Bases, alkalis and the pH scale
Bases, alkalis and strength
- Bases are metal oxides or hydroxides. An alkali is a base that dissolves in water.
- An acid is a proton ($\text{H}^{+}$) donor; a base is a proton acceptor.
- Strength = how much the acid splits into ions:
- strong acid — fully dissociated: $\text{HCl}(aq) \rightarrow \text{H}^{+}(aq) + \text{Cl}^{-}(aq)$,
- weak acid — only partly: $\text{CH}_3\text{COOH}(aq) \rightleftharpoons \text{H}^{+}(aq) + \text{CH}_3\text{COO}^{-}(aq)$.
Practice
An alkali is:
An alkali is a soluble base; it releases OH- ions in water.
Practice
A strong acid is one that:
Strength is about full vs partial dissociation, not concentration. HCl is strong (fully split).
The pH scale
| pH | Type | Universal indicator |
|---|---|---|
| below 7 | acidic | red / orange / yellow |
| 7 | neutral | green |
| above 7 | alkaline | blue / purple |
- More $\text{H}^{+}$ → lower pH. Neutralisation: $\text{H}^{+}(aq) + \text{OH}^{-}(aq) \rightarrow \text{H}_2\text{O}(l)$.
Practice
A solution with pH 2 is:
pH below 7 is acidic; 7 is neutral; above 7 is alkaline.
You've got it
Key idea
- a base is a metal oxide/hydroxide; an alkali is a soluble base
- strong acid = fully dissociated; weak = partly (strength ≠ concentration)
- pH <7 acidic, 7 neutral, >7 alkaline; universal indicator green at neutral