Atoms, Elements and Compounds
Cambridge O Level Chemistry 5070 — 2.1 Elements, Compounds and Mixtures — Cambridge O Level Chemistry 5070 Revision Notes & Topical Questions
5 min read 5 parts Lesson 1 of 7
Lesson summary
2.1 Elements, Compounds and Mixtures — Cambridge O Level Chemistry: classify substances by their particles, composition and methods of separation. An element is a pure substance whose atoms all have the same number of protons.
- Particle test
- Exam method
Section 1 of 5Elements
Elements
An element is a pure substance whose atoms all have the same number of protons.
- It cannot be broken down into simpler substances by chemical reactions.
- The periodic table lists the known elements.
- An element may consist of single atoms, such as helium, or molecules made from one kind of atom, such as oxygen gas, O2.
Particle test
If every particle in a sample contains only one type of atom, the substance is an element. Different isotopes of the same element still have the same proton number, so they remain atoms of that element.
Section 2 of 5Compounds
Compounds
A compound is a pure substance made from two or more different elements chemically combined in a fixed ratio.
- Its properties can be very different from those of its constituent elements.
- Examples are carbon dioxide (CO2), calcium carbonate (CaCO3) and copper(II) sulfate (CuSO4).
A physical separation method cannot split a compound into its elements because the atoms are chemically bonded. A chemical reaction is needed to break those bonds. A compound has a definite chemical formula showing its element ratio.
Section 3 of 5Mixtures
Mixtures
A mixture contains two or more substances that are together but not chemically bonded to one another. Its composition can vary. Each component keeps its own chemical identity, although its physical properties may be harder to observe in the mixture.
Components of a mixture can often be separated by physical methods based on different properties.
- Filtration separates an insoluble solid from a liquid;
- evaporation recovers a dissolved solid;
- a separating funnel can separate immiscible liquids.
- Sand and water, oil and water, and iron filings mixed with sulfur are examples.
Section 4 of 5Compare and classify
Compare and classify
O2 is an element because it contains only oxygen atoms. CO2 is a compound because carbon and oxygen atoms are chemically bonded in a fixed ratio. Air is a mixture because different gases share the same space without bonding into one substance.
Exam method
Ask two questions
- are there different kinds of atom, and are they chemically bonded in a fixed arrangement?
- One kind of atom indicates an element;
- different kinds bonded together indicate a compound;
- separate substances together indicate a mixture.
Section 5 of 5Check your understanding
- 1.
- Define an element using proton number.
- 2.
- Why is water a compound but sea water a mixture?
- 3.
- Which physical method separates sand from water?
- 4.
- Classify iron, iron sulfide and iron mixed with sulfur, explaining each answer.
Last updated: