Across
1
Biological catalyst
3
Positive particle in nucleus
5
Substance with pH below 7
6
Path of a planet around the sun
9
Nerve cell in the brain
13
Naturally occurring inorganic solid
Down
2
Curved glass that refracts light
4
Molten rock beneath Earth surface
6
Compound containing oxygen
7
Our home planet
8
Mass per unit volume
10
Second lightest element
11
Gas form of a liquid
12
Smallest unit of an element