Properties of Alkalis & Indicators
Chemistry: Cambridge International GCSE Supplement, topic 7.1.8
Acids, Bases & Salts: Properties of Alkalis and Indicators
Alkalis are soluble bases containing OH- ions. They turn red litmus blue, colourless thymolphthalein blue, and methyl orange yellow. Acids contain H+ ions.
What are alkalis?
- Alkalis are bases that dissolve in water to make a solution.
- The alkali solution contains OH- ions.
- Alkalis feel soapy and taste bitter.
- Alkalis react with acids to form salts and water.
Alkalis and litmus paper
- Litmus paper is a common indicator in chemistry.
- Alkalis turn red litmus paper blue.
- This change shows the solution is alkaline (basic).
Alkalis and thymolphthalein
- Thymolphthalein is a pH indicator.
- Alkalis change colourless thymolphthalein to blue.
- This colour change happens because thymolphthalein works in alkaline solutions above pH 9.
Alkalis and methyl orange
- Methyl orange is another indicator used in chemistry.
- Alkalis turn methyl orange indicator from red to yellow.
- Methyl orange changes colour at a pH around 3.1 to 4.4, indicating acidic or alkaline conditions.
Ions in acid and alkali solutions
- Aqueous acid solutions contain H+ ions.
- Aqueous alkali solutions contain OH- ions.
- These ions cause acids and alkalis to have different properties.