O-Level Experimental Chemistry

Pour the 100 cm³ of water (measured with a measuring cylinder) onto the solid blend held in a well-insulated container such as a polystyrene (foam) cup, ideally with a lid to cut heat loss. Place a thermometer in the mixture and record the starting temperature. Using a stopwatch, record the temperature at regular time intervals (for example every 30 s) while stirring, until the temperature stops rising and begins to fall. The recorded pairs of time and temperature let you follow how fast and how far the temperature changes.

Seawater is a solution: the salts are dissolved, so they would pass straight through filter paper and cannot be filtered out. The water itself has to be boiled off and then caught again. That method is simple distillation. Step 1 - put the seawater in a flask with a few anti-bumping granules and heat it with a Bunsen burner. Step 2 - the water boils at 100 °C and leaves the flask as steam. The dissolved salts are non-volatile solids with very high melting and boiling points, so they cannot vaporise and are left behind in the flask. Step 3 - the steam passes into a Liebig condenser. Cold water flows through the outer jacket, entering at the bottom and leaving at the top so that the jacket stays full. Step 4 - the steam is cooled below 100 °C, condenses back to liquid water and runs out of the condenser into a beaker. This distillate is the pure water. A thermometer placed at the neck of the flask, level with the side-arm, should hold steady at 100 °C, which confirms that only water is coming over.

For each job, ask what is being measured and how accurately. (a) transferring 150 cm3 of a liquid into a beaker - measuring cylinder. The volume is a whole 150 cm3 with no decimal places, so only an approximate measurement is needed. A measuring cylinder of a suitable size (250 cm3) is quick and accurate enough; read the bottom of the meniscus at eye level. (b) collecting a gas made in a reaction - gas syringe. The gas produced pushes the plunger of the syringe outwards, so the gas is trapped and its volume can be read off at the same time. (Collection over water is also possible, but only for a gas that is insoluble.) (c) delivering exactly 22.7 cm3 of alkali into a beaker - burette. 22.7 cm3 is quoted to 0.1 cm3, so an accurate, variable volume is needed. A burette is read before and after running the liquid out and the difference is the volume delivered. A pipette only gives one fixed volume, so it cannot deliver 22.7 cm3. (d) reading the boiling point of water - thermometer. A boiling point is a temperature, so a thermometer is used; hold the bulb in the vapour above the boiling water and read it when it stops rising.