Two or more atoms held together with bonds make up a molecule. Pure substances are made of only one type of atom. At least two types of atoms are required to make a compound. Mixtures can be made of two elements, two compounds, or an element & a compound.
A passing warm front decreases air pressure due to rising air.
On the contrary, a cold front would increase air pressure due to the cold air sinking.
According to an Answer written by World Citizen
If I understand you correctly; the two experimental treatments are the experimental condition and the control condition.
The thing differing between them should be what you want to examine, and especially the thing you want to know of how it influences something. So for example if you want to know how alcohol influences something, the difference should be that one condition will include alcohol and the other won't.