Elements Compounds and Mixtures
This topic covers the fundamental classification of matter into elements, compounds, and mixtures. Understanding these definitions is crucial for interpreting chemical formulae and describing substances in chemistry.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- A pure substance consists of only one element or one compound.
- Elements are the simplest form of pure substance, made of only one type of atom (e.g., O₂, Fe). They cannot be broken down by chemical reactions.
- Compounds are pure substances made of two or more different types of atoms chemically bonded in a fixed ratio (e.g., H₂O, CO₂). They can be broken down into simpler substances by chemical reactions.
- Mixtures consist of two or more substances (elements or compounds) that are not chemically bonded. Their composition can vary, and they can be separated by physical means.
- Diatomic or polyatomic molecules of a single element (like O₂, S₈) are still classified as elements, not compounds.
- Alloys, like steel or brass, are solid mixtures, not compounds.
Definitions
- Element
- A substance composed of only one type of atom, which cannot be broken down into a simpler substance by chemical means.
- Compound
- A substance formed from two or more different elements chemically bonded together in a fixed, definite proportion.
- Mixture
- A combination of two or more substances (elements or compounds) that are not chemically bonded and can be separated by physical methods.
- Pure Substance
- A substance consisting of a single element or a single compound, with a distinct set of chemical and physical properties.
Worked example
For each of the following, state whether it is an element, a compound, or a mixture: (a) Gaseous nitrogen (N₂) (b) Ethanol (C₂H₅OH) (c) Air (d) Cobalt (Co)
- 1
Analyse (a) Gaseous nitrogen (N₂):
The molecule contains only one type of atom (Nitrogen).
Therefore, it is an element.
- 2
Analyse (b) Ethanol (C₂H₅OH):
The formula shows three different types of atoms (Carbon, Hydrogen, Oxygen) chemically bonded in a fixed ratio.
Therefore, it is a compound.
- 3
Analyse (c) Air:
Air is primarily composed of nitrogen (N₂) and oxygen (O₂), with smaller amounts of other gases like argon and carbon dioxide.
These components are not chemically bonded together.
Therefore, air is a mixture.
- 4
Analyse (d) Cobalt (Co):
The chemical symbol 'Co' represents a single type of atom.
Therefore, it is an element.
Answer: (a) Element, (b) Compound, (c) Mixture, (d) Element
Common mistakes
- ×Confusing elements with similar chemical symbols for compounds. For example, 'Co' is the element Cobalt, while 'CO' is the compound Carbon Monoxide.
- ×Mistaking a molecule of an element, such as O₂ or Cl₂, for a compound. It is still an element because it is made of only one type of atom.
- ×Incorrectly classifying alloys like steel or bronze as compounds. They are mixtures because the constituent metals are not bonded in a fixed chemical ratio.
No-calculator tips
- ✓To quickly classify from a chemical formula: one capital letter (e.g., Na, F₂) indicates an element, while two or more different capital letters (e.g., NaCl, H₂SO₄) indicate a compound.
- ✓For substances described by name, think about their composition. 'Air', 'sea water', or 'salt solution' imply multiple components not chemically bonded, hence they are mixtures.