Ionic compounds are hard and brittle due to the strong force of attraction between the positive and negative ions, leading to the formation of a rigid crystalline structure.

Iron filings burn vigorously due to their fine particle size, enhancing reactivity with oxygen. Copper, when heated, does not burn, but it forms a black-colored layer of copper(II) oxide on its surface.

The thin layer of oxide acts as a protective layer, preventing further oxidation of the metal at ordinary temperatures.

Metals like potassium and sodium react vigorously with oxygen and can catch fire if exposed to air. Keeping them immersed in kerosene oil prevents accidental fires and protects them from reacting with atmospheric oxygen.

Carbon exists in different forms called allotropes. Diamond, the hardest natural substance with a high melting and boiling point, and graphite, a conductor of electricity, are two allotropes of carbon.