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Name: Zameer
Status: student
Grade: 6-8
Location: N/A
Country: N/A
Date: October 2006

Question:
I am 12 years old. How many different variables their are to the rate of dissolving something?



Replies:
Zameer,

There are quite a few variables controlling the speed at which something dissolves. We can split these up to two general classes: Thermodynamic (things that have to do with stability and energy) and Kinetic (things that have to do with the process or how the mixing is done) conditions. In the thermodynamic conditions would be things like: the type of intermolecular forces (how well the two molecules interact), whether there is a large entropy gain (if the molecule breaks apart and forms ions for example), the enthalpy lost or gained by the system (whether the solution formation is exothermic or endothermic) and so on. In the kinetic aspect, there are things like: how finely the solute is subdivided (especially if it is a solid), how hard does one stir the solution, and so on.

Greg (Roberto Gregorius)


Your question is a good one -- that means it is difficult to give an easy answer. But let us start by making a difference between "the rate of dissolving something" and "how soluble that something is". Those two things are not the same. However, your equating those two factors is a very common mistake. Very often we are asked about "how fast" or the "rate" of something when the person really means "how much" not "how fast". Of the two questions "how much" is easier to answer than "how fast". For example, "powdered sugar" dissolves much "faster" than "regular sugar" because "powdered sugar" is ground to a much smaller particle size than "regular sugar", even though both are the same substance and end up having the same amount dissolved in water.

It is usually much easier to answer the question "how much" than "how fast". The answer to "how much" means to measure the amount of a substance that dissolves. But when you ask "how fast" a lot of other variables enter in. For example: How much is the "something" stirred? What is the size of the particles? Is the "something" coated with another material? And a lot of other things. If you mean "how much", all of these things do not matter. You just have to wait long enough until all these variables do not matter.

To approach the question of "how much" there are some common "rules". The more alike the "something" is and the "solvent" is, then it likely that the "something" will be soluble. Chemists use the rule of thumb "likes-dissolve-likes". An example is sugar, which is very soluble in water, but is almost totally insoluble in alcohol or gasoline.

Usually, but not always, the higher the temperature, the more soluble a substance becomes. But you can find many exceptions. One very universal exception is the solubility of gases, which always (well, almost always) become less soluble as the temperature increases. But there is a large difference in the solubility of different gases. On the other hand, common table salt has almost the same solubility in cold, room, warm and hot temperatures.

I think that your question is really about "how much" and not really "how fast" because "how fast" depends upon many variables that can be changed or cannot be controlled.

Vince Calder



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