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Which substance shows the correct indicator colour? Substance pH Indicator Colour (A) Stomach acid 2 Methyl orange Yellow (B) Lemon juice 3 Phenolphthalein Pink (C) Soda water 4 Phenolphthalein Pink (D) Seawater 8 Methyl orange Yellow - HSC - SSCE Chemistry - Question 3 - 2010 - Paper 1
Question 3
Which substance shows the correct indicator colour? Substance pH Indicator Colour (A) Stomach acid 2 Methyl orange Yellow (B) Lemon juice 3 Phenolphthalein Pink (C)... show full transcript
Worked Solution & Example Answer:Which substance shows the correct indicator colour? Substance pH Indicator Colour (A) Stomach acid 2 Methyl orange Yellow (B) Lemon juice 3 Phenolphthalein Pink (C) Soda water 4 Phenolphthalein Pink (D) Seawater 8 Methyl orange Yellow - HSC - SSCE Chemistry - Question 3 - 2010 - Paper 1
Step 1
Identify the pH range for each indicator
Answer
Methyl orange typically changes color from red to yellow at a pH of around 3.1 to 4.4. Therefore, in acidic conditions (pH < 3.1), it appears red, and in neutral to alkaline conditions (pH > 4.4), it appears yellow.
Phenolphthalein changes from colorless to pink as the pH increases from about 7 to 10. It remains colorless in acidic conditions and turns pink only in basic conditions.
Step 2
Check the indicators against the provided substances
Answer
- Stomach acid (pH 2): Methyl orange should be red, not yellow.
- Lemon juice (pH 3): Phenolphthalein should still be colorless at this pH, so this is incorrect as it indicates pink.
- Soda water (pH 4): Phenolphthalein is still colorless at this pH, so it is incorrect to indicate pink.
- Seawater (pH 8): Methyl orange should be yellow, which is consistent since the pH is above 4.4.
Thus, seawater is the only substance correctly matching its indicator color.
Step 3