When sodium thiosulfate and hydrochloric acid are mixed together, they react to form
a suspension of sulfur. A student investigates the rate of this reaction using the apparatus shown in Figure
1. The following is the process the student follows: Measure 20 centimetres cubed of
hydrochloric acid and 20 centimetres cubed of sodium thiosulfate solution. 2) Add the solutions to the conical flask at the same time and start the
stopwatch. 3) Time how long it takes until the sulfur produced makes it impossible to see the
Before we move on to the question, let’s have a quick look at Figure 1. This is the black cross that is going to be obscured. The hydrochloric acid and the sodium thiosulfate solution are going to be measured
out using the measuring cylinders. Added to the conical flask, the stopwatch will be started and then stopped once
enough sulfur has been produced to hide the black cross.
To make this a fair test, the student should look through the top of the conical
flask rather than from the side so that they’re looking through a consistent amount
of liquid. Now, we can move on to the question.
The student repeats the experiment with different concentrations of sodium
thiosulfate. Why is it important to add the same volume of liquid each time? Tick one box. To ensure the cost of each experiment is the same, to ensure that measurement errors
are consistent, to ensure that the cross is viewed through the same amount of
liquid, or to ensure that the conical flask is stable.
Let’s go through the statements one by one. To ensure the cost of each experiment is the same: now this statement does not make a
great deal of sense. Any experiment will have a range of costs associated with it. And there is no need to make sure that each experiment has the same cost. This is, therefore, not a correct answer.
What about to ensure that measurement errors are consistent. While keeping measurement errors consistent is a valuable thing to do, it isn’t
important when it comes to measuring the liquid volume. The errors are dictated by the equipment used, not the volumes measured. Therefore, this is not correct answer.
What about to ensure that the cross is viewed through the same amount of liquid? Let’s have a look at two scenarios: one where there’s more liquid and one where
there’s less. Let’s have a think about what might happen if the volumes were different at certain
period after the start of the experiment.
Since the black cross is being viewed through the solution, a greater volume in the
flask means that more of the solution has to be looked through in order to see the
black cross. Therefore, with a different volume, you’re going to end up with a different time and
the experiments won’t be comparable.
Therefore, ensuring the cross is viewed through the same amount of liquid is a vital
part of making sure the experiment is a fair test. Therefore, this is the correct answer. But let’s look at the fourth statement just to be safe.
To ensure that the conical flask is stable: fundamentally, the way a conical flask is
designed is so that it is stable no matter how much liquid is in it. Therefore, this is not correct answer. So it is important to add the same volume of liquid each time to ensure that the
cross is viewed through the same amount of liquid.
Table 1 shows the time taken for the cross to be obscured with different
concentrations of sodium thiosulfate. Plot the data on Figure 2 and draw a line of best fit.
Since the axes already have values, we can start straightaway by plotting the
points. It may help you here to remember that every small square is worth one on the 𝑥-axis
and four on the 𝑦-axis.
Here’s our first point at eight grams per litre on the 𝑥-axis and 140 seconds on the
𝑦-axis. The second point is a little bit more tricky. We need to go one small square further than 15 on the 𝑥-axis and we need to go up to
77 on the 𝑦-axis. We know that 70 is halfway between 60 and 80 and we can make a good judgement as to
where 77 should be based on that. Now, we can move on to the third point and there we have it, and so on, and so
Now, what that remains is to draw the line of best fit. Now, it should be clear that these points describe a curve rather than a line. So our line of best fit should be curved. There are no obvious anomalies. So the line of best fit roughly follows each point.
Based on your line of best fit, how long would it take for the cross to be obscured
if the student used the solution of sodium thiosulfate with a concentration of 20
grams per litre?
To answer this question, we need to look at Figure 2, we need to find the value of 20
on the 𝑥-axis, mark up to the line of best fit, and then draw along to the 𝑦-axis
to find out the time taken for the cross to be obscured.
Here we are at 20 grams per litre of sodium thiosulfate. We use a ruler to draw up from 20 until we hit the line of best fit and then draw
along to the 𝑦-axis and we read off the value. I read this to be about 57, 58 seconds. So based on the line of best fit, it would take 58 seconds for a solution of sodium
thiosulfate with a concentration of 20 grams per litre to obscure the cross.
Of course, you may have drawn your line of best fit a little differently or read off
the graph a little differently. That’s ok. Anything in the range of 50 to 62 seconds would have been fine.
What concentration of sodium thiosulfate would be necessary to obscure the cross in
For this question, again, we’re going to use Figure 2. But this time, we’re going to start out at the 𝑦-axis, move along to the data, and
then down to the 𝑥-axis.
Here’s 40 seconds on the 𝑦-axis, the time taken for the cross to be obscured. Then using a ruler, we trace along to the data and then down to the 𝑦-axis where we
read off a value. I measure this to be 27.5 grams per litre. Therefore, the concentration of sodium thiosulfate necessary to obscure the cross in
40 seconds is 27.5 grams per litre.
Again, you may have drawn your line of best fit differently or read the graph
slightly differently. So any value in the range of 26 to 29 grams per litre would have been fine.
The equation for the reaction of sodium thiosulfate with hydrochloric acid is as
shown. Na2S2O3 aqueous plus 2HCL aqueous react to form 2NaCl aqueous plus H2O liquid plus
SO2 gas plus S solid. The student notices a smell similar to a burnt match. Which product causes the smell and why does its production cause the mixture in the
conical flask to lose mass?
Let’s have a look at the first clue: the student was able to smell something. This means the substance we’re looking for is probably a gas. Our starting materials sodium thiosulfate and hydrochloric acid will have the smell
because hydrochloric acid is volatile.
