Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
A compound is a pure substance made up of two or more different elements that are chemically bonded together in fixed proportions, such as water () or carbon dioxide ().
A mixture consists of two or more substances (elements or compounds) that are physically combined but not chemically bonded. The components of a mixture retain their individual properties.
In a compound, the properties of the substance are different from the elements it is made of. For example, sodium () is a reactive metal and chlorine () is a toxic gas, but together they form sodium chloride (), which is common table salt.
Mixtures can be separated by physical methods such as filtration, evaporation, distillation, or chromatography, while compounds can only be separated by chemical means (e.g., electrolysis).
Compounds have a specific chemical formula representing the ratio of atoms. For example, methane () always contains one carbon atom for every four hydrogen atoms.
📐Formulae
💡Examples
Problem 1:
Classify the following as a compound or a mixture: (a) Air, (b) Pure distilled water (), (c) A bowl of cereal and milk.
Solution:
(a) Mixture, (b) Compound, (c) Mixture.
Explanation:
Air is a mixture of gases like and that are not chemically bonded. Distilled water is a compound because hydrogen and oxygen are chemically bonded in a ratio. Cereal and milk is a heterogeneous mixture because the components can be easily seen and separated physically.
Problem 2:
Iron filings () and sulfur powder () are mixed in a beaker. A magnet is passed over the beaker. What happens? If the mixture is heated to form iron sulfide (), what happens if a magnet is passed over it then?
Solution:
In the initial mixture, the magnet will attract the iron filings. In the compound , the magnet will have no effect.
Explanation:
Before heating, the substances form a mixture, so the iron retains its magnetic properties. After heating, a chemical reaction occurs to form the compound iron sulfide (), which has entirely different properties from the original elements and is non-magnetic.