Which of the following is the condition of a solution with a small quantity of solute, compared with the amount of solvent? A) Dilute. B) Concentrated. C) Unsaturated. D) Saturated. E) Supersaturated.
To answer this question, we need to be sure of the meaning of the terms solution, solute, and solvent found within this question. In this question, the solute could be defined as the solid that will get dissolved. Examples of solutes are sugar or salt. The solvent is the liquid that will be doing the dissolving to the solute. The most common example of a solvent in everyday life is water. In this case, when all the solid solute has dissolved in the liquid solvent, we have a solution.
If we want to quantify how much solute has been dissolved into a given volume of solvent, we can refer to the concentration of a solution. Concentration is the amount of substance dissolved in a specified volume of liquid. An example of this is moles per liter. The solution in the left-hand diagram would be said to have a low concentration as there are not many solute molecules dissolved in a fixed volume of solvent. Conversely, the diagram on the right shows a solution with far more solute molecules per fixed volume of solvent. It would be said to be a high-concentration solution. An example of this in everyday life is antifreeze, which is used to protect engines in winter. It’s sold in concentrated form and must be diluted down with water for use.
A low-concentration solution would be one with a small quantity of solute molecules compared with the amount of solvent present. It would also be correct to describe the solution as dilute. In relative terms, a high-concentration solution will have a lot more solute molecules per volume of solvent. Therefore, we would refer to a solution such as this as being concentrated. In terms of the relative proportions of solute and solvent molecules present in a solution, the word dilute best fits the description in the question. It could be the correct answer here. Concentrated, on the other hand, certainly does not.
Before we commit to dilute as being the correct answer here, we need to explore what the terms unsaturated, saturated, and supersaturated actually mean. To do this, we need to explore the idea of solubility, which is the amount of solute that can dissolve in a given volume of solvent at a specified temperature. If we take a solute, for example, sugar, and add it to a fixed volume of water, that is the solvent, at a fixed temperature, we may find that it does not all dissolve in one go.
However, if we stir the solution well and leave it for a while, we may come back and find that all of the sugar has dissolved into the water. If all of the solute has dissolved into the water and no solute or, in this case, sugar is present in its solid physical state, then there is the potential for more solute to dissolve into this fixed volume of liquid. We would therefore say we have an unsaturated solution. This unsaturated solution could be dilute or it could be concentrated. The word unsaturated does not differentiate between these two terms. Therefore, it’s not the correct answer for the question here.
If we were to take our unsaturated solution and keep adding spoonfuls of sugar, we would be adding further solute until no more dissolves. Then, we would have a saturated solution. Evidence of a saturated solution is that there will be undissolved solid solute present at the bottom of the beaker. We have reached the limit of solubility at this fixed temperature. We would therefore say that this saturated solution is as concentrated as it is possible to be. A saturated solution contains a very large quantity of solute molecules compared with the amount of solvent present. It is certainly not a dilute solution, and it doesn’t match the description in the question. So, it’s not the correct answer here.
If we were to take our saturated solution at 25 degrees C and heat it up to a higher temperature, we would find that the solubility of our solute will increase. It’s quite possible that all of our undissolved solute will dissolve, and we now have an unsaturated solution at a new higher temperature. If we cool our unsaturated solution very slowly, it is possible that all of the solute will remain dissolved. We now have a solution that contains more solute molecules in a given volume of solvent than we would possibly expect to dissolve at this new low temperature.
Usually, when a single crystal is added to a supersaturated solution, crystallisation is induced and a mass of solid solute will appear. We would then have the saturated solution that we expect at the lower temperature. This idea is used in hand-warmer products that contain supersaturated solutions. When the crystals appear, heat energy is released. A supersaturated solution is a very concentrated solution. Therefore, there is a large quantity of solutes compared with the amount of solvent present. It is therefore not the correct answer either. The correct answer here is dilute.