Acids and bases
An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. A base is a substance that reacts with an acid to make a salt and water. A base that dissolves in water is an alkali, and it produces hydroxide ions (OH⁻). Common laboratory acids are hydrochloric (HCl), sulphuric (H₂SO₄) and nitric (HNO₃). Common alkalis are sodium hydroxide (NaOH), potassium hydroxide (KOH) and ammonia solution.
| Acids | Alkalis | |
|---|---|---|
| Taste (never test!) | Sour | Bitter and soapy |
| pH | Less than 7 | More than 7 |
| Litmus | Turns blue litmus red | Turns red litmus blue |
| Reaction with metals | Give hydrogen gas with reactive metals | No reaction (except with aluminium and zinc) |
| Examples | Lemon juice, vinegar, battery acid | Soap, baking soda solution, wood ash water |
Acid–base indicators
An indicator is a substance that changes colour depending on how acidic or alkaline a solution is. The pH scale runs from 0 (strongly acidic) to 14 (strongly alkaline), with 7 neutral. Universal indicator gives a range of colours. Plants such as red cabbage, hibiscus and turmeric give natural indicators.
| Indicator | In acid | In alkali |
|---|---|---|
| Litmus | Red | Blue |
| Methyl orange | Red | Yellow |
| Phenolphthalein | Colourless | Pink |
| Universal indicator | Red, orange, yellow | Blue, purple |
Test different solutions and match the colours to the pH scale.
Reactions of acids and bases
| Reaction | Equation (word) | Example |
|---|---|---|
| Acid + metal | salt + hydrogen | |
| Acid + metal oxide or hydroxide (base) | salt + water | |
| Acid + carbonate | salt + water + carbon dioxide | |
| Acid + alkali (neutralisation) | salt + water |
Applications in daily life
| Use | Acid or base | How it helps |
|---|---|---|
| Indigestion tablets | Bases (e.g. magnesium hydroxide) | Neutralise excess stomach acid |
| Toothpaste | Weak alkali | Neutralises acids made by bacteria in the mouth |
| Treating acidic soil | Lime (calcium hydroxide or oxide) | Raises the soil pH for crops |
| Treating a wasp sting | Vinegar (acid) | Neutralises the alkaline sting |
| Car batteries | Sulphuric acid | The electrolyte |
| Cleaning and soap making | Alkalis | React with fats and grease |
Choose a pH. The concentration of hydrogen ions is [H⁺] = 10^(−pH) and of hydroxide ions [OH⁻] = 10^(pH−14). Notice that each step on the scale is a factor of ten.
Key terms
- Acid
- A substance that gives hydrogen ions in water; pH below 7.
- Alkali
- A base that dissolves in water and gives hydroxide ions; pH above 7.
- Indicator
- A substance that changes colour with the acidity of a solution.
- Salt
- A compound formed when an acid is neutralised by a base.
- Neutralisation
- The reaction of an acid with a base to make a salt and water.
Summary
- Acids give H⁺ (pH < 7); alkalis give OH⁻ (pH > 7); pH 7 is neutral.
- Indicators such as litmus, methyl orange, phenolphthalein and universal indicator show acidity.
- Acid + metal → salt + hydrogen; acid + base → salt + water; acid + carbonate → salt + water + carbon dioxide.
- Neutralisation is used in medicine, farming and everyday products.