Question

# A student takes about 6 ml of distilled water in each of the four test tubes $$P$$, $$Q$$, $$R$$ and $$S$$, then dissolves an equal amount of four different salts namely sodium chloride in $$P$$, potassium chloride in $$Q$$, calcium chloride in $$R$$ and magnesium chloride in $$S$$. He then adds 10 drops of soap solution to each test tube and shakes its contents. The test tubes in which scum (insoluble substance) is formed with soap are:

A
P and Q
B
Q and R
C
R and S
D
Q and S

Solution

## The correct option is D $$R$$ and $$S$$Soap forms insoluble precipitates with calcium and magnesium ions forming a scum. Hence, test tube $$R$$ and $$S$$ containing calcium chloride and magnesium chloride respectively will form scum.Option C is correct.Chemistry

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