In $H_{3}O^{+}$,there is a coordinate covalent bond between $......$ and $......$.

Which of the following is used for the dissolution of gold?

In an attempt to demonstrate electrical conductivity through an electrolyte,the following apparatus was set up. Which among the following statement$(s)$ is(are) correct?
$(i)$ Bulb will not glow because electrolyte is not acidic.
$(ii)$ Bulb will glow because $NaOH$ is a strong base and furnishes ions for conduction.
$(iii)$ Bulb will not glow because circuit is incomplete.
$(iv)$ Bulb will not glow because it depends upon the type of electrolytic solution.
Solution: The apparatus shows a beaker containing a dilute $NaOH$ solution with two nails fixed on a rubber cork. $NaOH$ is a strong base that dissociates completely in water to provide $Na^+$ and $OH^-$ ions. These free ions act as charge carriers,allowing the electric current to flow through the solution and complete the circuit. Therefore,the bulb will glow. Statement $(ii)$ is correct.

Which of the following salts does not contain water of crystallisation?

$(a)$ Write the chemical name and chemical formula of washing soda.
$(b)$ How is it obtained from sodium chloride? Give equations of the reactions.
$(c)$ Why is it called a basic salt? Give any one of its uses.

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