What type of reaction occurs when sulfur dioxide $(SO_2)$ reacts with an acidic solution of potassium dichromate $(K_2Cr_2O_7)$?

An alloy is

Explain the following statements:
$(a)$ Most metal oxides are insoluble in water but some of these dissolve in water. What are these oxides and their solutions in water called?
$(b)$ At ordinary temperature,the surfaces of metals such as magnesium,aluminium,and zinc are covered with a thin layer. What is the composition of this layer? State its importance.
$(c)$ Some alkali metals can be cut with a knife.

What are the constituents of solder alloy? Which property of solder makes it suitable for welding electrical wires?

$NH_{3}$ is used in the preparation of nitric acid by $.....$ process.

Metals are refined by using different methods. Which of the following metals are refined by electrolytic refining?
$(i)$ $Au$
$(ii)$ $Cu$
$(iii)$ $Na$
$(iv)$ $K$