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Chemistry · Chapter 1: Introduction to Chemistry
Elements, Compounds and Mixtures
Elements, compounds and mixtures explained with particle diagrams, symbols of elements, and the differences between a compound and a mixture.
Key termRememberExampleWatch out
01Concept
Elements
An element is a pure substance made of only one kind of atom. It cannot be broken down into simpler substances by chemical methods. Iron (Fe), oxygen (O), carbon (C), gold (Au).
- 118 elements are known today. About 90 of them occur naturally; the rest have been made in laboratories.
- Each element has a symbol of one or two letters. The first letter is always a capital: H for hydrogen, Na for sodium (from Latin natrium), Fe for iron (from Latin ferrum).
- Elements are grouped as metals (iron, copper), non-metals (oxygen, sulphur) and metalloids, which have some properties of both (silicon, boron).
Compounds
A compound is a pure substance formed when two or more elements combine chemically in a fixed ratio by mass.
- The ratio is always the same: in water, hydrogen and oxygen always combine in the ratio 1 : 8 by mass.
- A compound's properties are completely different from its elements: sodium is a soft metal that reacts violently with water, chlorine is a poisonous green gas, but together they make sodium chloride, the common salt we eat.
- A compound can be separated into its elements only by chemical methods, such as electrolysis.
Mixtures
A mixture is two or more substances mixed together but not chemically combined. Air, sea water, soil, a fruit chaat.
- The parts can be mixed in any proportion.
- Each part keeps its own properties: in a mixture of iron filings and sulphur, a magnet still picks out the iron.
- The parts can be separated by physical methods such as filtration, evaporation, distillation or using a magnet.
Compound or mixture?
| Compound | Mixture |
|---|---|
| Elements are chemically combined | Substances are only mixed |
| Fixed ratio by mass | Any ratio |
| Properties are different from its elements | Each part keeps its own properties |
| Separated only by chemical methods | Separated by physical methods |
| Energy is usually given out or taken in when it forms | Little or no energy change on mixing |
| Fixed melting and boiling points | Melts or boils over a range |
| Water, salt, carbon dioxide | Air, soil, salt water |
02Real World Examples
Salt on your food
Sodium chloride is safe to eat, though sodium and chlorine on their own are dangerous: proof that a compound has new properties.
Air we breathe
Air is a mixture: about 78% nitrogen, 21% oxygen and small amounts of argon, carbon dioxide and water vapour.
Gold jewellery
Pure gold (an element) is soft. Jewellers mix it with copper or silver to make it harder: a mixture called an alloy.
Separating scrap
Scrap dealers use magnets to pull iron out of mixed metal waste: a physical method.
03Practice Questions
Try to answer in your own words first, then open the model answer.
Short questions
Q1.Define element and compound with two examples each.Show answer
Answer
- Element: a pure substance made of one kind of atom. Iron, oxygen.
- Compound: a pure substance of two or more elements chemically combined in a fixed ratio. Water, sodium chloride.
Q2.Why does sodium chloride not behave like sodium or chlorine?Show answer
Answer
Because a compound has properties completely different from its elements. When sodium and chlorine combine chemically, a new substance forms with new properties.
Q3.Write the symbols of: sodium, potassium, iron, copper.Show answer
Answer
- Sodium: Na (Latin natrium)
- Potassium: K (Latin kalium)
- Iron: Fe (Latin ferrum)
- Copper: Cu (Latin cuprum)
Q4.What are metalloids? Give two examples.Show answer
Answer
Metalloids are elements that have some properties of metals and some of non-metals. Silicon and boron.
Long questions
Q5.Differentiate between a compound and a mixture. Explain with the example of iron and sulphur.Show answer
Answer
| Compound | Mixture |
|---|---|
| Elements chemically combined | Substances only mixed |
| Fixed ratio by mass | Any ratio |
| New properties | Parts keep their properties |
| Separated by chemical methods | Separated by physical methods |
| Energy change on forming | Little or no energy change |
Example: iron filings and sulphur powder stirred together make a mixture; a magnet pulls out the iron. When the mixture is heated, they react to form iron sulphide (FeS), a compound; a magnet can no longer pull out the iron.