Video Transcript
The diagram below shows five beakers containing tea at various temperatures. An equal mass of sugar is placed into each beaker. Some of the beakers are stirred. What is the most likely order, from quickest to slowest, for the sugar in each beaker to completely dissolve?
The question is asking us to determine the different speeds at which the sugar will dissolve in these various beakers under these different sets of conditions. In other words, we’re determining the rate of dissolution, which can be defined as the rate at which the solute dissolves. In a solution, the solute, in this case the sugar, is the substance in lower amount and is being dissolved in a solvent, which in this case is the tea. The rate of dissolution is affected by the amount of collisions between the particles of solute and particles of solvent. The amount of successful collisions between particles of solute and solvent and therefore the rate of dissolution can be affected by three main factors: particle size, stirring, and temperature.
Let’s first look at how particle size impacts the rate of dissolution. The diagram shows the solvent and solute interactions when solute particles are very large, such as a sugar cube. When solute particles are large, interactions with solvent particles are much more limited, occurring only on the outer surface of the large particles called the surface area. If these larger particles are broken down into smaller particles, the surface area for possible collisions is increased. Therefore, a smaller particle size allows for quicker dissolving of the solute. The time it takes to dissolve a solute is also impacted by stirring. By agitating the particles in a solution by stirring, more collisions between solute and solvent particles occur and speed up the rate of dissolution. Therefore, stirring a solution increases the rate of dissolving.
Finally, let’s look at how temperature impacts the dissolution rate. The temperature can be defined as the average kinetic energy of the particles in the solution. So, if the temperature of a solution is low, the average kinetic energy of the particles is also low. This means the solute and solvent particles will be colliding less frequently. However, if the temperature of the solution increases, the average kinetic energy of the particles is greater. Because the speed at which the particles are moving is greater, there will be more collisions between solute and solvent particles in the solution. Therefore, a higher temperature will speed up the rate of dissolution.
Let’s now have a look at the conditions in each beaker. In each beaker, the sugar is either a cube, which would be a larger particle size, or powdered, which would be a smaller particle size. Next, the tea is either at a temperature of 20 degrees Celsius or at 80 degrees Celsius. Comparing these two possible temperatures, the tea at 20 degrees Celsius would be the lower temperature, while the tea at 80 degrees Celsius would be the higher temperature. Finally, the tea and sugar is either not stirred or stirred. Knowing that smaller particles, stirring a solution, and a high temperature increases the rate at which a solute dissolves, let’s first identify the beaker that has all three variables in favor of dissolving the sugar quickly.
We can see that beaker (E) has small particle size, a high temperature, and stirring. It is likely then that the sugar in beaker (E) would dissolve the quickest of the five beakers. Let’s look now for the beaker that has the opposite conditions with large particle size, low temperature, and no stirring. We can see that beaker (A) fits all of these conditions and would likely dissolve the sugar the slowest. Now, we can order the remaining three beakers by determining how they compare to the slowest and fastest dissolving beakers (A) and (E). Beaker (B) is nearly identical to beaker (A), which contains the slowest dissolving conditions, with the exception of particle size. Because beaker (B) has small particle size, it is likely it would dissolve the sugar slightly faster than beaker (A).
This leaves beakers (C) and (D). Both beakers have large particle size. However, they also have high temperature, which would increase the rate of dissolution. Their difference lies in the stirring. Because the solution is not stirred in beaker (D), the sugar in beaker (D) would likely dissolve more slowly. Therefore, the most likely order from quickest to slowest for the sugar in each beaker to completely dissolve is (E), (C), (D), (B), and (A).
