Question

Ideally, when you measure the vapour pressure of your system, it should increase steadily then reach a plateau (constant value). If it is observed to increase initially and then decrease, what might be the reason?

		a)The vapour pressure is lower than the level that can be accurately detected by the probe.
		b)The intermolecular forces between the molecules in the solution are too strong to enable any to enter the vapour state.
		c)There is a leak in seal for the probe in the experimental system.
		d)It is a dynamic equilibrium so the vapour pressure will be constantly changing.

Answer 1

The answer is Part "d" of the question.

During vaporazation process, after certain time it reaches to a condition of the dynamic equllibrium that cause no further increase in the vapor pressure.  

Once the dynamic equlibrium is reached i.e. rate of vaporization and rate of condensation is same that causes steadily change or drop in vapor pressure.

The main reason causing dyanamic equilibrium is the satuared vapor pressure of system at particular operating tempearture.