1Acids and alkalis at home
Lemon juice contains an acid, oven cleaner contains an alkali, and pure water is neutral. Litmus shows which is which.
acidalkalineutral
Lesson 14.1 notes
Acids, alkalis and indicators
Acids and alkalis at home, litmus and red cabbage indicators, bases and alkalis, concentration and safety.
2Litmus
An indicator changes colour in acids and alkalis. Litmus turns red in an acid and blue in an alkali.
indicatorlitmus
3Test with both papers
Red litmus turns blue in an alkali. Blue litmus turns red in an acid. If neither paper changes, the substance is neutral.
neutral
4Red cabbage indicator
Soak chopped red cabbage in hot water and filter it. The purple liquid is an indicator: red or pink in acids, purple when neutral, and blue, green or yellow in alkalis.
red cabbage
5Bases and alkalis
A base is a substance that can neutralise an acid. An alkali is a base that dissolves in water.
basealkali
6Concentrated or dilute
A concentrated acid has lots of acid particles in a little water. Adding water makes it more dilute: the same particles spread through more water.
concentrateddilute
7Staying safe
Concentrated acids and alkalis can be corrosive: they attack skin, eyes and metals. Wear eye protection and wash splashes off with lots of water.
corrosive
Explore: Indicator tester
Pick a substance and an indicator, then watch the colour change.
Common mistake: Alkalis are not safer than acids
People often think only acids burn. Concentrated alkalis, like the sodium hydroxide in oven cleaner, are corrosive too: they can burn skin and badly damage eyes.
Remember
- An indicator changes colour in acids and alkalis. Litmus turns red in acid and blue in alkali.
- Neutral substances, like pure water, are neither acidic nor alkaline.
- An alkali is a base that dissolves in water. Concentrated acids and alkalis can be corrosive, so wear eye protection.
Going further at GCSE