Here's why:

* Pure substances are made up of only one type of molecule. Nitrous oxide consists only of nitrogen and oxygen atoms bonded together in a specific ratio (N₂O).

* Colloids are mixtures where particles are dispersed throughout a medium, but they don't settle out. Nitrous oxide doesn't have separate components that are dispersed.

* Heterogeneous mixtures have visibly different components that are not evenly distributed. Nitrous oxide is a uniform gas.

Therefore, nitrous oxide is a pure substance.