Answer:
1. only in systems in which gases are involved
2. only when the chemical reaction produces a change in the total number of gas molecules in the system
Explanation:
According to Le Chatelier's principle, pressure will only affect a system in equilibrium containing gaseous reactants and products. However, change in the pressure will only affect the gaseous system in which the total number of moles of the reactants are different from the total number of moles of the products.