Explain the following statements:

i. Most metal oxides are insoluble in water but some of these dissolve in water. What are these oxides and their solutions in water called?

ii. At ordinary temperature, the surface of metals such as magnesium, aluminium and zinc, etc. is covered with a then layer. What is the composition of this layer? State its importance.

iii. Some alkali metals can be cut with a knife.

i. The metal oxides that dissolve in water are alkali metals. The solution of metal oxides in water is called as a diluted solution or alkaline solution.

Explanation:

• Metal oxides react with water to form metal hydroxides.

• Eg: Na₂O + H₂0 → NaOH

• Metal hydroxide or the bases that are soluble in water are called alkali and they generate hydroxide ions (OH^-)

• Example: NaOH + H2O → Na ⁺ + OH^-

ii. At ordinary temperature, the surface of metals such as magnesium, aluminium and zinc is covered with a thin layer. The thin layer is the oxide layer.

Importance of oxide layer:

• The oxide layer covers the metal fully and makes it less reactive.

• This covering is a way of protecting the metal from corrosion.

• Hence, with this layer the metal do not corrodes and can remain in its original form.

iii. Some alkali metals such as lithium, sodium, potassium can be cut with a knife. These metals are so soft that they can be cut with a knife easily.