Colloids

Figure 47.

Salts of alkali metals and ammonium, and salts containing nitrate and acetate are always soluble. Carbonates, sulfates, hydroxides, phosphates, and heavy metal salts are typically insoluble in water.

COLLOIDS

A colloid is one of three main types of mixtures (with solutions and suspensions being the other two). It is a solution with particles that range from 1 to 1000 nanometers in diameter but that are able to remain evenly dispersed throughout the solution. These are also referred to as colloidal dispersions because they will not settle to the bottom of the container.

As you can see, a colloid is going to be similar to a solution but involves particles that are not seen with a naked eye but that are larger than a molecule. Particles larger than 1000 nanometers will be a suspension and particles less than 1 nm will be a solution.

There are four types of colloid classifications:

• Sol—this is a colloidal suspension with solid particles in a liquid.

• Emulsion—this is a colloid between two liquids.

• Foam—this is a colloid where gas particles are trapped in a liquid or solid

• Aerosol—this is a liquid or solid dispersed in gas.

When the solvent is water, the resulting colloid is referred to as a hydrocolloid. Hydrocolloids are often used to make medical dressings.

A simple way of determining whether or not a mixture is a colloid or not is to examine the Tyndall effect. When light is passed through a solution, it will pass through it directly. When light passes through a colloid, it will be scattered in all directions, making it readily seen. Fog is an example of a colloid so a headlight beam can be seen through it because of the Tyndall effect. This reflection of light is seen only in colloids. Another example is attempting to pass the colloid through a semipermeable membrane. Colloids will not be able to pass through the semipermeable membrane.