which condition must always be kept constant?

The enthalpy of a substance is defined as the heat content of the substance under constant pressure.

Therefore, in an experiment to measure the enthalpy change, pressure must be kept constant.

The answer: A. Pressure

I agree with the answer of 'pressure' given by wanszeto. Although, I would tend to want more of a reason other than "because that is how it is defined".

An import factor in this explanation is that this it is an experiment and you have to measure something. The something you measure is not the rise in enthalpy (From my understanding), it is really the measure of the heat into or out of the system. This detail is importante...enthalpy and heat are two differnt things but how do you insure you get what you want...which is the rise in enthalpy? There is one way to do it..and that is get it so that enthalpy change is equal to heat. This equality happens to occur when the pressure is held constant. If the pressure were not constant then enthalpy doesnt cease to exist..i.e even if the pressure changed you will still have an enthalpy change but I am not sure if it can be measured in a nice way. In that case you would need to calculate it instead or measuring it. So, once pressure is held constant....which happens if you were measuring it in your laboratory under earth's more or less constant atmospheric pressure, then a measure of heat directly tells you a change in enthalpy.


If you are familiar with diffential calculus and if you care to know, this can be seen mathematically in the definition of infentesimal enthalpy change.

dH = dU + p*dv+v*dp
where dU = dQ - p*dv making
dH = dQ - p*dv + p*dv + v*dp which reduces to
dH = dQ + v*dp
if pressure were constant then it reduces to
dH = dQ
which is that the enthalpy change will equal heat.

(NOTE: just being picky.... dQ is an inexact differential in this case)