Solutions

A Solution is a mixture of solvent and one or more solutes. Solvent + Solute = Solution

In a table salt solution the molecules of water, the sodium ions and the chloride ions are evenly distributed.

In air the molecules of nitrogen, oxygen, carbon dioxide and the atoms of argon are evenly distributed.

In a solution the substances present have not changed

The Solvent (usually liquid) is the substance in which the solutes dissolve. To dissolve means to mix perfectly with the solvent. A solid dissolves by breaking up and separating into its particles that become evenly distributed throughout the solvent. The solid seems to disappear into the solvent. Water and alcohol are two great solvents.

The Solute is the substance that dissolves in the solvent. A solute is said to be soluble. Some substances may be soluble in one substance but insoluble in others. Sand, oil, plastic, hair, wax, and paper are insoluble in water.

A Dilute solution contains a small amount of solute in comparison to the quantity of solvent.

A Concentrated solution has a high amount of solute in comparison to the amount of solvent.

A Saturated solution cannot dissolve any more of the solute at that temperature i.e. it is at its maximum concentration at that temperature.

Investigate the Solubility of a Variety of Substances in Water

1. A variety of solid and liquid substances are supplied.

2. Half fill a test-tube with water and add a small amount of the substance.

3. Stopper the test tube. Shake the test-tube vigorously up and down for 20 seconds.

4. Examine the test tube to see if any of the substance ‘disappeared’ into the water.

5. Decide from the results if the substance is insoluble, slightly soluble or very soluble in water.

Investigate The Effect of Temperature on the Solubility of Copper Sulphate

The solubility of a substance is the mass of that substance that will dissolve in 100cc of water at a temperature.

1. Using a pipette put 25cc of water in a clean dry beaker. The water is at room temperature, 20°C. Gradually add copper sulphate to the water and stir to dissolve. Keep adding until no more copper sulphate will dissolve.

By heating, raise the temperature to 30°C and stir. The excess copper sulphate dissolved.

By heating, increase the temperature to 40°C, then to 50°C and so on up to 90°C.

If the mass of solute dissolved to saturation at each temperature is measured and recorded then a solubility graph can be drawn placing temperature on the x-axis and grams of the solute per 100cm3 of water on the y-axis.

The solubility of copper sulphate is 47g at 70°C. At room temperature, 20°C, it is 20g.

If the temperature of the hot saturated copper sulphate solution is allowed to cool to room temperature then the water will not be able to hold all that extra 27 g copper sulphate in solution. The extra copper sulphate will ‘fall out of solution’ reappearing as solid copper sulphate. The excess solid copper sulphate will be in the form of crystals. A crystal is a solid with a specific geometric shape that is due to a particular regular arrangement of its fundamental particles. The excess particles of copper sulphate, that could no longer stay in solution, became arranged into a particular pattern forming a solid with a particular shape. This is known as Crystallisation.

MandPractivity Grow Crystals of Copper Sulphate

1. Put about 50cm3 of water into a clean dry beaker.

2. Dissolve as much copper sulphate as possible in the water at room temperature.

3. Then heat the beaker to 60°C and dissolve more copper sulphate until the solution is again saturated.

4. Pour the copper sulphate solution in a petri dish.

5. Allow the solution to cool and evaporate slowly. Slower results in big crystals of copper sulphate(blue).