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AQA GCSE · Question 02.3 · Atomic structure and the periodic table
Potassium hydroxide solution is produced when potassium reacts with water.
What is the colour of universal indicator when added to potassium hydroxide solution?
Give one reason for your answer.
Colour of universal indicator:
Reason:
Potassium hydroxide solution is produced when potassium reacts with water.
What is the colour of universal indicator when added to potassium hydroxide solution?
Give one reason for your answer.
Colour of universal indicator:
Reason:
How to approach this question
1. Identify the type of substance potassium hydroxide is. Is it acidic, alkaline, or neutral?
2. Recall the colour of universal indicator in that type of substance.
3. Explain *why* it is that type of substance, referring to its chemical nature or the ions it produces in solution.
Full Answer
When potassium reacts with water, it forms potassium hydroxide (KOH) and hydrogen gas. Potassium hydroxide is a metal hydroxide, which is a type of alkali. Alkalis are substances that produce hydroxide ions (OH⁻) in solution and have a pH value greater than 7. Universal indicator is used to measure pH. In strongly alkaline solutions (like potassium hydroxide, pH 11-14), universal indicator turns blue or purple.
Common mistakes
✗ Colour: Green (this indicates a neutral pH of 7).
✗ Reason: "It is a base" - alkali is a more precise term for a soluble base. "It is alkaline" is better.
✗ Reason: "It is not an acid" - this doesn't explain why it is alkaline.
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