Acids and indicators
Acids
- An acid forms hydrogen ions ($\text{H}^{+}$) when dissolved in water.
- Three typical reactions:
- acid + metal → salt + hydrogen,
- acid + base → salt + water (neutralisation),
- acid + carbonate → salt + water + carbon dioxide.
Practice
An acid forms which ion when dissolved in water?
Acids release H+ ions in water; alkalis release OH- ions.
Practice
Acid + carbonate produces:
Acid + carbonate → salt + water + carbon dioxide (the fizzing gas is CO2).
Indicators
| Indicator | In acid | In alkali |
|---|---|---|
| litmus | red | blue |
| thymolphthalein | colourless | blue |
| methyl orange | red | yellow |
- Acids contain $\text{H}^{+}$ ions; alkalis contain hydroxide ions ($\text{OH}^{-}$).
Practice
Litmus in an alkali is:
Litmus is red in acid and blue in alkali.
You've got it
Key idea
- an acid makes $\text{H}^{+}$ ions in water
- acid + metal → salt + hydrogen; acid + base → salt + water; acid + carbonate → salt + water + CO₂
- litmus: red in acid, blue in alkali