Which variable is held constant when testing for the thermal expansion relationship of a gas?

In the reaction between a crushed antacid tablet and vinegar what gas is emitted

Answer explain why it is not possible to deduce a complete order of reactivity.

Liquid ammonia is produced at high temperatures and under great pressure in a tank by passing a mixture of nitrogen gas and hydrogen gas over an iron catalyst. the reaction is represented by the following equation. n2(g) + 3h2(g) → 2nh3(g) changing all but one experimental condition will affect the amount of ammonia produced. that condition is a) increasing the concentration of both reactants b) changing the temperature within the tank c) decreasing the pressure within the tank. d) increasing only the amount of nitrogen present.