However, since these are reactants and the student would have had them to start with,
the student would not have noticed the smell of hydrochloric acid as a new
smell. As the solution reacts, sodium chloride will be produced along with water and sulfur
and sulfur dioxide gas would escape the flask.
Since solutions like sodium chloride solution, liquids like water, and solids like
sulfur are very likely to stay in the flask, the only real candidate for the smell
is sulfur dioxide. Therefore, the product that causes the smell is sulfur dioxide.
The only question remaining is why producing sulfur dioxide makes the mixture in the
conical flask lose mass. The reason the conical flask mixture loses mass is because sulfur dioxide is a gas
and it escapes.
Now, we need to put all this into full sentences. The smell is caused by SO2, which is a gas. The mixture loses mass because the SO2 gas escapes.
The student used the black cross to measure a key point of progress of the
reaction. Explain how else the student could have measured the progress of the reaction.
The question you have to ask yourself is what else besides the opacity of the
solution changes during the reaction. The first thing you might consider is whether the reaction is endothermic or
exothermic. Either way, you could use the change in temperature to monitor the reaction.
If you think back to part five, there’s a clue to what else changes; that’s the mass
of the reaction mixture. As gas is produced, the mass of the mixture decreases. Again, this could be used to monitor the reaction progress.
The other thing that changes is that we produce a gas during the reaction. Therefore, if we measure the volume of gas, we can monitor the progress of the
reaction. To match up with using the black cross, we could measure the change in each of these
at a specified time after the reaction has started.
I’m going to walk you through how to answer one of these — how the volume of gas
produced changes during the reaction. So here, we have our freshly mixed reaction mixture. We have sodium thiosulfate and hydrochloric acid. And as the reaction proceeds, sulfur dioxide is being produced.
If we just left the reaction mixture like this, we would have no way of measuring the
volume of gas; it would simply escape. So we need an instrument that measures the volume of a gas.
That instrument is a gas syringe. As gas is produced, the gas syringe plunger moves outwards and the graduations on the
gas syringe show us how much volume has been produced. We could then use a stopwatch to time how long it takes to produce a certain volume
of gas or to measure the volume of gas produced in a certain amount of time.
Now, we have to put all this into full sentences. A gas syringe could be used to collect the sulfur dioxide gas. The volume of gas produced in a fixed time after the start of the reaction could be
measured. Now, as I said before, there were three possible routes you could have taken. Now, as I mentioned before, there were three possible routes to an answer and I went
with just one of them.
If you went with either of the other two, all you would have needed to do would be
mentioned the right measurement apparatus, for instance, thermometer for temperature
or balance for mass, and mentioned that you would have to make a measurement after a
fixed time interval.
Another student planned to investigate how heating the mixture during the reaction
would affect the reaction rate. What two things should this student keep the same as the first student to ensure that
the investigation is a fair test? Tick two boxes. The concentrations of the reactants, the volumes of the reactants, the size of the
beakers, the shape of the white tile, or the size of the black cross.
Let’s go through the statements one by one. Remember that the objective of the second student is to be able to compare their
results with the results of the first student. So we’re looking for factors that if they were to change would significantly affect
The concentrations of the reactants: if the second student would pick different
concentrations of the reactants, they would not be able to compare directly with the
first student’s results. This is because changing the concentration affects the rate of reaction.
The student is investigating how heating affects the rate of reaction. If they change the rate of the reaction by changing the concentration, they won’t be
able to tell which factor is causing the change. Therefore, the second student should keep the concentration of the reactants the same
as the first student. Therefore, this is a correct answer.
What about the volumes of the reactants? As we saw in part one, viewing the black cross through a different volume of liquid
is going to produce a different endpoint. Therefore, if the student changes the volumes of the reactants as compared to the
first student’s investigation, they’re not going to know the effect of heating. Therefore, they should keep this the same as the first student. And this is, therefore, a correct answer.
This, therefore, gives us two correct answers. But I’m going to go through the other three statements, just in case.
The size of the beakers: if we think back to Figure 1, the beakers were only used to
store the solutions. Therefore, the size of the beaker doesn’t matter. And the student doesn’t have to worry about keeping it the same as the first
What about the shape of the white tile? Well, the purpose of the white tile is to provide contrast for the black cross and
also to support the conical flask. Therefore, the shape doesn’t really matter; it could be circular and it will perform
the same job. Therefore, this is not a correct answer.
And finally, what about the size of the black cross? Well, as long as the cross is big enough to begin with, the size of the cross does
not affect how long it takes for the solution to hide it. Therefore, this is also not correct answer.
So the two things that the student should keep the same as the first student to
ensure that the investigation is a fair test are the concentrations of the reactants
and the volumes of the reactants.
The student performs a different experiment, reacting samples of magnesium with
oxygen and weighing the result. The student times how long it takes for the magnesium to burn completely and measures
the increase in mass. They recorded their results in Table 2. The equation for calculating the mean rate of a reaction like this is mean rate of
the reaction equals mass gained divided by time taken. What is the mean rate of this reaction? Give your answer to three significant figures.
So here, we have a student burning magnesium. Once burned, what’s left behind is a pile of magnesium oxide powder. This is the reaction equation. The question has asked us to work out what the mean rate of this reaction is and it’s
giving us the equation to use. We just need to substitute these values into the equation and evaluate.
Therefore, the mean rate of the reaction is equal to the mass gained 1.02 grams
divided by the time taken 9.82 seconds. This is equal to 0.10387 grams per second.
The question also asks us to give the answer to three significant figures. Therefore, the mean rate of reaction is equal to 0.104 grams per second.