Question

Sulphurous acid turns ___ litmus paper___.
A. Blue, red
B. Red, blue
C. Red, red
D. Blue, blue

Answer 1

Hint:
Classification of compounds as acids or bases based on their interaction with acid-base indicators can help us in deducing the right indicator and the change accordingly.

Complete step by step solution
We can encounter acids and bases all around us in our daily lives and not just chemistry labs. For example, vinegar and lemon juice contains acetic acid and citric acid respectively whereas baking soda is basic in nature. Now, what if we encounter an unknown compound, how can we find out whether it is acidic, basic or just neutral? For this purpose, we have acid-base indicators which give different observations upon reacting with acid, base or neutral substances.

We can have natural as well as synthetic indicators based on the source of origin. For example, litmus, turmeric and red cabbage leaves are some natural acid-base indicators whereas methyl orange and phenolphthalein are synthetic acid-base indicators. Every acid-base indicator shows characteristic change when added to acid or base.
Here we will talk about litmus indicators. Litmus solution is basically a purple dye, and remains purple in neutral solution. However, it turns red with an acid and blue with a base.

So, we can list the following points for litmus test:
1. An acid would change blue litmus to red
2. An acid won’t bring any change to red litmus
3. A base would change red litmus to blue
4. An acid won’t bring any change to blue litmus
5. In neutral solution, litmus would be purple

Now, as here we are given sulphurous acid. So, being an acid it will turn blue litmus to red.
Hence, the correct option is A.

Note:
We can use litmus in solution as well as paper form with similar color changes in both the forms.