Across
1
Gas essential for breathing
5
Hottest planet in solar system
6
Fourth state of matter
7
Substance with pH below 7
8
The Red Planet
9
Second lightest element
Down
2
Tissue that contracts for movement
3
Natural satellite of Earth
4
Center of an atom or cell
10
Measure of matter in an object
11
Blocking of light by a celestial body
12
Preserved remains of ancient life
13
Substance with pH above 7