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.

(N/A) Most metal oxides are basic in nature. Metal oxides that are soluble in water are called basic oxides,and their aqueous solutions are known as alkalis.
$(b)$ Metals like magnesium,aluminium,and zinc react with atmospheric oxygen to form a thin,protective oxide layer on their surface. This layer is composed of the metal oxide. Its importance lies in the fact that it prevents further oxidation and corrosion of the underlying metal.
$(c)$ Alkali metals like $Li$ (Lithium),$Na$ (Sodium),and $K$ (Potassium) have a low density and weak metallic bonding due to their high electropositive character,making them soft enough to be cut with a knife.