The solubility of a substance in water is measured by determining the maximum amount of that substance that can dissolve in a specific amount of water at a given temperature.
Here's a breakdown of the process:
1. Preparation:
- Solvent: Use a known volume or mass of purified water (often 100 grams). The purity of the water is crucial for accurate measurements.
- Solute: The substance you want to measure the solubility of.
2. Saturation:
- Gradually add the solute to the water while stirring continuously.
- Keep adding solute until no more dissolves and solid solute remains undissolved at the bottom of the container. This indicates the solution is saturated.
- Ensure thorough mixing to facilitate dissolution. Temperature control is vital, as solubility is temperature-dependent.
3. Separation:
- Carefully separate the saturated solution from the undissolved solute. This can be done through:
- Decantation: Carefully pouring off the liquid, leaving the solid behind.
- Filtration: Using filter paper to separate the solid from the liquid. This is usually the preferred method. The filter paper must be pre-weighed.
4. Quantification:
- Evaporation: Accurately measure the volume of the saturated solution. Then, carefully evaporate the water from the separated saturated solution (the filtrate if using filtration). A pre-weighed container should be used for evaporation.
- Drying: Ensure all water is evaporated, usually by heating in an oven at a low temperature.
- Weighing: Weigh the container with the dried solute residue.
5. Calculation:
- Subtract the weight of the empty container from the weight of the container plus dried solute to determine the mass of the dissolved solute.
- Calculate the solubility. Solubility is typically expressed as grams of solute per 100 grams of water (g/100g H₂O). For example, if you evaporated water from 50g of saturated solution and found 10g of solute, then to extrapolate to solubility you can calculate:
- If 50g saturated solution contains 10g solute, it also contains (50g-10g) = 40g water
- Solubility = (10g solute / 40g water) * 100g water = 25g solute/100g water
Factors Affecting Solubility Measurement:
- Temperature: Solubility generally increases with temperature for most solids in water. Always record the temperature at which the solubility is measured. A water bath can be used to maintain a constant temperature.
- Pressure: Pressure has a significant effect on the solubility of gases in water, but little effect on the solubility of solids or liquids.
- Purity: The purity of both the solute and the solvent (water) is crucial for accurate measurements. Impurities can affect solubility.
Example:
Imagine you are measuring the solubility of sugar in water at 25°C. You add sugar to 100g of water until no more dissolves. You then filter the solution to remove any undissolved sugar. After evaporating the water from the filtrate, you find that 200g of sugar remained. Therefore, the solubility of sugar in water at 25°C is 200g/100g H₂O.
