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Name: Tia
Status: educator
Grade: 6-8
Location: WA
Date: May 2008


Question:
How does heating a solution effect the concentration of the solution?



Replies:
The answer is, it depends. This is a very simple question but like many simple questions, the answer is pretty complex because of the huge range of possibilities.

If a nonvolatile solute (something which does not evaporate) is dissolved in a liquid solvent, heating the solution will cause some of the solvent to evaporate. *If nothing else happens* (but see below), the concentration will then increase. This would happen for, say, salt (NaCl) dissolved in water.

However, if the solute is a gas when not dissolved, or is more volatile than the solvent, heating the solution can cause more solute to evaporate than solvent. Then the concentration will decrease. This would happen for carbon dioxide dissolved in water.

A complication is that some solutes dissolve better when the solution is hot, and some solutes dissolve more poorly when the solution is hot (we say that the solubility may be either an increasing or decreasing function of temperature). If the solubility decreases as the temperature increases, the solute may completely drop out of solution and the concentration will decrease despite solvent evaporation. So, concentration can either increase or decrease as you heat the solution, just depending on the specific properties of the solution components.

There are probably other factors I have forgotten to discuss here but I hope this is a bit helpful.

Robert Topper


Tia,

The concentration of a solution is commonly defined as the mass of solute divided by the volume of total solution. Also, if the units moles per liter are used, then this concentration is known as a Molar solution (M). i.e. 5 moles of solute in 1 liter of total solution is a 5 Molar solution. Assuming that heating the liquid does not evaporate any significant volume, then the only thing that could possibly change the concentration is expansion of the solvent due to the heating. This is common, but the amount at which it expands depends greatly on the solvent.

Speaking from personal observation, I could estimate that many organic solvents can expand at least 5-10%. For an example, if you had a 1 Molar solution and the solvent expanded from 1 Liter to 1.1 Liters, then the resulting concentration would then be 0.91 Molar instead of 1 Molar.

Matt Voss


Tia,

Since concentration is often a description of the ratio of the amount of solute to that of the solvent and since amounts are not affected by temperature, then temperature should have no effect on concentration.

However, some concentration units describe the amount of solute in a *volume* of solution, and since volume can expand as a function of temperature, then concentration changes -becomes lower- as the volume increases with temperature. This is especially observable in gases if the concentration unit is one involving the volume of the container. The volume of the container can change dramatically with temperature (at constant pressure) and so concentration can change dramatically.

In solutions involving liquids, the volume change as a function of temperature is usually small and therefore not usually taken into consideration.

Greg (Roberto Gregorius)


The "usual" answer is that heating a solution of solutes (dissolved substance) increases their solubility. However, that "rule of thumb" is not at all universal. The solubility of some substances increases with increased temperature. The solubility of other substances decreases with increasing temperature. The solubility of other substances, the classic case being NaCl (common table salt), varies very little with changes in temperature. I am not aware of any model that predicts the change in solubility with changes in temperature.

Vince Calder


In a closed system, all that heating does is increase the volume, which will decrease the concentration slightly as measured in grams or moles of solute per volume of solution. One exception to this is water between 0 C and 4 C, which actually contracts as temperature increase, so the trend would go the other way over that temperature range for very dilute solutions. (More concentrated aqueous solutions may not share this negative thermal expansion.)

When systems are not closed, other things can happen. For example, gases tend to be less soluble in liquids as the temperature increases. this is because heat drives them out of the liquid and into the gas phase. So, heating a solution of a gas in a liquid (such as carbon dioxide in water) will greatly decrease the concentration. Increasing the temperature of a solution also can enhance chemical reactions producing gases: a classic example is the "boiler-scale reaction," which produces lime scale deposits in hot-water systems and creates travertine in hot springs. In this reaction, dissolved calcium bicarbonate decomposes to insoluble calcium carbonate, water, and gaseous carbon dioxide:

Ca(HCO3)2 (aq) --> CaCO3 (s) + CO2 (g) + H2O

In this reaction, the carbon dioxide escapes (especially if the solution is hot--carbon dioxide, a gas, is not very soluble in hot water) and the calcium carbonate precipitates out of solution, so the concentration of calcium bicarbonate (and calcium ion) in the water drops dramatically.

Richard Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming



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