Required Practical: Analysis & Purification of Water Samples
Boiling Point Elevation: The presence of dissolved impurities typically increases the boiling point of water. Monitoring the temperature at which water boils is a primary method for determining its purity.
Phase Changes in Distillation: Distillation relies on the different boiling points of substances in a mixture. Water is turned into water vapor (evaporation) at , leaving behind non-volatile solid impurities, and is then cooled back into a liquid (condensation).
Solubility and Residue: When water is evaporated, the concentration of dissolved solutes increases until the solution becomes supersaturated and the solids precipitate. The mass of this residue indicates the 'hardness' or mineral content of the original sample.
pH Testing: Use a pH meter or universal indicator paper to determine the acidity or alkalinity of the sample. Pure water must record a pH of exactly 7.
Testing for Dissolved Solids: Weigh an empty evaporating basin accurately. Add a fixed volume (e.g., cm) of the water sample and heat over a Bunsen burner until all water has evaporated. Re-weigh the basin; any increase in mass represents the dissolved solids in that volume.
Setup: Place the water sample in a conical flask connected to a delivery tube and a condenser (or a test tube immersed in ice water).
Heating: Heat the flask gently. Ensure the water boils steadily but does not 'bump' violently into the delivery tube. Using anti-bumping granules helps promote smooth boiling.
Collection: The steam travels through the delivery tube, cools, and condenses into a separate container. This collected liquid is the distillate (pure water).
| Feature | Pure Water | Potable Water |
|---|---|---|
| Composition | + safe minerals/ions | |
| pH | Exactly 7.0 | Usually 6.5 to 8.5 |
| Boiling Point | Exactly | Slightly above |
| Residue on Evaporation | None | Small amount of white solid |
Mass Calculations: Always subtract the mass of the empty vessel from the final mass to find the mass of dissolved solids. Ensure units are consistent (usually grams).
Temperature Monitoring: In exam questions, if the temperature shown on a thermometer during distillation is not exactly , the water being distilled is likely impure or the pressure is non-standard.
Safety Precautions: Mention the use of a safety mat, goggles, and the risk of 'spitting' when the evaporating basin is nearly dry. Reducing heat toward the end of evaporation prevents the solid from jumping out of the dish.
Verification: To prove the distillate is pure, you must re-test its pH (should be 7) and its boiling point (should be ).