Y11 Term 1 Investigation - Separation Techniques

Mixture

Substances made from two or more pure substances. Can be separated. Not chemically combined.

Pure Substance

a substance made up of only one type of particle and have a fixed structure. Has distinct measurable properties that can be used to identify it.

Properties of a mixture

• The components can be separated physically

• The components retain their properties

• The ratio of elements and compounds are not fixed

• depend on both identity and relative amounts that make up the mixture.

Homogeneous mixture

a mixture that is uniform in composition and properties; components are evenly distributed. E.g dishwashing liquid, air

Heterogeneous mixture

A mixture that is not uniform in composition or properties; components are not evenly distributed throughout the mixture. E.g sand, water, oil

Physical Properties

the characteristics of a substance that can be observed or measured without changing the chemical composition of a substance. E.g melting and boiling points, density

Physical Change

Changes in the physical properties, such as density or state of matter. There is no change in the chemical composition of the substance. E.g freezing water, tearing paper, dissolving salts.

Chemical Properties

relate to a substance's ability to participate in chemical reactions to form new substances. To determine chemical properties, the chemical composition of the original substance is changed. e.g toxicity, acidity

Chemical Change

Change in which at least one new substance is formed.

Ways to tell a chemical change has occurred

- a solid is formed (called a precipitate).
- a gas is produced
- there is a colour change. E.g iron exposed to air or water changes to a reddish brown
- there is a significant change in temperature, energy is released or absorbed.
- an insoluble solid disappears. E.g calcium carbonate in acid.

Decanting

Pouring liquid off the top when sediment has settled to the bottom of the container. A method of separating immiscible liquids by pouring the top layer into another container. Also used to separate liquid from undissolved solids quickly.

Filtration

Separates materials based on the size of their particles. Most often used to separate liquid or a solution from an insoluble solid. E.g separating sand and salt (NaCl). salt is soluble in water, but sand is insoluble. sand can then be filtered off from the solution using filter paper.

Evaporation

Recovery of a dissolved solid from a solution. E.g dissolved solid such as salt in sea water cannot be separated by filtration. is possible because salt wont evaporate forst becauase it has a much higher boiling point then water (the liquid)

Crystallisation

Formation of crystals as a dissolved substance solidifies. Depends on the components of a mixture having different solubilities in a selected solution. E.g a mixture of salt and baking soda can be separated by dissolving in hot water and then cooling the resultant solution. Both are soluble in hot water, the baking soda is less soluble in cold water and will crystalise from cool solution. The crystals can then be filtered off, leaving the salt.

Recrystallisation

A process of repeated crystallisation in order to purify a solid or to obtain more satisfactory crystals of a solid that is already pure. Used to purify impure substances. e.g sugar refining. Impure substance is dissolved in a minimum amount of solvent at high temperature. Hot mixture is filtered to remove insoluble impurities. Filtrate is cooled, causing crystals of the pure substance to form. Crystals can be filtered, leaving any impurities in the filtrate.

Distillation

Used to purify liquids. Most effective when components have very different boiling points. Mixture is heated, component with lowest boiling point boils off first, vapour is cooled in a condenser and collected as a pure liquid, components with high boiling points remain in distillation flask.

Simple Steps of Distillation

In the distillation flask, water boils and enters the condenser. In the condenser, water vapour condenses. Pure water is collected as a distillate.

Solvent Extraction

Used to separate substances because of differing solubilities in two immiscible liquids. E.g salt and iodine, petroleum refinery. dissolve mixture in water and places in separating funnel. Non-polar solvent is added (e.g hexane) and the mixture shaken. Mixture is allowed to settle, salt will dissolve in aqueous layer, iodine in upper hexane layer. Two liquids can be separated by running the lower layer through the tap at the bottom of the separating funnel.

Polar Substance

A substance whose molecules have electric charges that are not equally distributed. An example is water.

Non-polar substance

a substance that has molecules whose electric charges are equally distributed

Fractional Distillation

- separation of two or more liquids from a mixture
- a fractionating column is added to the distilling flask. Glass beads cause the higher boiling point liquid to condense. Is possible due to difference in boiling points of liquids. e.g alcohol from water.

Uses of fractional distillation

- Used in the petroleum industry to separate components of crude oil: gaseous oil is allowed to pass through the column, the temperature gradually increasing along its length and components of different boiling points condense into pure oils.

Chromatography

Used to separate components in small quantities of mixtures e.g food additives and dyes.
- chromatography types - High Performance Liquid (HPLC) and thin layer (TLC) and paper chromatography and gas (GC).
- mixture is passed over an inert substance such as alumina, silica or chromatography paper.
- the separation of components in the mixture occurs because the components adhere, or cling to the surface of the inert substance with different strengths

Compound

a substance formed by two or more different elements that are chemically combined in a fixed whole number ratio