Acids, Alkalis & pH
Define acids and alkalis in terms of hydrogen ion concentration, describe the pH scale (0–14), and explain how indicators are used to identify and measure acidity or alkalinity
What a learner can do afterwards
- States that acids have pH below 7, alkalis have pH above 7, and neutral is pH 7
- Explains what hydrogen ions (H⁺) have to do with acidity
- Names common indicators (litmus, universal indicator) and describes colour changes
- Identifies everyday acids (vinegar, lemon juice) and alkalis (bleach, bicarbonate of soda)
1 · Read
Chemists sort liquids into three groups: acids, alkalis, and neutral. What decides the group is how many hydrogen ions, written H⁺, are floating around in the liquid. A hydrogen atom that has lost its one electron becomes an H⁺ ion. Acids are substances that release H⁺ ions when they dissolve in water, and the more H⁺ ions crowded in, the more acidic the liquid is. Alkalis work the other way. They release hydroxide ions, OH⁻, which soak up H⁺ ions.
The pH number tells you how strong an acid or alkali is. Low numbers mean lots of H⁺ ions: lemon juice sits around pH 2 and vinegar around pH 3. High numbers mean lots of OH⁻ ions: a solution of bicarbonate of soda is around pH 9, and bleach is higher still. Pure water, at exactly 7, is neither acid nor alkali.
Priya tests two kitchen liquids with universal indicator paper. She dips a strip into vinegar, and it turns red-orange, showing a pH of about 3, an acid. She dips a fresh strip into a solution of bicarbonate of soda, and it turns blue-green, showing a pH of about 9, an alkali.
Mix an acid with an alkali and they cancel each other out. The H⁺ ions from the acid join the OH⁻ ions from the alkali to make plain water. Keep adding alkali to an acid and the mixture climbs the pH scale until it reaches green, pH 7: neutral. This balancing act is called neutralisation, and it is how indigestion tablets calm an over-acidic stomach.
Indicators are dyes that change colour depending on the number of H⁺ ions around them. Litmus gives a simple acid-or-alkali answer: red for acid, blue for alkali. Universal indicator goes further and gives a different colour for every number on the pH scale.
pH tells you how many H⁺ ions are in a liquid: below 7 is acid, above 7 is alkali, 7 is neutral. Indicators show you which by changing colour, and mixing acid with alkali can balance both out.
Adapted from OpenStax Chemistry (CC-BY 4.0), openstax.org · license
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