Solution:

H202 (Hydrogen peroxide) which is harmful and can cause death.

Solution:

Mercury

Solution:

The nail would be at the same height since trees grow at their tops.

Solution:

In vacuum both will reach in same time.

whereas in case of air density medium it will depend on the surface area which is exposed to gravity opposing force.
gravity opposing force will increase as it is coming downwards and the amount of force is directly proportional to the area exposed to it.

Solution:

Fm.

These are the chemical elements whose atomic numbers are perfect squares.

1, 4, 9, 16, 25, 36, 49, 64, 81, 100

Solution:

When you mix bleach and ammonia [found in most cleaning products] it creates a deadly gas that can kill.

Solution:

An Echo

Solution:

Charcoal

Solution:

Throw the ball straight up in the air.