Answer:
1. We add salt to our food to make it taste better.
salt and food - both commnon nouns
2. A heap of books is lying on the table.
heap of books - collective noun; table - common noun
3. The shopkeeper is known for his honesty.
shopkeeper - common noun; honesty - abstract noun
4. Iron is a hard and strong metal.
iron and metal - both material nouns
5. The Bushmen live in the Kalahari Desert.
Bushmen and Kalahari Desert - both Proper nouns
Explanation: