Matter and Its Classification
Every substance you will ever meet sits somewhere on one simple family tree.
On a ward in Enugu, a nurse hangs a clear bag labelled 0.9% Sodium Chloride. It looks exactly like water. Next to it on the trolley sits a packet of glucose powder and a small cylinder of oxygen. One of these is an element, one is a compound, and one is a mixture. Could you say which is which, and why? By the end of this lesson you will, and you will never look at a drip bag the same way again.
Definition of Matter
Chemistry is the study of matter and the changes it undergoes. Matter is anything that has mass and occupies space (has volume). Your bones, the air in your lungs, the harmattan dust on your windscreen: all matter. Light, heat and sound are not matter; they are forms of energy.
Chemists sort matter by asking one question first: is it one substance, or several substances mixed together?
Elements, Compounds and Mixtures
An element is a substance that cannot be split into simpler substances by chemical means; all its atoms have the same atomic number. Oxygen in that cylinder is an element.
A compound is a substance made of two or more elements chemically combined in a fixed ratio by mass. Glucose, , is always 6 carbon : 12 hydrogen : 6 oxygen atoms, whether it came from a Kano sugar factory or your liver. A compound's properties are new: sodium is a soft metal that fizzes in water, chlorine is a choking green gas, yet sodium chloride is the salt on your rice.
A mixture contains two or more substances that are not chemically combined, in no fixed proportion. The saline drip is a mixture: salt dissolved in water. A homogeneous mixture (a solution) looks the same everywhere; a heterogeneous one does not. Blood looks uniform, but spin it in a centrifuge and red cells settle beneath the yellow plasma: heterogeneous.
| Feature | Compound | Mixture |
|---|---|---|
| Composition | Fixed ratio by mass | Any proportion |
| Properties | New, unlike its elements | Keeps properties of its parts |
| Separation | Only by chemical reactions | By physical methods (filtration, distillation, chromatography) |
| Energy on forming | Usually given out or taken in | Little or no energy change |
| Example | Water, | Air; saline; palm wine |
Physical and Chemical Changes
A physical change produces no new substance and is usually easily reversed: ice melting, sugar dissolving, water boiling. A chemical change produces one or more new substances and is usually not easily reversed: an iron roof rusting in Port Harcourt's damp air, firewood burning, food being digested. Clues to a chemical change are a gas given off, a colour change, a precipitate, or heat and light released.
Pause & think
Back to the trolley. Is the 0.9% sodium chloride drip an element, a compound or a mixture? And the sodium chloride inside it?
Quick check
Did it land?
Spotted a mistake?