Which indicator in the table would be best for distinguishing between a face cleanser (pH = 5.0) and a soap (pH = 9.0)?
| Indicator | Colour (low pH – high pH) | pH range |
|-------------------|---------------------------|-----------|
| A - HSC - SSCE Chemistry - Question 2 - 2020 - Paper 1
Question 2
Which indicator in the table would be best for distinguishing between a face cleanser (pH = 5.0) and a soap (pH = 9.0)?
| Indicator | Colour (low pH – high... show full transcript
Worked Solution & Example Answer:Which indicator in the table would be best for distinguishing between a face cleanser (pH = 5.0) and a soap (pH = 9.0)?
| Indicator | Colour (low pH – high pH) | pH range |
|-------------------|---------------------------|-----------|
| A - HSC - SSCE Chemistry - Question 2 - 2020 - Paper 1
Step 1
Identify the required pH range.
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Answer
To distinguish between the face cleanser (pH = 5.0) and soap (pH = 9.0), we need an indicator that changes color within this pH range.
Step 2
Evaluate the indicators.
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Answer
From the table:
A. Bromophenol blue: pH range 3.0–4.6 (does not cover pH 5.0 or 9.0)
B. Methyl orange: pH range 3.1–4.4 (does not cover pH 5.0 or 9.0)
C. Phenol red: pH range 6.4–8.0 (includes pH 5.0 but not pH 9.0)
D. Thymolphthalein: pH range 9.4–10.6 (does not cover pH 5.0)
Only Phenol red provides a clear distinction between a solution with a pH of 5.0 and one with a pH of 9.0.
Step 3
Conclude the best indicator.
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Answer
Therefore, option C (Phenol red) is the best indicator to distinguish between a face cleanser and soap, as it allows for a clear color change in the relevant pH range.
