3 beakers labelled I, II & III, were each filled with 50 cm^3 of distilled water. The temperature of the water was set at 20, 35 & 60 deg. ..
C respectively. 1 g of copper(II) sulphate crystal was added to each beaker. In which beaker did copper(II) sulphate take the shortest time & longest time to completely diffuse throughout the water? Explain.
Difffusion is a process where a substance moves from an area of greater concentration to an area of lesser concentration. A good example is perfume being released into the air. As the perfume molecules move about into the air they go from a great concentration right where they were released to a much lower concentration throughout the air.
In the example you have given the small amount of copper sulfate when placed into the water first has to become dissolved - where the solid goes into solution. Initially this will result in a relatively deep blue color at the bottom of the beakers.
What then determines the rate of diffusion is how fast the molecules are moving in the solution. Since molecules move faster at a higher temperature, the beaker which has the highest temperature will see the fastest diffusion and the beaker at the lowest temperature will take the longest to diffuse.
The solubility of a solid in liquid increases with the increase in temperature of the liquid. Therefore the solubility of copper (II) sulphate crystal in water will increase with the temperature of water.
In the given example the quantity of water in all the three beakers is same, but the temperature of the water in different beakers is different. Therefore the there will be no difference in the maximum quantity of copper (II) sulphate that can be dissolved by water in different beaker due to difference in quantity of water. However because of temperature difference the solubility of copper (II) sulphate and the maximum quantity that can be dissolved will be higher in beaker with water at higher temperature as compared to beaker with water at lower temperature.
Therefore 1 gram of copper (II) sulphate will dissolve fastest in beaker containing water at 60 degrees C, and slowest in beaker containing water at 20 degrees centigrade.