Types of Mixtures

1. Mixtures and Pure Substances

Mixture: Two or more substances that are physically combined and not chemically joined.

A pure substance, such as an element or compound, contains only one kind of matter and has fixed properties. Pure water, for instance, boils at 100 °C at sea level. In a mixture, each substance keeps its own properties. Sugar in water still tastes sweet, and iron filings in sand are still attracted to a magnet. The amounts can vary too: you can add a little or a lot of sugar to your tea. In a compound, however, the elements are chemically joined in a fixed ratio.

2. Mixtures and Compounds Compared

3. Mixtures in Everyday Life

Air is a mixture of gases: about 78% nitrogen, 21% oxygen and small amounts of others. Sea water contains water and dissolved salts. Soil contains minerals, humus, water and air. Palm soup contains oil, water, tomato and spices. Concrete contains cement, sand, stone and water. Knowing that these are mixtures explains how they can be changed or separated.

4. Physical Changes

Forming a mixture is normally a physical change, since no new substance is produced and it can usually be reversed. If you heat iron and sulfur together, however, they form a new compound called iron(II) sulfide, which is not attracted by a magnet. That difference is the key to telling a mixture from a compound.

1. Two Main Types

2. Identifying the Type

To identify a mixture, look at it closely. Can you see separate parts? Does a sample from the top have the same make-up as one from the bottom? In salt water, every drop is equally salty. In a bowl of groundnut soup with pieces of meat, some spoonfuls contain more meat than others. The word "homogeneous" comes from Greek words for "same kind", and "heterogeneous" from "different kind".

3. Size of Particles

Particle size explains the difference. In homogeneous mixtures, particles are tiny, the size of molecules and ions, so you cannot see them. In heterogeneous mixtures, the particles are large enough to be seen, or they scatter light. Muddy river water is heterogeneous, with visible soil particles. Over time, gravity pulls heavier particles down, so the bottom of the bottle gets thicker with mud.

4. Water Quality in Ghana

Whether water is homogeneous or not does not tell us whether it is safe. Clear water can contain dissolved chemicals or invisible germs. Illegal mining has made many rivers muddy with soil and sometimes polluted with mercury. Water treatment plants must remove such substances. Science helps us to understand the nature of the problem and how to solve it.

1. Three Types

2. Solutions

A solution has a solute dissolved in a solvent. In salt water, salt is the solute and water is the solvent. Water is often called the universal solvent, because so many substances dissolve in it. A solution can be concentrated (with a lot of solute) or dilute (with a little). A saturated solution has dissolved all the solute it can at that temperature.

3. Suspensions and Colloids

A suspension contains large particles that settle out. Medicines labelled "shake well before use" are suspensions, as are pap and muddy water. In a colloid, particles are too small to settle but large enough to scatter a beam of light. This is called the Tyndall effect. Fog, milk, mayonnaise and blood plasma are colloids. Shine a torch through milk and water, and you can see the beam.

4. Kinds of Solutions

Solutions are not all liquids. Air is a gaseous solution. Brass, an alloy of copper and zinc, is a solid solution. Soda is a gas dissolved in a liquid. Alloys like steel and brass are made since they are stronger or more useful than their pure metals. Solids dissolve faster in hot water, when stirred or when crushed, since these increase contact between solute and solvent.