Factors Affecting Rates of Dissolving

You already know that sugar dissolves in water. But what do you know about the rate at which it dissolves? Like rates of chemical reactions, rates of dissolving depend on the frequency and energy of collisions that occur between very small particles. During a chemical reaction, collisions occur between particles of the reactants. During the formation of a solution, collisions occur between particles of the solute and solvent. Factors that affect the rate of dissolving include surface area, stirring, and temperature.

Figure 7 The rate of dissolving can be increased by reducing the particle size of the solute, by stirring, and by heating the solvent. Predicting How does changing the temperature setting on a washing machine affect how fast the detergent will dissolve?

When a sugar cube dissolves in water, the dissolving takes place at the surfaces of the cube. The greater the surface area of a solid solute, the more frequent the collisions are between solute and solvent particles. More collisions result in a faster rate of dissolving. You can increase the surface area of a solid by dividing it into smaller particles. The more finely divided a solid solute, the faster it dissolves. For example, one gram of granulated sugar dissolves faster in water than a 1-gram sugar cube.

You can also make sugar dissolve faster by stirring the mixture. Stirring moves dissolved particles away from the surface of the solid, and allows for more collisions between solute and solvent particles.

Another way to speed up the rate of dissolving is to increase the temperature of the solvent. For example, sugar dissolves faster in warm water than it does in cold water. Increasing the temperature of a solvent causes its particles to move faster, on average. As a result, both the number of collisions and the energy of these collisions with solute particles increase. The solute goes into solution more quickly.