Across
1
Mass per unit volume
4
Basic unit of life
6
Nerve cell in the brain
8
Second lightest element
9
Substance with pH below 7
12
Hottest planet in solar system
Down
2
Natural satellite of Earth
3
Form of thermal energy
5
Blocking of light by a celestial body
7
Unit of force
10
Curved glass that refracts light
11
Absolute temperature scale unit