Carbon and its Compounds — Science STD 10 — Question

Why is a normal eye not able to see clearly the objects placed closer than 25 cm?

Give two differences between food chain and food web.

Why is an equitable distribution of resources essential in a society? List two forces which are against such distribution.

Three elements A, B and C have 3, 4 and 2 electrons respectively in their outermost shell. Give the group number to which they belong in the Modern Periodic Table. Also, give their valencies.

Give two general properties of ionic compounds and two those of covalent compounds.

Keeping the potential difference constant, the resistance of a circuit is doubled. By how much does the current change?

Explain the terms analogous and homologous organs with examples.

Describe with the help of a ray-diagram, the size, nature and position of the image formed by a convex lens when an object is placed beyond 2f in front of the lens.

When a solution becomes more acidic, pH gets lower.

State how the valency of elements varies.

1. In a group.
2. In a period, of the periodic table.