Which technique could be used to obtain pure liquid acetaldehyde, boiling point of 20 degrees Celsius, from a mixture of acetaldehyde and ethanol, boiling point of 80 degrees Celsius? Evaporation, centrifugation, filtration, crystallization, distillation.
First, let’s consider exactly what the question is asking us to do. It’s asking us to separate liquid acetaldehyde from a mixture of acetaldehyde and ethanol, both of which are liquids. We also know from the question that the two liquids have very different boiling points. So, this might be important in which procedure we select. Now, we have an idea of what the question is asking us about. Let’s look at each of the techniques suggested in turn.
Let’s begin with evaporation. You would use this technique when you have a solution of something. This is something that’s dissolved, the solute, into a solvent. During this technique, you heat the solution and the solvent starts to evaporate. Once all of the solvent has evaporated, you’ll be left with a solid. So, this technique is best used when you want to separate a soluble solid from the solvent that it’s dissolved in. This is not the case with the example given in our question, where we’re given two liquids. So, this cannot be the answer.
Now let’s think about centrifugation. During this technique, a sample is placed into a small container and into a centrifuge. Once the centrifuge is switched on, it spins all of the samples round really quickly. The centrifugal force created by their spinning separates particles from our solution. They’re separated according to size, shape, and density. This type of process is very common with biological samples. Because this technique specializes in separating particles from a solution, it is not appropriate for the example we’re given in the question.
Let’s move on to filtration. A typical setup for filtration will often involve a conical flask inside which is placed a funnel, and in the funnel, a piece of filter paper. Into this set-up, you’ll pour your mixture. And anything insoluble will get stuck in the filter paper and will be separated from whatever is in the solution. So, this technique is great for separating insoluble solids from a solution. An example of this might be removing sand from water. Again, this technique is not appropriate for the example given in the question.
Let’s move on to look at crystallization. This begins in a similar way to evaporation. We begin by heating our solution, which causes some of the solvent to evaporate. In evaporation, we would remove all of the solvent in this way. But crystallization is different. In crystallization, we only remove a bit of the solvent in order to create a more concentrated solution. We then allow this to cool. And as it does so, crystals form and come out of solution. So, a bit like evaporation, this technique is useful for separating a soluble solid, which, again, is not suitable for the example given in the question.
As a side note, you might wonder what the difference is between evaporation and crystallization, and when you would pick one over the other. Generally speaking, crystallization is great if you want really pure crystals and that there’s a chance there may be impurities in your solution. When you crystallize your compound, all of the impurities remain in the solution. And so, you can easily filter them off.
So, let’s look at the final technique, distillation. So, here’s a possible setup for a simple distillation experiment. We would place a mixture of liquids inside this round bottom flask. And then we would heat it. As the solution starts to heat up, whichever of the liquids has the lowest boiling point will start to evaporate. This will travel upwards and reach a thermometer, which can tell us what temperature the gas is at. This can help us identify which solvent is coming off, if we’re not sure.
As the solvent travels up the tube, the evaporating solvent can travel further up the tube, where it cools down. Or it can pass down the slanted tube and into the condenser. This cools the gas and condenses it. Once the solvent has turned back into a liquid, it can run down the slanted tube and into a collection vessel. Here, I’ve drawn it as a conical flask. We would carry on heating the system until all of our low-boiling-point solvent has evaporated, condensed, and been collected. If our original mixture contained more than one liquid, we would perform fractional distillation. Essentially, repeating this process for every solvent that we wanted to remove.
So, let’s look at our question. Would this process be suitable? We’re told that we have a mixture of acetaldehyde, which has a boiling point of 20 degrees, and ethanol, with a boiling point of 80 degrees. So, if we were to put our mixture of acetaldehyde and ethanol into our flask and begin to heat it, the lowest-boiling-point liquid would start to evaporate first. In this case, acetaldehyde. We would see a reading of approximately 20 degrees Celsius on our thermometer. And our collection vessel would collect the acetaldehyde once it had condensed. Finally, we’d be left with pure ethanol in the original flask.
Bear in mind that if you were to carry on heating the flask, eventually the ethanol would also evaporate. And it would condense and collect in the same container as the acetaldehyde, if you hadn’t moved it. So, distillation is a technique whereby we can separate liquids according to their boiling point. This makes this the correct answer for this question.