What three factors increase the rate of dissolution?

#"(i) particle size of the solute:"# smaller particle size will favour dissolution.

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Rate of Dissolution
Surface Area
Agitation of the Solution
Temperature

Explanation:

#"(ii) temperature of the solvent:"# dissolution is generally a bond-breaking process, which is favoured by higher temperatures.........

#"(iii) intrinsic solubility of the solute:"# a solute might have good or indifferent solubility in the solvent.

So there are three. These are both kinetic and thermodynamic factors. Do I win £5-00?

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Many people enjoy a cold glass of iced tea on a hot summer day. Some like it unsweetened, while others like to put sugar in it. How sugar dissolves in the tea depends on two factors: how much sugar was put into the tea, and how cold it is. Tea usually has to be stirred for a while to get all the sugar dissolved.

Rate of Dissolution

Dissolution is the process by which a solute dissolves into a solvent and forms a solution. We know that the dissolution of a solid by water depends upon the collisions that occur between the solvent molecules and the particles in the solid crystal. Anything that can be done to increase the frequency of those collisions and/or to give those collisions more energy will increase the rate of dissolution. Imagine that you were trying to dissolve some sugar in a glassful of tea. A packet of granulated sugar would dissolve faster than a cube of sugar. The rate of dissolution would be increased by stirring, or agitating the solution. Finally, the sugar would dissolve faster in hot tea than it would in cold tea.

Surface Area

The rate at which a solute dissolves depends upon the size of the solute particles. Dissolution is a surface phenomenon, since it depends on solvent molecules colliding with the outer surface of the solute. A given quantity of solute dissolves faster when it is ground into small particles, rather than in the form of large pieces, because more surface area is exposed. A packet of granulated sugar exposes far more surface area to the solvent and dissolves more quickly than a sugar cube.

Agitation of the Solution

Dissolving sugar in water will occur more quickly if the water is stirred. The stirring allows fresh solvent molecules to continually be in contact with the solute. If it is not stirred, then the water right at the surface of the solute becomes saturated with dissolved sugar molecules, meaning that it is more difficult for additional solute to dissolve. The sugar cube would eventually dissolve because random motions of the water molecules would bring enough fresh solvent into contact with the sugar, but the process would take much longer. It is important to realize that neither stirring nor breaking up a solute affect the overall amount of solute that dissolves—these actions only affect the rate of dissolution.

Temperature

Heating up a solvent gives the molecules more kinetic energy. The increased rapid motion means that the solvent molecules collide with the solute with greater frequency, and that the collisions occur with more force. Both factors increase the rate at which the solute dissolves. As we will see in the next section, a temperature change not only affects the rate of dissolution, but also affects the amount of solute that dissolves.

Summary

Some solids dissolve easily and rapidly in liquid solvents such as water, whereas others require a prolonged period to fully dissolve. Dissolution fundamentally involves the separation of molecules or ions by their binding to solvent molecules. The rate at which a substance dissolves, therefore, operates as a function of the frequency of collisions between the dissolving substance and the solvent. Consequently, anything that increases the frequency of collisions also increases the rate of dissolution. If you are trying to dissolve a substance, you have three primary avenues to increase the dissolution rate: decreasing the particle size of the solid, increasing the temperature and/or increasing the mixing or stirring rate.

Consult a reference book, such as the “CRC Handbook of Chemistry and Physics,” to verify that the substance being dissolved exhibits reasonable solubility in your chosen solvent. For purposes of demonstration, the handbook entry for aspirin -- chemical name 2-(acetyloxy)benzoic acid -- indicates “s H2O, eth, chl; vs EtOH; sl benzene.” This means aspirin exhibits solubility in water, ether and chloroform, very good solubility in ethanol and slight solubility in benzene. Make certain the substance you are dissolving is at least slightly soluble in your chosen solvent.

Crush or grind the solid to be dissolved into a fine powder with a mortar and pestle.

Place the crushed powder into a saucepan, beaker or flask and fill the pan, beaker or flask about halfway with solvent. Stir the mixture and note the rate at which the substance dissolves. If the substance exhibits good solubility in the solvent, it may dissolve within a few seconds with stirring alone.

Heat the pan, beaker or flask gently on an electric burner or hot plate if the substance has not dissolved within 1 minute with stirring alone. Continue stirring as the substance heats. The rate of dissolution should noticeably increase as the temperature of the solution rises.

Things You'll Need

Mortar and pestle
Sugar
Saucepan, beaker or flask
Stirring spoon

Warnings

Do not attempt these processes without training. Never use organic solvents near an open flame or source of ignition, as these solvents are highly flammable. If you must use an organic solvent, wear safety glasses and do not place a container with the solvent directly on a heating surface. Instead, prepare a hot water bath by placing tap water into a beaker or large pan, and place the container with the solvent into the water bath. Never heat an organic solvent to boiling.