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)
The lesson
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. The more H⁺ ions crowded into a liquid, the more acidic it is.
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.
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, and indicators like litmus and universal indicator show you which by changing colour.
Watch it
Where it sits
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Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.