Question

A white solid imparts a violet colour to a bunsen flame. On being heated with concentrated ${H_2}S{O_4}$, the solid gives violet vapors that turn starch paper blue. The salt may be:
(A) KI
(B) NaI
(C) $Mg{I_2}$
(D) $CaB{r_2}$

Answer 1

Hint: With the colour of the flame i.e. flame test, we can predict the cation present in the salt. When iodine gas reacts with starch, it forms a complex which is blue in colour.

Complete answer:
We are given two observations of the tests performed on a white solid salt. We will take a look at both the observations in order to find which salt would give that.
- First it is given that the white solid gives violet flame on a Bunsen flame. When a salt is heated on bunsen flame to observe the colours emitted, the test is called flame test. With the flame test, we can predict the cation present in the salt. Here, as the salt is giving violet colour, the cation present in the salt should be potassium ions because only potassium ion gives violet colour in flame test. Thus, from this observation we can say that ${K^ + }$ is present in the salt.
- When we heat the salt with concentrated ${H_2}S{O_4}$, it is given that it turns the starch paper blue. We know that the starch paper turns blue only when iodine gas reacts with it. So, iodine gas should have been produced here. We can give the possible reaction as:
     \[8KI + 5{H_2}S{O_4} \to 4{K_2}S{O_4} + 4{I_2} + {H_2}S + 4{H_2}O\]
Here, we can see that the iodine gas is produced. This gas makes a complex with starch present on the starch paper. This turns the starch paper blue.
- Thus, we can conclude that the salt should be KI.

So, the correct answer is (A).

Note:
Remember that if sodium or calcium is present in the salt, then they give golden yellow and brick red colour in the flame test. Magnesium ion does not give any characteristic colour in flame test.