Mixtures
A mixture is made when two or more substances are combined, but they are not combined chemically.
Characteristics of Mixtures
The constituents of a mixture are not present in a fixed ratio. The various characteristics of mixtures are discussed below.
The components of a mixture can be easily separated
The components each keep their original properties
The proportion of the components is variable
There is no chemical force acting between the two or more substances that are mixed, but they still exist together.
They can either be heterogeneous or homogeneous in nature.
The proportions of the substances vary in an indefinite manner.
The properties of the mixture depending upon the individual components.
The constituents of the mixture can be separated by physical methods.
Boiling point and the melting point of the mixture depends upon the characteristic of the constituents.
During the formation of a mixture, there is no change in energy.
All the states of matter (solid, liquid, gases) can combine to form mixtures.
It can be concluded that almost everything in our vicinity is nothing but a mixture.
For example, the food we eat is a mixture of ingredients, the atmospheric air we breathe is a combination of gases and the fuel we use in locomotives is a heterogeneous mixture.
Types of Mixtures
There are two main categories of mixtures: homogeneous mixtures and heterogeneous mixtures. In a homogenous mixture all the substances are evenly distributed throughout the mixture (salt water, air, blood). In a heterogeneous mixture the substances are not evenly distributed (chocolate chip cookies, pizza, rocks)
Within the categories of homogeneous and heterogeneous mixtures there are more specific types of mixtures including solutions, alloys, suspensions, and colloids.
Solutions (homogeneous)
A solution is a mixture where one of the substances dissolves in the other. The substance that dissolves is called the solute. The substance that does not dissolve is called the solvent.
An example of a solution is salt water. These components can be easily separated through evaporation and they each retain their original properties. However, the salt is dissolved into the water to where you can't see it and it is evenly distributed in the water. In this example the water is the solvent and the salt is the solute.
Alloys (homogeneous)
An alloy is a mixture of elements that has the characteristic of a metal. At least one of the elements mixed is a metal. One example of an alloy is steel which is made from a mixture of iron and carbon.
Suspensions (heterogeneous)
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 key characteristic of a suspension is that the solid particles will settle and separate over time if left alone.
An example of a suspension is 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.
Colloids (heterogeneous)
A colloid is a mixture where very small particles of one substance are evenly distributed throughout another substance. They appear very similar to solutions, but the particles are suspended in the solution rather than fully dissolved. 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.
An example of a colloid is milk. Milk is a mixture of liquid butterfat globules dispersed and suspended in water.
Colloids are generally considered heterogeneous mixtures, but have some qualities of homogeneous mixtures as well.
Methods for Separating Mixtures
Mixtures can be separated using a variety of techniques, we can use the differences in the physical properties of the components of the mixture to separate the components from each other.
Hand sorting is a suitable separation method for a mixture that contains a relatively small number of large items.
For Example sorting out a variety of colored beads.
Sieving is a suitable separation method when the pieces to be separated are sized differently. Sieving is a simple technique for separating particles of different sizes. A sieve such as used for sifting flour has very small holes.
Filtration is a good method for separating an undissolved solid from a liquid.
When the particles in a mixture are too small to be caught by a sieve and when the components of the mixture are in different states, we can separate them by filtration using a filter.
Components with different magnetic properties can be separated using magnetic separation.
The following diagram shows how magnetic separation can be used to separate a mixture of components. In the example, mineral ore that contains two compounds (one magnetic, and the other non-magnetic) is being separated. The ore grains are fed onto a revolving belt. The roller on the end of the belt is magnetic. This means that all the magnetic grains in the ore will stick to the belt when it goes around the roller, while the non-magnetic grains will fall off the end. As soon as the magnetic grains move past the magnetic roller, they will also fall down.
Evaporation is a suitable separation method for removing a liquid from a solid.
Do you know where most of the salt that we use in South Africa comes from? South Africa gets it salt from inland salt pans, coastal salt pans and seawater. A salt pan is a shallow dam in the ground where salt water evaporates to leave a layer of dry salt.
Distillation is a suitable method for separating two liquids with different boiling points.
Distillation is the separation of one substance from another by evaporation followed by condensation. The apparatus used in this technique is called a still.
Here in Trinidad, we have the Angostura Rum Distilleries.
In the oil and gas industry, Crude oil is separated into different components using distillation. The components are evaporated, starting with lighter fuel (which has the lowest boiling point), then jet fuel, then petroleum, then motor car oil, until only tar is left. We call the separated components fractions, and the process, fractional distillation.
Chromatography is a good method for separating coloured pigments from each other. Most inks are a mixture of different pigments, blended to give them just the right colour. A pigment is a chemical that gives colour to materials. When a mixture contains colourful compounds, it is often possible to separate the different components using a separating method called chromatography.
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