Acids, Bases and Salts
Use indicators and the pH scale to classify solutions and neutralise acids.
Acids and bases are all around us, from citrus fruit to soap.
- Define acid, base and salt.
- Use the pH scale and indicators.
- Write the word equation for neutralisation.
Acids and Bases
An acid produces hydrogen ions in water and has a pH below 7. A base neutralises an acid; a soluble base is called an alkali and has a pH above 7.
The pH Scale
The pH scale runs from 0 to 14. Litmus turns red in acid and blue in alkali. Universal indicator gives a colour for each pH value.
Neutralisation
When an acid reacts with a base, a salt and water form: acid + base → salt + water. This is how antacids relieve stomach acid and how farmers treat acidic soil.
Lemon juice — acidic (pH about 2).
Pure water — neutral (pH 7).
Soap solution — alkaline (pH about 10).
Hydrochloric acid + sodium hydroxide → sodium chloride + water.
Practice
- Classify as acid, base or neutral: lemon juice, soap, pure water, vinegar.
- Write the word equation for the reaction between hydrochloric acid and potassium hydroxide.
- Why is universal indicator more useful than litmus?
Recap
Acids have pH below 7, alkalis above 7. Neutralisation produces a salt and water.
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Chemical Reactions
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