What's the matter?

Chemistry is the study of the properties and the composition of matter. This has historically been difficult because observations could not be directly made due to the extremely small size of matter. As early as 400B.C. Democritus, a Greek philosopher, believed that a substance (e.g., piece of chalk) could be continuously cut into smaller and smaller pieces until you reached the smallest possible component of the substance. He called these invisible particles atomos, meaning indivisible.

Over the centuries, scientists observed the behaviour of particles in water and smoke and they formulated the Particle Theory of Matter which states that:

• Matter is made of very small particles.
• Particles are in constant motion.
• Particles attract each other.
• Particles move faster at higher temperatures and slower at lower temperatures.
• Particles of each pure substance are identical to one another and different from the particles of all other pure substances.

Classification of matter

The material that makes up your computer, mp3 player, and the air you breathe is all made up of matter. Matter exists in one of three states: solid, liquid, or gas. All matter could be classified into two different groups: mixtures or pure substances.

Pure substances

Pure substances contain only one type of particle: Elements are composed of only type of atom. For example sodium (Na), carbon (C), sulphur (S), and helium (He). All of the elements are found in the Periodic Table (discussed in Activity 5). Compounds are composed of one type of molecule with two or more atoms from different elements, e.g., sodium chloride (NaCl), also known as table salt; molecules are composed of sodium and chlorine atoms bonded together.

Mixtures

Mixtures contain two or more kinds of particles. There are two general classes:

• Homogeneous mixtures, or solutions, are mixtures with only one visible phase; that is you can only see one component. For example, when you make cherry Kool-Aid, the crystals are dissolved into the water and the solution appears as a single red liquid. A solution has two components – a solute, which is the substance that is being dissolved and the solvent, which is the substance that is dissolving the solute. In the Kool-Aid example, the crystals are the solute and water is the solvent.
• Heterogeneous mixtures have more than one visible phase. Heterogeneous mixtures may be easily identifiable, as with granite rock; however in some instances, the initial observation and identification of a suspension, colloid, or emulsion may be that of a solution.

  In a suspension the small particles are barely visible with the eye, and if left undisturbed the suspended particles will eventually settle out. Many of the products in your fridge that instruct you to shake well before use are suspensions; the shaking causes the particles to become suspended. For example if tomato juice is left for a long period of time, it will develop two layers as the tomato settles to the bottom.
  Colloids contain suspended particles that are so small they do not settle out. For example, in fog the water droplets are not small enough to be dissolved by the air, and they are not large enough to fall as rain droplets.
  Emulsions are a type of colloid in which tiny liquid droplets are mixed into another liquid. Milk is an example of an emulsion in which drops of cream are mixed with water. An emulsifying agent could be used to prevent separation. For example, an egg yolk is used in a mixture with oil and vinegar to produce mayonnaise; a chemical in the egg yolk keeps the oil and vinegar from separating.
  In order to tell the difference between a solution and a colloid, you could shine a light through the solution; if the light is scattered it is a colloid. This is called the Tyndall Effect.