash text\{salt\}\; +\; \backslash text\{water\}$$

For example, $\mathrm{H}\mathrm{C}\mathrm{l}(\mathrm{a}\mathrm{q})+\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}(\mathrm{a}\mathrm{q})\to \mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}(\mathrm{a}\mathrm{q})+{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\backslash mathrm\{HCl(aq)\; +\; NaOH(aq)\; \backslash rightarrow\; NaCl(aq)\; +\; H\_2O(l)\}$. What really happens is that the hydrogen ions of the acid and the hydroxide ions of the alkali join to form water:

$${\mathrm{H}}^{+}(\mathrm{a}\mathrm{q})+\mathrm{O}{\mathrm{H}}^{-}(\mathrm{a}\mathrm{q})\to {\mathrm{H}}_2\mathrm{O}(\mathrm{l})\backslash mathrm\{H^+(aq)\; +\; OH^-(aq)\; \backslash rightarrow\; H\_2O(l)\}$$

You can watch neutralisation with an indicator. Put 10 cm³ of dilute sodium hydroxide in a flask with two drops of phenolphthalein; it turns pink. Add dilute hydrochloric acid drop by drop, swirling. The pink colour suddenly disappears when exactly enough acid has been added.

Table 2.1: Properties of acids and alkalis compared