mixture

Learning objectives

1. Define a mixture

2. General properties of a mixture

3. Two main categories of mixtures: homogenous and heterogeneous

4. Classification of mixtures based on the particle size of their components or substances: alloys, solution, colloids, and suspension

 

What is a mixture?

A mixture is when you combine two or more substances and each of the substances retains its own chemical makeup. To be a mixture, the substance cannot form or break chemical bonds with each other.

In order to be classified as a mixture, the substance must satisfy three general properties:

1. The components in a mixture can easily be separated.

2. Each of the components in a mixture keeps its own chemical properties.

3. The proportion of the components in a mixture is variable.

 

Characteristics of Mixture

Mixtures are different from chemical compounds. This is because:

• There is very little or no energy change when a mixture forms.
• The substances in a mixture can be separated by the use of physical methods like distillation, freezing and filtration.
• Mixtures have variable compositions, while compounds have a fixed, definite formula.
• In a mixture, individual substances maintain their properties while in a compound, the properties may change.

Mixtures can be homogeneous or heterogeneous.

 

Homogenous mixture

A homogenous mixture is one where all the substances in the mixture are distributed evenly within the entire mixture. This type of mixture can be sampled in any area and get the same composition of the substances. An example of this is salt and water. The salt dissolves in water and forms a homogenous mixture throughout the entire container it is in.

One example of a mixture is air. It is a homogeneous mixture of gases and small amounts of other substances. Sugar, salt and other substances dissolve in water forming homogeneous mixtures. A homogenous mixture where there are both a solute and a solvent is known as a solution.

 

Heterogeneous mixture

A heterogeneous mixture is basically the opposite of a homogeneous mixture. It’s a mixture of two or more items that do not produce evenly distributed substances throughout the container, even though the mixture has the same composition throughout. A good example of this is mixed cereal or chocolate chip cookies, in which you can see different items with the naked eye. Sand in water is another example of a heterogeneous mixture. 

 

Classification of mixtures

Based on the particle size of their components or substances, mixtures are further classified as follows

1. Alloys

An alloy is a homogenous mixture of elements that has the characteristics of a metal. At least one of the elements mixed is a metal. For example, steel is an alloy made from a mixture of iron and carbon.

2. Solution

A solution is a mixture where one of the substances dissolves in the other. The substance that is being dissolved is called a solute, while the dissolving medium is called a solvent.  A solution has tiny particle sizes that are less than 1 nanometer in diameter. Components of a solution can’t be separated by centrifuging or decanting the mixture. An example of a solution is saltwater with salt as the solute and water as the solvent.

In chemistry, a solution is actually a type of mixture. A solution is a mixture that is the same or uniform throughout. Think of the example of saltwater – it is a homogenous mixture. A mixture that is not a solution is not uniform throughout, for example, sand in water. It is a heterogeneous mixture.

3. Colloids

A colloid is a mixture where very small particles of one substance are evenly distributed throughout another substance. Particle sizes of colloids are from 1 nanometer to 1 micrometer. They appear very similar to solutions, but the particles are suspended in the solution rather than fully dissolved. The separate substances in a colloid can be isolated by a centrifuge. The difference between a colloid and a suspension is that the particles will not settle to the bottom over a period of time, they will stay suspended or float.

Examples of colloids are hair spray, milk.

- In a hair spray, the liquid is an aerosol that combines with a gas.

- In milk, fat globules are dispersed and suspended in water.

4. Suspension

A suspension is a mixture between a liquid and particles of a solid. In this case, the particles do not dissolve. The particles and the liquid are mixed up so that the particles are dispersed throughout the liquid. They are “suspended” in the liquid. A suspension has larger particles than the above two mixtures. At times, it appears heterogeneous. A key characteristic of a suspension is that the solid particles will settle and separate over time if left alone. Both decantation and centrifugation can separate suspensions.

Examples of suspensions are

• A mixture of water and sand. When mixed up, the sand will disperse throughout the water; if left alone, the sand will settle to the bottom.
• Salad dressing with vinegar and oil. Oil and vinegar separate into layers after some time because vinegar has a higher density compared to oil which will float. 

The table below shows the main properties of the three families of mixtures and examples:

DISPERSION MEDIUM	DISSOLVED OR DISPERSED PHASE	SOLUTION	COLLOID
Gas	Gas	Gas mixture: air (oxygen and other gases in nitrogen)	None
Liquid	None	Liquid: aerosol, fog, mist, vapor, hair, sprays
Solid	None	Solid: aerosol, smoke, ice, cloud, air particulates
Liquid	Gas	Solution: oxygen in water	Liquid: foam, whipped, cream shaving cream
Liquid	Solution: alcoholic beverages	Emulsion: milk
Solid	Solution: sugar in water	Liquid solution: pigmented ink, blood
Solid	Gas	Solution: hydrogen in metals	Solid foam: aerogel, Styrofoam, pumice
	Liquid	Solution: mercury in gold, hexane	Gel: agar, gelatin, silicagel, opal
	Solid	Solution: alloys, plasticizers in plastics	Solid sol: cranberry glass