Which scientist introduced the $pH$ scale?

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

State the chemical property in each of the following cases,on which the following uses of baking soda are based:
$(a)$ Applied on an ant stung area.
$(b)$ As a constituent of baking powder.
$(c)$ In soda-acid fire extinguisher.

Match the important chemicals given in Column $(A)$ with the chemical formulae given in Column $(B)$.

Column $(A)$	Column $(B)$
$(a)$ Plaster of Paris	$(i)$ $Ca(OH)_{2}$
$(b)$ Gypsum	$(ii)$ $CaSO_{4} \cdot 1/2 H_{2}O$
$(c)$ Bleaching Powder	$(iii)$ $CaSO_{4} \cdot 2H_{2}O$
$(d)$ Slaked Lime	$(iv)$ $CaOCl_{2}$

Give reasons for the following:
$(a)$ Dry $HCl$ gas does not change the colour of dry litmus paper.

What is the $pH$ value of a basic aqueous solution?