Lecture 4.2 – The Dissolving Process and the Rate of Dissolving

Why do substances dissolve?

I. The Dissolving Process The dissolving process is facilitated by motion of liquid molecules and intermolecular forces. Liquid molecules are moving in random motion. When a molecule is dissolved in water, water molecules hydrogen bond with one another to form a cage to dissolve.

II. Like Dissolves Like Like substances dissolve like substances If something has a charge, then it will not be able to dissolve something that does not have a charge.

How does temperature, pressure, and surface area impact rate of dissolving?

I. Dissolving We can alter the rate at which the solute dissolves by: 1. Changing the temperature 2. Changing the surface area of the solute 3. Changing the pressure of the solution.

II. Temperature and Dissolving As you increase the temperature, molecules start moving faster. By moving faster, the molecules are better able to grab the solute, form the cages around them and pull them into the solution. Therefore, the faster molecules are moving, the faster something dissolves.

Remember What Dissolving Looks Like?

Temperature and Dissolving Rate Cold liquid Hot liquid

III. Surface Area and Dissolving As you increase the surface area, the rate at which the solute dissolves increases. As you increase the surface area of a substance, you are providing more sites for the solvent to interact with the solute.

Surface Area and Dissolving

IV. Pressure and Dissolving As you increase the pressure, the rate at which the solute dissolves increases. The solubility and pressure relationship is clearly seen when a gas dissolves in a liquid. Gases can dissolve into liquids.

IV. Pressure and Dissolving When a gas is above a liquid and the pressure is increased, the gas molecules collide more often with the wall of the liquid and this causes more gas to be pushed into the liquid.