Calcium phosphate is present in tooth enamel. Its nature is:

Write true or false for the following statement:
The process of dissolving an acid or a base in water is a highly exothermic process.

The metal salt $A$ is blue in colour. When salt $A$ is heated strongly over a burner,a substance $B$ present in it is eliminated and a white powder $C$ is left behind. When a few drops of a liquid $D$ are added to powder $C$,it becomes blue again.
$(a)$ Identify $A, B, C, D$.
$(b)$ Write the chemical equations involved.
$(c)$ Give an example of a salt which also shows the above property.

At which value of $pH$ of the inner side of the mouth does the decay of teeth take place?

Match the acids given in Column $(A)$ with their correct source given in Column $(B)$.

Column $(A)$	Column $(B)$
$(a)$ Lactic acid	$(i)$ Tomato
$(b)$ Acetic acid	$(ii)$ Lemon
$(c)$ Citric acid	$(iii)$ Vinegar
$(d)$ Oxalic acid	$(iv)$ Curd

The Bronsted-Lowry acid-base theory was based on which of the following?