Mixtures and Separation

• Matter can be classified into two main groups: pure substances and mixtures.
• Pure substances have the following general characteristics.

  • They have a fixed, constant composition.
  • Their properties are fixed and constant.
  • The component parts of a pure substance cannot be separated by physical means.
  • Have a fixed boiling and melting point.
  • Have a fixed density.
• Mixtures have the following general characteristics.

  . They have a variable composition.

  · Their properties are variable since their components retain their own,

  individual properties.

  . The component parts of mixtures can be separated by physical means.
• Pure substances can be further classified into elements and compounds.
• Mixtures can be further classified into homogeneous mixtures and

  heterogeneous mixtures.
• To test if a substance is pure or not the melting/ boiling point is used. An impure substance has a lower or increase the melting/boiling point compared to it's pure state.
• An element is the simplest form of matter, and it can't be broken down into anything simpler by chemical or physical means.
• The smallest particle in

  an element that has the same properties as the element is an atom.
• Each element is composed of only one kind of atom.
• Compounds are composed of more than one kind of atom. They are combined together chemically, they are always present in the same

  proportions by mass and they cannot be separated by physical means.
• A compound can be represented by a chemical formula, which indicates the elements that the compound is made up of and the ratio in which they have combined.
• The properties of a compound are fixed and are different from the properties of the individual elements that form the compound. For example, hydrogen and oxygen are both gases at room temperature, however, water is a liquid.
• Mixtures are composed of two or more substances which are not chemically combined, therefore their components can be separated by physical means.
• Some of the physical methods for separating mixtures are filtration, evaporation, crystallisation, distillation, fractional distillation and chromatography.
• A homogeneous mixture is one in which the properties and composition

  are uniform throughout the mixture.
• Component of a homogeneous mixture can’t be distinguished from each other. A solution is a homogeneous mixture.
• Examples of homogeneous mixtures are air, salt dissolved in water and metal alloys such as brass, a mixture of copper and zinc.
• A heterogeneous mixture is a non-uniform mixture, for example, a mixture in which the component parts are in different states.
• Parts of a heterogeneous mixture can be distinguished from each other, although not always with the naked eye.
• An example of a heterogenous mixture is sand and water or oil and vinegar.
• . If there is only one kind of atom, then it is an element.

  . If there are two or more kinds of atoms joined together in the same ratio then it is a compound.

  . If there is a combination of two or more elements and/or compounds,

  then it is a mixture.
• A solution is a homogeneous mixture.
• The major component of a solution is known as the solvent and the minor component is known as the solute.
• The solute and solvent can be gases, liquids or solids.
• When a gas or a solid dissolves in a liquid, the liquid is always the solvent, e.g. in a mixture of salt in water, salt is the solute and water is the solvent.
• A suspension is a heterogeneous mixture containing minute particles which are visible to the naked eye.
• If left undisturbed, the particles in a suspension eventually settle.
• The components of a suspension can be separated by filtration.
• Examples of suspensions

  . Dust in air is a suspension of a solid

  in a gas.

  . Powdered chalk in water is an example of a suspension of a solid in a liquid.

  . Muddy water is another example of a solid suspended in a liquid.

   
• A colloid is a heterogeneous mixture containing particles that are

  intermediate in size between those of a solution and those of a suspension.
• The particles in a colloid cannot be seen even with a microscope, and if left undisturbed they do not settle.
• Colloids have properties intermediate between solutions and suspensions.
• Examples of colloids

  . Smoke in air is a colloid of a solid in a gas, also known as a solid aerosol.

  . Fog and aerosol sprays in air are colloids of a liquid dispersed in a gas,

  also known as liquid aerosols.

  . Milk and mayonnaise are colloids of a liquid dispersed in a liquid, also

  known as emulsions.

  . Gelatine and jelly are colloids of a solid dispersed in a liquid, also known as gels.
• The solubility of a solute is an indication of how much of the solute can

  dissolve in a fixed mass of solvent at a particular temperature.
• When no more solute can be dissolved in the solvent, the solution reaches saturation point and we say the solution is saturated. If

  any more solute is added to the solvent, the solute will remain in solid form and will be mixed with the saturated solution.
•  The solubility of a solute in water is usually measured as the mass of solute that will saturate 100g of water.
• Each solute and solvent combination has a specific solubility at a

  given temperature.
• For most solid solutes in water, solubility increases as temperature increases.
• As the temperature increases, a greater mass of solute will saturate a fixed mass of water.
• The unit for solubility is g per 100 g water.