Who defined acid and base on the basis of chemical properties?

Which gas is released when acid reacts with metal?

If the $pH$ of an aqueous solution is $6$,then its $pOH$ is:

Crystals of copper sulphate are heated in a test tube for some time.
$(a)$ What is the colour of copper sulphate crystals:
$(i)$ before heating, and $(ii)$ after heating?

Write true or false for the following statement:
The universal indicator shows the same colour at different concentrations of hydrogen ions in a solution.

Which of the following is an oxide of metal?