Atomic Foundations of Matter

Place a clean and dry 100 mL beaker on a digital weighing balance. Set the balance reading to zero by pressing the tare or reset button. Pour about 50 mL of water into the beaker. Add a spatula full of common salt to the water contained in the beaker. Record the reading on the weighing balance (Fig. 9.1a). Swirl until the added salt dissolves and record your observations (Fig. 9.1b). What do you observe? You may notice that the mass of the solution is equal to the sum of the masses of water and salt taken. This shows that there is practically no change in the mass during the formation of a solution, which is a physical change. This is true for all physical changes. You can repeat the above activity by weighing a piece of paper before and after tearing it into pieces, and observe whether its mass changes or not.

Let us investigate a chemical change. Experimental set-up 1: Place a clean, dry 100 mL conical flask and a medium-sized balloon on a weighing balance. Set the balance reading to zero. Pour about 20 mL of vinegar or lemon juice into the conical flask. Take about 2 g of baking soda and put it into the balloon. Keep the balloon filled with baking soda on the weighing balance next to the conical flask. Record the initial reading (Fig. 9.2a). Carefully transfer the baking soda from the balloon into the conical flask containing vinegar (Fig. 9.2b). Place the conical flask and balloon back on the weighing balance, and record the final reading (Fig. 9.2c). What do you observe? Are the initial and the final readings same? A brisk effervescence is observed. The final reading does not match the initial reading. What can be the reason for this? Repeat the above experiment in a slightly modified way as explained below. Experimental set-up 2: Place a clean, dry 100 mL conical flask and a medium-sized balloon on a weighing balance. Set the balance reading to zero. Pour about 20 mL of vinegar or lemon juice into the conical flask. Place about 2 g of baking soda in the balloon. Fix the balloon to the mouth of the conical flask using a thread, without allowing the baking soda to mix with the vinegar. Weigh the conical flask containing vinegar and the balloon containing baking soda, and record the reading (Fig. 9.3a). Lift the other end of the balloon upwards, allowing the baking soda to fall into the vinegar (Fig. 9.3b). What do you observe? As in experimental set-up 1, a brisk effervescence occurs, which inflates the balloon during the reaction. Record the final reading (Fig. 9.3c). Are the initial and the final readings same in this case? In this experiment, the final reading matches the initial reading. You have noticed that the total mass of vinegar and baking soda before the chemical reaction is equal to the total mass of carbon dioxide and other substances formed after the reaction. In experimental set-up 1, the mass difference occurs because the gas produced by the chemical reaction escapes, resulting in a difference between the initial and the final readings.

Let us verify the law - Group activity. Place two clean and dry 100 mL conical flasks on a weighing balance, and mark them A and B. Set the balance reading to zero by pressing the tare or reset button. Pour about 10 mL of 1% m/v sodium sulfate solution into the Conical Flask marked A. In the Conical Flask B, pour about 10 mL of 1% m/v barium chloride solution. Leave both Conical Flasks A and B on the weighing balance undisturbed, and record the total mass of both the solutions (Fig. 9.4a). Transfer the solution from Conical Flask B to Conical Flask A and mix the two solutions carefully. What do you observe? Now, place both the Conical Flasks A and B on the weighing balance again as shown in Fig. 9.4b, and note the reading. Do you observe any change in the reading after mixing the solutions?