Four students I, II, III and IV were asked to examine the changes for blue and red litmus paper strips with dil. HCl (Solution A) and dil. NaOH (Solutions). The following observations were reported by the 4 students. The sign …………. indicates no colour change
| Litmus | A | B | Litmus | A | B |
| I | Blue | - | red | Blue | red | - |
| II | Red | - | blue | Red | - | blue |
| III | Blue | red | red | blue | blue | blue |
| IV | Red | blue | blue | Red | red | red |
Correct Answer: (c) III
Explanation: The correct observation is taken by student III because HCl (solution A) is an acid that turns blue litmus red and dil. NaOH (solution B) is a base which turns red litmus blue. 
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