Will the solubility of a solute increase over the time a solute dissolve?

would like to ask about a question that I came across in one of my assessment books. Appreciate the help that I can get.

Qns: In an investigation, two substances X and Y of the same mass are dissolved in the same volume of water. It is found that X dissolves faster than Y. Which of the following graphs correctly shows the solubility of the substances against time in this investigation? The answer given is A.

I would like to ask why is that so. Intuitively, I would think that the solubility would be a constant since solubility is defined as "the maximum amount of solute that will dissolve in a given volume of solvent at a given temperature and pressure", therefore it will not change with time as the solute is dissolving.

Even if we're talking about rate of dissolving here, shouldn't the rate be decreasing, as when a higher concentration of particles are dissociated, there would be more of the dissociated particles in the solvent which will shift the equilibrium to the left?

Thanks for all your help!

• Yes, we are talking about the rate of dissolving. Yes, the graphs are named wrongly, because solubility is a constant; it should have been something like "Amount dissolved by the time t". Yes, in any realistic setup the rate should be decreasing. This is a dumb question intended to sort out even more dumb people; you're overthinking it. – Ivan Neretin Jun 1 '17 at 5:54

Ivan's comment really nailed the problem, but I'll throw in a bit more.

In the question the term solubility would be ok in an English class, but the term is incorrectly used within chemistry. The more correct tense would be solvation. Likewise the diagrams should have solvation rather than solubility.

In an investigation, two substances X and Y of the same mass are dissolved in the same volume of water. It is found that X dissolves faster than Y. Which of the following graphs correctly shows the solubility solvation of the substances against time in this investigation?

According to question the rate of dissolving of X is higher than that of Y so the slope of the solubility-time curve will be higher for X that's why A should be the answer!

It is 'A', the graph is showing the solubility of a substance over time , the answer 'A' shows that 'X' completely dissolves in less time than 'Y'.

Different substances have different solubility values in a solvent, so the same mass of two different substances in the same volume of the same solvent can have different solubility values.

A more soluble substance will dissolve at a higher rate.

• Thanks for the answer! Would like to know the reason why a more soluble substance would dissolve at a higher rate assuming all other conditions are held the same? I know that a larger mass of it will dissolve but I can't find any articles that state that a higher solubility would naturally equate to a higher rate of dissolving. – truetoall Jun 1 '17 at 3:35
• Also, what I meant to say is that if solubility is defined as "the maximum amount of solute that will dissolve in a given volume of solvent at a given temperature and pressure", even throughout the experiment, the maximum amount that is able to dissolve wouldn't change so shouldn't it be that solubility remains constant? Thanks so much for trying to clarify! – truetoall Jun 1 '17 at 3:35
• Note that as very often in chemistry, solubility (the amount of solute that would be in equilibrium with its solid) and rate of dissolving (the rate at which a solid is incorporated into the solution) respond to different characteristics and don't always go in parallel. Solubility is thermodynamic, rate of dissolving is kinetic; the first tells you what's the limit, the second tells you how fast you get there; you can find situations where compounds which are less soluble (the limit is higher) get to a given concentration before more soluble compounds (the rate of dissolving is higher). – user41033 Jul 1 '17 at 20:55