Is Matter Around Us Pure?

The One Thing

Matter may be a pure substance containing one kind of particle or a mixture containing two or more substances. Mixtures retain the properties of their components and can be separated by physical methods selected according to differences in properties such as solubility, particle size, density and boiling point.

Reactions, Processes and Experiments

Key Terms

• Pure substance: Matter made of only one kind of particles and having a fixed composition and definite properties.
• Element: A pure substance made of only one kind of atom that cannot be broken into simpler substances by chemical methods.
• Compound: A pure substance formed when two or more elements combine chemically in a fixed ratio and show new properties.
• Mixture: A combination of two or more substances in any proportion, where each substance generally retains its own properties.
• Homogeneous mixture: A mixture with uniform composition and properties throughout, such as salt solution.
• Heterogeneous mixture: A mixture with non-uniform composition, such as sand mixed with water.
• Solution: A homogeneous mixture in which a solute dissolves in a solvent.
• Solute: The substance that is dissolved in a solution.
• Solvent: The substance that dissolves the solute and is usually present in the larger amount.
• Concentration: The amount of solute present in a given amount of solution or solvent.
• Saturated solution: A solution that contains the maximum amount of solute that can dissolve at a particular temperature.
• Unsaturated solution: A solution that can dissolve more solute at a given temperature.
• Solubility: The maximum amount of solute that dissolves in a given amount of solvent at a specified temperature.
• Suspension: A heterogeneous mixture containing large, insoluble particles that settle down when left undisturbed.
• Colloid: A mixture in which very small particles are dispersed through another substance and remain suspended for a long time.
• Dispersed phase: The particles distributed throughout a colloid.
• Dispersion medium: The substance in which the colloidal particles are distributed.
• Tyndall effect: The scattering of light by colloidal particles, making the path of a light beam visible.
• Filtration: A method used to separate an insoluble solid from a liquid using a filter.
• Evaporation: A method used to recover a dissolved solid by vaporising the solvent.
• Centrifugation: A method that separates suspended particles by rapidly rotating a mixture.
• Separation funnel: An apparatus used to separate immiscible liquids with different densities.
• Sublimation: The direct change of a solid into vapour, used to separate sublimable solids from non-sublimable substances.
• Chromatography: A technique that separates components based on their different movement through a stationary medium.
• Distillation: A method that separates a solvent from a solution by evaporation followed by condensation.
• Fractional distillation: A method used to separate miscible liquids having close boiling points.
• Crystallisation: A method of obtaining pure solid crystals from a solution by controlled cooling or evaporation.
• Mass percentage of a solution: $(\text{Mass of solute}/\text{Mass of solution})\times 100$.
• Volume percentage of a solution: $(\text{Volume of solute}/\text{Volume of solution})\times 100$.
• Mass of solution: Mass of solute + Mass of solvent.
• Immiscible liquids: Liquids that do not mix uniformly.
• Miscible liquids: Liquids that mix completely.
• Concentrated solution: A solution containing relatively more solute.
• Dilute solution: A solution containing relatively less solute.

Easily Confused

• Pure substance and mixture: A pure substance contains one kind of particle and has fixed composition; a mixture contains two or more substances and generally has variable composition.
• Element and compound: An element contains one kind of atom and cannot be chemically broken down; a compound contains two or more elements chemically combined in a fixed ratio.
• Compound and mixture: A compound has new properties produced by chemical combination; a mixture retains the properties of its components and is formed by physical mixing.
• Homogeneous and heterogeneous mixture: A homogeneous mixture has uniform composition throughout; a heterogeneous mixture has non-uniform composition.
• Solution and suspension: Solution particles are extremely small, do not settle and cannot be separated by ordinary filtration; suspension particles are larger, settle on standing and can be separated by filtration.
• Solution and colloid: Solution particles are generally less than 1 nanometre and do not scatter light; colloidal particles are approximately 1 to 1000 nanometres and show the Tyndall effect.
• Suspension and colloid: Suspension particles are large and settle on standing; colloidal particles remain dispersed for a long time and generally do not settle.
• Saturated and unsaturated solution: A saturated solution cannot dissolve more solute at a particular temperature; an unsaturated solution can dissolve more.
• Miscible and immiscible liquids: Miscible liquids mix completely; immiscible liquids form separate layers.
• Evaporation and distillation: Evaporation recovers the dissolved solid by removing the solvent; distillation also condenses and collects the solvent.
• Distillation and fractional distillation: Distillation separates a solvent from a solution; fractional distillation separates miscible liquids with close boiling points.
• Physical separation and chemical combination: Physical separation produces no new substance; chemical combination in a compound produces a substance with new properties.

What Gets Asked

• Classifying examples as solutions, suspensions or colloids: Use particle size, visibility, settling, filtration and the Tyndall effect. A common error is assigning milk to a true solution or treating a colloid as a suspension because both may appear uniform.
• Calculating solution composition: Apply the stated formulae for mass percentage and volume percentage, using mass of solution = mass of solute + mass of solvent. A frequent mark-losing error is using the mass of solvent instead of the mass of solution in the denominator.
• Selecting a separation method: Match the method to the relevant difference in physical properties, such as solubility, particle size, density, boiling point, magnetism or ability to sublime. No single method applies to every mixture.
• Distinguishing compounds from mixtures: State that compounds form by chemical combination in fixed ratios and have new properties, whereas mixtures form by physical mixing in variable ratios and retain component properties.
• Explaining the Tyndall effect: Identify scattering of light by colloidal particles and the resulting visible path of the beam. The effect should not be attributed to true solution particles.
• Identifying suitable separation apparatus or conditions: For immiscible liquids, use a separation funnel; for miscible liquids with close boiling points, use fractional distillation; for a dissolved solid, use evaporation or crystallisation as appropriate.