Solve these clues to fill the grid.
- A heavy odorless colorless gas formed during respiration and by the decomposition of organic
- 4 Across
- Layer of air around the Earth
- 7 Across
- An arrangement of people or things acting as a unit
- 8 Across
- A substance that can be consumed to produce energy
- 1 Down
- A process in which a substance reacts with oxygen to give heat and light
- 2 Down
- Occurring or formed with the liberation of heat
- 3 Down
- A compound that exists in forms having different arrangements of atoms but the same molecular weight
- 5 Down
- A chemical substance that is present at the start of a chemical ___ (plural)
- 6 Down