What acids and bases have in common: ions in water

What do all acids have?

HCl, $\text{H}_2\text{SO}_4$, $\text{HNO}_3$ and $\text{CH}_3\text{COOH}$ are different, but they behave in the same way with indicators and metals. What do they have in common? Hydrogen.

But not every substance with hydrogen is an acid. Glucose and alcohol have hydrogen, and they are not acids. The hydrogen must be able to leave as an ion in water.

Acids give hydrogen ions

In water, an acid gives hydrogen ions, $\text{H}^+$.

The $\text{H}^+$ cannot stay alone. It joins a water molecule to make a hydronium ion, $\text{H}_3\text{O}^+$.

Without water, an acid does not show acidic behaviour. Dry HCl gas does not change dry blue litmus paper. It changes it when it is wet.

Bases give hydroxide ions

A base that dissolves in water gives hydroxide ions, $\text{OH}^-$.

Not all bases dissolve. Those that do are the alkalis.

Why does the solution carry a current?

The ions in the water are free to move. A solution of an acid, or an alkali, conducts electricity. A solution of glucose or alcohol does not, because it has no ions.

Neutralisation, in terms of ions

In neutralisation, the $\text{H}^+$ ions of the acid and the $\text{OH}^-$ ions of the base join to make water.

Strong and weak

A strong acid, such as HCl, gives almost all its hydrogen ions in water. A weak acid, such as ethanoic acid in vinegar, gives only a few. The same is true of a strong base, such as NaOH, and a weak base, such as ammonium hydroxide.

You should find that a weak acid needs a hundred times the concentration of a strong one to reach the same pH.

Diluting an acid, and safety

When we add water to an acid, there are fewer ions in each unit of volume. The concentration of $\text{H}_3\text{O}^+$ falls, and the acid is weaker.

Mixing an acid or a base with water gives out a lot of heat. So:

• Always add the acid to water, slowly, with stirring. Never add water to a concentrated acid.
• If water is added to the acid, the mixture may heat so much that it spits out and burns you. Glass may break.

Practise with new numbers

Try it in Python

This program dilutes $10\ \text{mL}$ of $0.1\ \text{M}$ hydrochloric acid to bigger and bigger volumes, and it finds the pH.

Remember these

• Acids give $\text{H}^+$ (as $\text{H}_3\text{O}^+$) in water. Alkalis give $\text{OH}^-$ in water.
• Ions in the solution let it carry a current. Neutralisation is $\text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O}$.
• Strong: almost all ions. Weak: few ions. Always add acid to water.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is the hydronium ion, and how does it form?

2. Which ion do alkalis give in water?

3. Ethanoic acid and hydrochloric acid have the same concentration. Which gives more hydrogen ions?

4. What happens to the concentration of hydrogen ions when an acid is diluted?