Chemistry: Changes to materials, mixtures and separation

In chemistry, all substances can be classified into three main groups: elements, compounds, and mixtures. An element is the simplest form of a substance, made up of only one type of atom (e.g., Oxygen, O; Iron, Fe). A compound is formed when two or more different elements are chemically bonded together in a fixed ratio. These chemical bonds create a new substance with properties that are completely different from its constituent elements (e.g., water, H₂O, is a liquid formed from two gases). A mixture consists of two or more substances (elements or compounds) that are physically mixed but not chemically bonded. Because there are no chemical bonds, the components of a mixture keep their own individual properties and can be separated by physical means like filtration or evaporation. The composition of a mixture can vary, unlike the fixed composition of a compound.

An alloy is a mixture of elements, where at least one element is a metal. Alloys are created to enhance the properties of pure metals, often making them harder, stronger, or more resistant to corrosion. In a pure metal, the atoms are all the same size and are arranged in regular, orderly layers. These layers can easily slide over one another, which makes pure metals relatively soft and malleable. When other elements with different-sized atoms are introduced to form an alloy, this regular structure is disrupted. The different-sized atoms distort the layers, making it much more difficult for them to slide. This is why alloys are typically harder and stronger than their main component metal. Common examples include steel (iron mixed with carbon), brass (copper mixed with zinc), and bronze (copper mixed with tin).

Filtration is a physical separation technique used to separate an insoluble solid from a liquid. An insoluble substance is one that does not dissolve in the liquid (the solvent). The process relies on a barrier with pores small enough for the liquid particles to pass through, but large enough to trap the solid particles. The standard laboratory setup involves a filter funnel lined with filter paper, placed over a conical flask or beaker. The mixture is poured into the filter funnel. The liquid that passes through the filter paper is called the filtrate. The insoluble solid that gets trapped in the filter paper is called the residue. This method is used in water treatment plants to remove solid impurities and is also the first step in separating a mixture like sand and salt water.

To separate a soluble solid (a solute) from the liquid it has dissolved in (the solvent), we use evaporation or crystallisation. The choice depends on whether we need to collect the solvent and the quality of the solid crystals required.

Evaporation: This method is used to obtain the solute from a solution when the solvent is not needed. The solution is heated in an evaporating basin, causing the solvent to turn into a gas and escape, leaving the solid solute behind. It is a fast method but may produce tiny, impure crystals. It is unsuitable for substances that decompose when heated, like sugar.

Crystallisation: This is a better method for obtaining large, pure crystals of a solute. The solution is gently heated to evaporate some of the solvent until the solution becomes saturated (it can't dissolve any more solute). You can test for a saturated solution by dipping a cold glass rod into the solution; if small crystals form on the rod, it is ready. The hot, saturated solution is then left to cool slowly. As it cools, the solubility of the solute decreases, and it forms pure crystals. The crystals can then be collected by filtration and dried.

Distillation is a powerful separation technique that works by separating substances with different boiling points. There are two main types:

Simple Distillation: This is used to separate a pure liquid solvent from a dissolved solid (e.g., obtaining pure water from salt water). The solution is heated in a flask. The liquid with the lower boiling point (the water) evaporates, turning into vapour. This vapour travels into a condenser, which is a tube cooled by a jacket of flowing cold water. The cold surface causes the vapour to condense back into a pure liquid, which is collected in a beaker. The dissolved solid is left behind in the flask.

Fractional Distillation: This is used to separate a mixture of liquids with different but close boiling points (e.g., ethanol and water, or crude oil). The apparatus includes a fractionating column placed between the flask and the condenser. The column is packed with glass beads or rings, providing a large surface area. As the mixture boils, both liquids evaporate, but the vapour will be richer in the substance with the lower boiling point. As this vapour rises up the hot column, it cools, condenses, and re-evaporates many times. With each cycle, the vapour becomes more and more enriched in the more volatile component (the one with the lower boiling point). This substance eventually reaches the top of the column, passes into the condenser, and is collected as a pure liquid (the distillate).

Chromatography is a technique used to separate and identify the components in a mixture of soluble substances, such as different coloured inks or food dyes. The separation depends on the different affinities of the components for two phases: a stationary phase and a mobile phase.

In paper chromatography, the stationary phase is a sheet of absorbent paper, and the mobile phase is a solvent (like water or ethanol). A baseline is drawn in pencil near the bottom of the paper, and a spot of the mixture is placed on it. The paper is then suspended in a beaker containing the solvent, ensuring the solvent level is below the baseline. As the solvent (mobile phase) soaks up the paper, it dissolves the mixture and carries it up the paper. The components that are more soluble in the solvent and have a weaker attraction to the paper will travel further up. The components that are less soluble and have a stronger attraction to the paper will travel shorter distances. This results in the separation of the mixture into a series of distinct spots.

The Rf (retention factor) value can be calculated for each spot to help identify the substance. It is a ratio and has no units.