PURE AND IMPURE SUBSTANCES

PURE SUBSTANCES – These are substances that consist of a single particle (atoms or molecules), with uniform structure and definite composition. They are usually either elements or compounds.

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CHARACTERISTICS OF PURE SUBSTANCES

• Homogeneous in nature.
• Uniform/definite composition.
• Consistent properties.
• Exhibits chemical stability.
• Cannot undergo physical separation.

IMPURE SUBSTANCES – Also known as mixtures. These are substances which contains two or more constituents (different kinds of particles) that are not chemically bounded.

CHARACTERISTICS OF IMPURE SUBSTANCES

• Heterogeneous in nature.
• Indefinite composition.
• Undergoes physical separation of constituents.

1. CRITERIA FOR PURITY

1. BOILING & MELTING POINT – Impurities raises the boiling point and lowers the melting point. (NB: This is fixed at a certain temperature for pure substances).
2. CHROMATOGRAPHY – A pure substance gives one spot on the paper chromatograph.

1. ELEMENTS, COMPOUNDS AND MIXTURES

ELEMENTS – These are substances which cannot be split into simpler units by an ordinary chemical process. There are six types of elements grouped in form of a periodic table: the reactive metals, transition metals, lanthanides and actinide, pure metals, non­­-metals, and noble gas. They are either naturally occurring or made in the laboratory.

COMPOUNDS – These are substances which contains two or more elements chemical combined. Examples include water, H₂O, limestone, CaCO_3, SiO₂ etc.

MIXTURES – These are substances that contains two or more constituents which can easily be separated by a physical method. Examples include air, sea water, urine, blood, crude oil, soil, milk, palm wine etc.

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1. COMPARISON OF MIXTURES AND COMPOUNDS

1. CHANGES IN MATTER

PHYSICAL CHANGE – One which is easily reversed, and which no new substances are formed. There is no change in mass and requires little amount of heat for changes to take effect. Examples:

• Dissolution of solutions.
• Change in the state of matter.
• Separation of mixture.
• Magnetization and demagnetization of substances.

CHEMICAL CHANGE – One which cannot be easily reversed, and new substances are formed. There is a change in mass and requires a great amount of heat for changes to occur. Examples:

• Burning of materials.
• Rusting of iron.
• Slaking of lime.
• Changes in electrochemical series.
• Fermentation and decay of substances.

1. SEPARATION TECHNIQUES

Separation techniques involves a system which makes use of the physical properties of the constituents of a mixture for separation. They include:

• SEIVING – Used to separate solid particles of different sizes.
• MAGNETIC SEPARATION – Used to separate magnetic substances from non-magnetic ones.
• SUBLIMATION – Used to separate solids that can sublime from other solids.
• DECANTATION – Used to separate mixtures containing solid and liquid particles with two (or more) distinct layers.
• FILTRATION – Used to separate solid particles from liquids using a filter paper.
• CENTRIFUGATION – Using a centrifuge (a machine that spins test-tubes containing suspensions at high speed) to separate suspensions into heavy solids at the bottom, and clear liquid at the upper layer, which can easily be decanted.
• CRYSTALLIZATION – Used to separate salt which decomposes easily on heating from their solutions. (NB: used where purity is important e.g drug and sugar industries).
• FRACTIONAL CRYSTALLIZATION – Used to separate two or more solid solutes present in the same solution, but with different solubilities at different temperatures.
• PRECIPITATION – Mixtures are separated using the difference in the solubility of a solid in two miscible liquids.
• DISTILLATION – Used to recover solvents from its solution.
• FRACTIONAL DISTILLATION – Involves the use of a fractionating column to separate a mixture of two or more miscible liquids based on their boiling point.
• SEPARATING FUNNEL METHOD – Used to separate two immiscible liquids e.g kerosene and water, petrol and water etc.
• CHROMATOGRAPHY – A technique used to separate and analyze mixtures based on their chemical properties. It is carried out by moving a solvent over a porous absorbent medium (e.g paper, gel).

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1. APPLICATION OF SEPARATION TECHNIQUES

• Sieving in mining industries.
• Filtration in water purification plants.
• Evaporation in salt making industries.
• Distillation in gin and water distilleries.
• Crystallization in drugs and sugar manufacturing.
• Fractional distillation in petroleum industries.
• Chromatography in laboratories etc.