Matter & Its Properties
Pure Substances and Mixtures
1st Year · 2nd Year · 3rd Year (Junior Cert)
- ✓By the end of this lesson students will be able to define and distinguish between elements, compounds, and mixtures.
- ✓By the end of this lesson students will be able to provide examples of elements, compounds, and mixtures.
- ✓By the end of this lesson students will be able to describe and explain common laboratory techniques used to separate mixtures.
- ✓By the end of this lesson students will be able to identify appropriate separation techniques for different types of mixtures.
Key concepts
An element is a pure substance that cannot be broken down into simpler substances by chemical means. It is made up of only one type of atom. Each element has a unique chemical symbol.
A compound is a pure substance formed when two or more different elements are chemically combined in fixed proportions. The properties of a compound are different from the properties of the elements from which it is formed.
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 and can be separated by physical means. Mixtures can be homogeneous (uniform composition throughout, e.g., salt solution) or heterogeneous (non-uniform composition, e.g., sand and water).
Filtration is a separation technique used to separate an insoluble solid from a liquid. The mixture is passed through a filter medium (like filter paper) which allows the liquid (filtrate) to pass through but retains the solid (residue).
Evaporation is a separation technique used to separate a soluble solid from a liquid solvent. The solution is heated, causing the solvent to turn into a gas and escape, leaving the solid behind. This method only recovers the solid component.
Distillation is a separation technique used to separate a soluble solid from a liquid, or to separate two liquids with different boiling points. The liquid is heated to its boiling point, turning it into a vapour. The vapour is then cooled and condensed back into a liquid (distillate) in a separate container. This method allows for the recovery of both the liquid and, if applicable, the solid.
Chromatography is a separation technique used to separate components of a mixture based on their differential movement through a stationary phase by a mobile phase. In paper chromatography, a solvent (mobile phase) moves up a piece of filter paper (stationary phase), carrying the components of the mixture at different rates, causing them to separate.
Key facts to remember
- 1Elements are the simplest pure substances and cannot be broken down chemically.
- 2Compounds are formed when elements chemically combine in fixed ratios, resulting in new properties.
- 3Mixtures are formed when substances physically combine, retaining their individual properties, and can be separated physically.
- 4Filtration separates insoluble solids from liquids.
- 5Evaporation separates soluble solids from liquids, recovering only the solid.
- 6Distillation separates soluble solids from liquids (recovering both) or miscible liquids with different boiling points.
- 7Chromatography separates components based on their differential movement through a stationary phase by a mobile phase.
- 8The properties of a compound are different from its constituent elements, while the properties of substances in a mixture are retained.
Worked examples
Example 1
Classify the following substances as an element, a compound, or a mixture: Iron (Fe), Water (H₂O), Air, Sodium chloride (NaCl), Oxygen (O₂), Sand and water.
Answer
Iron (Fe): Element Water (H₂O): Compound Air: Mixture Sodium chloride (NaCl): Compound Oxygen (O₂): Element Sand and water: Mixture
Remember that chemical symbols represent elements, and chemical formulas represent compounds.
Example 2
Describe, step-by-step, how you would separate a mixture of sand and salt.
Answer
The mixture of sand and salt can be separated by first dissolving the salt in water, then filtering out the insoluble sand, and finally evaporating the water from the salt solution to recover the pure salt.
This process uses a combination of dissolving, filtration, and evaporation.
Example 3
Explain how paper chromatography can be used to separate the different coloured components of black ink.
Answer
Paper chromatography separates the components of black ink by allowing a solvent to move up a strip of paper. The different coloured dyes in the ink have varying solubilities in the solvent and different affinities for the paper. As the solvent travels up, it carries the dyes with it, but at different rates, causing them to separate into distinct coloured spots along the paper.
The pencil line is used because pencil lead (graphite) is insoluble in most solvents and will not interfere with the separation.
Common mistakes
- ✗Confusing compounds with mixtures; remember compounds involve chemical bonding, mixtures involve physical mixing.
- ✗Believing that elements can be broken down into simpler substances by ordinary chemical reactions.
- ✗Incorrectly choosing a separation technique for a given mixture (e.g., using filtration for a soluble solid).
- ✗Confusing evaporation and distillation; evaporation only recovers the solid, while distillation recovers both the liquid and solid (or separates two liquids).
- ✗Not understanding that the components of a mixture retain their original properties, unlike compounds.
Exam tips
- ★Learn the precise definitions of element, compound, and mixture, and be able to provide clear examples for each.
- ★Understand the underlying principle of each separation technique (e.g., difference in solubility, boiling points, particle size).
- ★Practise identifying the correct separation technique for various types of mixtures.
- ★Be able to describe the steps for common separation techniques and, if required, draw and label simple diagrams for apparatus like filtration or distillation.
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