Water is a compound of hydrogen and oxygen, and life as we know it depends on it. Pure water is colourless, odourless and tasteless, melts and freezes at 0°C, has a density of 1g/cm³, and is neutral to litmus.
Solutions, Solutes and Solvents
A solution is a uniform (homogeneous) mixture of a solute in a solvent. The solute is the substance being dissolved — always the minor component; the solvent is what it dissolves into — always the major component. Water is called the “universal solvent” because it dissolves more substances than any other. A solution with water as the solvent is called an aqueous solution.
By contrast, a suspension is a heterogeneous, cloudy mixture of small insoluble solid particles in a liquid — muddy water and chalk-in-water are examples. Left undisturbed, the particles slowly settle to the bottom.
Experiment: Preparing a Saturated Solution at a Set Temperature
Requirements: two beakers, a thermometer, common salt, a teaspoon, distilled water, Bunsen burner, filter paper and funnel.
Procedure: Half-fill a beaker with distilled water and heat it to 25°C, monitoring with the thermometer. Add a teaspoon of salt at a time, stirring, until some salt remains undissolved — at that point the solution is saturated. Filter out the undissolved salt; the filtrate collected is the saturated solution.
Solubility
The solubility of a substance is the mass, in grams, needed to saturate 100g of water at a given temperature. A substance that dissolves easily is soluble (e.g. sugar, salt, ethanol); one that doesn’t is insoluble (e.g. sand, kerosene); one that dissolves only slightly is sparingly soluble.
To measure solubility: prepare a saturated solution at the chosen temperature, find the mass of substance that saturated the water and the mass of water used, then scale up (or down) to find how much would saturate 100g.
Temperature affects solubility — most solids dissolve more easily as the solvent warms up, while gases dissolve more easily as it cools down. Pressure affects only the solubility of gases: raising the pressure of a gas above a liquid increases how much of it dissolves.
Non-Aqueous Solvents
Solvents other than water — alcohols (e.g. ethanol), benzene, turpentine, and paraffins like kerosene and petrol, plus carbon tetrachloride — dissolve substances that won’t dissolve in water. Everyday uses include: petrol, kerosene and ammonia solution for removing grease and oil stains; turpentine for paint stains; borax solution for tea and coffee stains; ethanol for dissolving aromatic oils in perfume-making; benzene for making vulcanising solution (used to repair tyres and make raincoats); and ethanol for dissolving iodine into tincture of iodine, a mild antiseptic.
Crystals and Crystallisation
A crystal is a solid with a fixed geometric shape — flat surfaces, straight edges and sharp angles. Crystals of the same substance always share the same shape, whatever their size; common shapes include cubic, octahedral, needle-like (prismatic), triclinic and rhombic.
Crystallisation is the process of forming crystals by cooling a hot, saturated solution. As the solution cools, the solvent can hold less and less dissolved solute, so the excess separates out as crystals.
Process: heat the solution to drive off some solvent and produce a hot saturated solution, let it cool so crystals form, then filter them out, wash with distilled water, and dry between sheets of filter paper.
Crystallisation also serves as a purification method: pure crystals melt at one fixed temperature, while impure ones melt gradually over a range. First-pass crystals usually aren’t fully pure; dissolving and re-crystallising them one or more times (re-crystallisation) leaves most impurities behind in solution and produces progressively purer crystals.
