# Does the amount of solute affect rate of solvation?

I know that in order to increase the rate of solvation, you need to increase how often solute and solvent particles come into contact.

You can do it by:

1. agitating the mixture
2. increasing the surface area of the solute
3. heating up the solvent

But my question is: Does adding more solute (like adding more salt to water) increase the rate of solvation since the contact surface area is larger?

Edit: The original question in the book describes dissolving a certain amount of $$\ce{NaCl}$$ into water and asks, “What can be done to increase the rate of dissolution without affecting solubility?” I assume the rate is measured by how fast the mass of $$\ce{NaCl}$$ is consumed, which is $$-\frac{dW}{dt}$$.

• Could you write down the chemical equation? Are we going from pure solid substance to completely dissolved substance? If so, point 2 is increasing the surface area of the solid substance, right? And how do you measure rate? Is it how fast the concentration of solute particles increases? I think technically, the process is called dissolution, not solvation. Solvation is more general, like when an acid dissociates and the products are now surrounded by more solvent molecules, or when a cation was bound to a protein and now is surrounded by bulk water. – Karsten Theis Apr 25 '19 at 16:56
• Just made an edit. – Alex Apr 26 '19 at 5:36

## 2 Answers

If the answers are to be exclusive, then obvious choice is agitating the mixture.

If the answers are not to be exclusive, then agitating the mixture + increased surface by adding more solute increases $$\frac{\mathrm{d}c}{\mathrm{d}t}$$

Adding more solute does not increase rate of solvation, but as the absolute surface increases, it increases $$\frac{\mathrm{d}c}{\mathrm{d}t}$$.

Note also without affecting solubility from your comment, so increasing temperature is out.

• Thanks! I was curious because the question in my homework was like “What can you do to increase rate of solvation and not affect the solubility?”, and the option “Adding more solute” wasn’t one of the answers, so I went asking around. – Alex Apr 25 '19 at 10:28
• @Alex I think Poutnik didn't get the point of the Q. Increasing amount of solute does not speed up the process. – Alchimista Apr 25 '19 at 11:17
• @Alchimista Increasing of amount does not speed up diisolution. Increasing of surface does. Even if achieved by increasing the amount.Best if combined with agitating of mixture. I agree if solute sits passively on the bottom, the effective surface may stay about the same. – Poutnik Apr 25 '19 at 11:35
• See also the answer update. – Poutnik Apr 25 '19 at 11:56
• The original answer was wrong. Op ask if adding more solute speed up dissolution and answer is " it does. ...do you have any reason why it may not". The correct answer is no, from whatever point of views. The answer you gave is that to an hypothetical multiple choice exercise. Still the opening of your A is wrong. I do no bother, but there are major misunderstanding (while in your side it is likely a misunderstandng of the Q) that can convince the OP of something wrong (as OP accepted too quickly your A). – Alchimista Apr 25 '19 at 12:59

But my question is: Does adding more solute (like adding more salt to water) increase the rate of solvation since the contact surface area is larger?

Not really. If I get the point, you would like to know if x + y grams of a substance dissolve faster (as dn/dt) than an amount x. No. Ignoring activity of the solute, or in sufficiently diluted solution, the speed of solubilisation per surface stays the same, or decrease as activity increase if you look for an overall picture.

Thus, as per amount of substance the kinetic of solubilisation just profit of an effective augmented surface, ie the ratio between surface and mass of the solute (point 2 in OP list). Smaller grains dissolve faster, and this is the basis for precipitate digestion to get bigger crystals.

This answer assume that the meaning of solvation is that of solubilisation, and point to a relative speed rather than the absolute time required to dissolve a substance. The latter goes up for both principle and, I would say so, practical reasons.

Edit after reading an OP comment. The answer to the book Q "how to increase the rate of solvation without affecting solubility" are right stirring or finely grinding the substance to be dissolved, for instance.