Example 1 · level 1
Red litmus paper dipped in sodium hydroxide solution turns:
- yellow
- blue
- colourless
- red (no change)
Show solution
- Answer: blue.
Answer: blue
CLASS 10 SCIENCE · ACIDS, BASES AND SALTS
Acids, bases and the pH scale
pH < 7 acidic, pH = 7 neutral, pH > 7 basic. Lower pH = more H⁺ ions; each step of 1 in pH is a 10-fold change in H⁺ concentration.
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Worked examples
Example 2 · level 2
Which of these is the most basic (highest pH)?
- sodium hydroxide solution
- pure water
- lemon juice
- milk of magnesia
Show solution
- pure water: pH ≈ 7
- sodium hydroxide solution: pH ≈ 14
- lemon juice: pH ≈ 2.2
- milk of magnesia: pH ≈ 10.5
- Most basic: sodium hydroxide solution.
Answer: sodium hydroxide solution
Example 3 · level 3
Solution A has pH 3 and solution B has pH 4. How many times greater is the H⁺ ion concentration in A than in B?
Show solution
- [H⁺] ∝ 10^(−pH).
- Ratio = 10^(4 − 3) = 10.
Answer: 10