2 2 methods of separating and purifying substances

A pure substance consists of only substance (this can be an element or a compound), while a mixture contains two or more different substances that are not chemically combined.

False. A solution of salt dissolved in water is a mixture.

The melting point is the temperature at which a solid turns into a liquid.

The boiling point is the temperature at which a liquid turns into a gas.

Melting point and boiling point can be used to distinguish a pure substance from a mixture.

False. Pure substances have sharp, specific melting and boiling points, while mixtures have a range of melting and boiling points.

A cooling curve is a graph that shows how a substance changes from a gas to a liquid to a solid as temperature decreases.

A horizontal line on a cooling curve indicates where a substance is changing from a gas to a liquid or from a liquid to a solid.

A pure substance is a single element or compound that contains no other substances.

Pure substances have specific and sharp melting and boiling points, while mixtures have a range of melting and boiling points.

Melting point analysis is used to assess the purity of a substance.

Substance A is impure because it melts over a range of temperatures.

Simple distillation is used to separate:

Simple distillation is a technique used to separate a liquid from a solution (e.g., water from a salt solution) by boiling the solution and condensing the vapour.

Fractional distillation is a technique used to separate two or more liquids that have different boiling points from a mixture.

Filtration separates an undissolved solid from a mixture of solid and liquid.

Crystallisation is used to separate a dissolved solid from a solution.

It is important that the solid (solute) is much more soluble in hot solvent than in cold.

Paper chromatography depends upon the solubility of different substances.

Paper chromatography is used to separate substances that have different solubilities in a given solvent, such as identifying the components of a mixture.

The mobile phase in chromatography is the solvent / liquid that travels up the paper.

The baseline (starting line) should be drawn in pencil because it will not contaminate the results / will not run.

False. In paper chromatography, a pure substance only produces one spot.

The technique that separates solutions with more than one type of dissolved solid is chromatography.

A single spot on a chromatogram indicates that the substance is pure.

The equation for the retention factor (Rf value) is: Rf =

An Rf value close to 1 indicates that the component is very / highly soluble in the solvent used.

False. Multiple spots on a chromatogram indicate that the substance is a mixture.

False. The Rf value of a compound depends on the solvent used. If the solvent is changed, the Rf value will change.

The purpose of calculating Rf values is to allow chemists to identify unknown substances.

The Rf values ranges between 0 and 1.

The initial line on a chromatogram should be drawn with a pencil.

False. The solvent level should not start above the pencil line, as this would ruin the chromatogram.

There are 3 substances in D.

True. If two substances are the same, they will produce identical chromatograms.

An Rf value does not have units because it is a ratio.

The mobile phase is the water and the stationary phase is the chromatography paper.

The black food colouring contains A, E and an unknown.

The first step in the purification of water is filtration to remove large particles.

Desalination is the process of removing salt from seawater to make it suitable for drinking and irrigation.

False. Distillation is not used in water purification.

Sedimentation allows heavy particles to settle at the bottom of the water treatment tanks.

Chlorination is the process of adding chlorine to water to disinfect it by killing bacteria and other pathogens.

False. Filtration removes large particles but not dissolved impurities.

Reverse osmosis is a water purification process that uses a semi-permeable membrane to remove ions, molecules, and larger particles.

Deionised water is used as the ions could react with the substances under analysis and would give the experiment a false result.

The process is expensive as a lot of energy is required to heat the large volumes of water to 100 ºC.

Potable water is water that is clean enough for human consumption.