Figure 1 shows the reaction of potassium metal with water. Identify one feature of Figure 1 that indicates a chemical reaction is taking
Looking at Figure 1, the most obvious feature is this fire or explosion in the
centre. This is definitely an indication that some kind of chemical reaction is taking
place. So how do we put that in a sentence? We could simply write “A flame is produced.”
You might be tempted to put that “Light is produced,” but this would not be the
correct answer. If you think about a light bulb, for example, light is produced by a bulb, but that’s
not an indication of a chemical reaction taking place. So a good answer to this question might simply be “A flame is produced.”
One product of the reaction in Figure 1 is a compound of potassium. Give the name of this compound.
First, let’s look back at the information we’ve already been given. The question tells us that this reaction is between potassium and water. But what does potassium and water react to form? Remember that the chemical symbol for potassium is K and water is H₂O. So we know that the products that form have to be a mixture of potassium, hydrogen,
and oxygen atoms.
Hopefully, looking at this will remind you that the product of potassium and water is
potassium hydroxide. So the answer to this question is simply potassium hydroxide. Out of interest, this is not quite the complete equation for this reaction. There’s also hydrogen gas formed. And then it needs some tweaking to make sure it’s balanced. So this is the complete equation for the reaction.
The reaction shown in Figure 1 takes place rapidly even if the water is cold. What conclusion can be drawn from this observation? Tick one box. The activation energy is high. The reaction is exothermic. The reaction is endothermic. The activation energy is low.
A good way to think about this is to draw some energy level diagrams for each of the
possible options. Let’s start with the centre two: the reaction is exothermic or the reaction is
endothermic. So here are some energy level diagrams for both an exothermic reaction and an
Remember that, in an exothermic reaction, the energy exits, whereas in an endothermic
reaction, energy enters. If we look at our energy level diagram for the exothermic reaction, we can see that
the energy of the reactants is much higher than that of the products, which means
that, during the course of this reaction, energy is released.
Compare this to our endothermic reaction and we can see that the energy level of the
products is higher than that of the reactants, which means that, for this reaction
to occur, energy must have been taken in. So how does this relate to our question?
The question says that this reaction takes place rapidly even if the water is
cold. This tells us that the water is low in energy. However, the amount of energy that a reactant has at the beginning doesn’t change
whether it’s exothermic or endothermic during the reaction. So neither of these can be correct. So let’s rule them out and look at the activation energy.
Again, let’s use energy level diagrams. So here we have two diagrams showing a reaction with a high activation energy and one
with a low activation energy. Remember that the term “activation energy” is sometimes written 𝐸𝑎.
Remember that an activation energy is the barrier to a reaction occurring. That means it’s kind of like a hill that you have to get over. In the diagram showing a high activation energy, we can see that this barrier is
really big. This means it takes an awful lot of energy for this reaction to occur. You can think of this like trying to drive a car up a steep hill. It takes an awful lot of fuel and energy to get it up that hill. In the diagram showing low activation energy, this barrier to the reaction is much
smaller, which means it takes very little energy at all to get over that barrier and
complete the reaction. So let’s go back to our question.
The question tells us that this reaction takes place rapidly even if the water is
cold, which means that the water has low energy. If this reaction had a high activation energy, it will need a lot of energy to get
started, but that’s not the case in this example. So what this tells us is that this reaction has a low activation energy because it
doesn’t need a big push of energy to get over that hill. So the correct answer to this question is that the reaction takes place rapidly even
if the water is cold because the activation energy is low.
A sample of the reacted water in Figure 1 was tested with a universal
indicator. The universal indicator turned purple. What is the pH of the solution? Tick one box. 13, nine, five, one.
So let’s recap what we know about pH. pH is a measure of how acidic or basic
something is. A pH value of zero is strongly acidic, a pH of seven is neutral, and a pH of 14 is
strongly basic. Now let’s try to remember what colour each one goes when tested with universal
indicator. You should know that acidic solutions turn red and very basic solutions turn
So looking at our potential answers, a pH of one, which is close to zero, would turn
red in the universal indicator, whereas a pH of 13 would turn purple. So it’s possible that 13 is the answer to our question, but let’s just check the
other two. Both pH five and pH nine are close to neutral. And hopefully, you know that neutral turns green with universal indicator. So something slightly more acidic than neutral at pH five would turn a pale green,
and something slightly more basic than neutral at pH nine turns a shade of blue.
So now we have all the information we need to answer our question. In our question, the universal indicator turned purple, and the only answer on our
list which turns purple is pH 13. Incidentally, if you think back to earlier in this question, we identified that the
product of this reaction was potassium hydroxide. And you should know that hydroxides are strongly basic. So the answer of pH 13 makes sense.
The rate of reaction in Figure 1 can be increased by dividing the potassium metal
into nanoparticles. Why do nanoparticles react more rapidly than large pieces of metal? Tick one box. They have a greater solubility in water. They have a greater surface area to volume ratio. They have a lower mass-to-volume ratio. They have a lower melting point.
So let’s consider the difference between potassium metal in a block or a sheet and
potassium nanoparticles. By changing the potassium metal into nanoparticles, we haven’t changed the metal. It’s still potassium. But we’ve just made it into a much smaller size.
You should realize that what this does is increases the surface area of the
potassium. This means that there’s more surface area of potassium to carry out the reaction. And this is why the reaction goes more quickly if the potassium is in
nanoparticles. So looking on our list of possible answers, the only one which could be correct is
the second one. They have a greater surface area to volume ratio.
Nanoparticles are increasingly used for drug delivery. Give one advantage of using nanoparticles in drug delivery.
So let’s think about what we know about nanoparticles. We know that nanoparticles are really, really small. Being so small, nanoparticles are much more easily absorbed by the body. You can think of this like a doorway. Because nanoparticles are so much smaller than traditional drug delivery methods,
it’s easier for the nanoparticles to slip into the body and then into the cells. So now let’s try and put this into a sentence. Nanoparticles are absorbed more easily by the body than larger particles.
Give one disadvantage of using nanoparticles in drug delivery.
Perhaps the most obvious disadvantage of using nanoparticles for drug delivery is
that they’re still reasonably new. Because this work on nanoparticles is still recently new, that means that scientists
haven’t yet fully explored the possible side effects or hazards of taking
nanoparticles. This means that the risks to your long-term health if you take nanoparticles are
So let’s form this into a sentence. So a possible answer to this question could be “The long-term health effects of
nanoparticles are unknown.” Of course, there may be other disadvantages of using nanoparticles for drug
delivery. But since the question only asks for one, this will do.
Fine particles in a powder have an average radius of 1.5 times 10 to the minus six
metres. Chemical processing divides the fine particles into nanoparticles with an average
radius of 5.9 times 10 to the minus nine metres. Calculate how many times smaller the average particle radius becomes because of the
So let’s pull out the important information from the question. We know that the fine particles in a powder have an average radius of 1.5 times 10 to
the minus six metres. We’ve also been told that, after chemical processing, we can form nanoparticles with
an average radius of 5.9 times 10 to the minus nine metres. The question asks us to calculate how many times smaller the average particle radius
becomes after this processing.
So what it’s asking us to do is to calculate the scaling factor that converts the
fine particle radius to the nanoparticle radius. So to calculate the scaling factor, we’re going to do the radius of the fine
particles divided by the radius of the nanoparticles. So this is 1.5 times 10 to the minus six divided by 5.9 times 10 to the minus
nine. This works out to be 2.5 times 10 to the power two times smaller.
It’s worth noting that if we’d done this calculation the opposite way round and done
the radius of the nanoparticles divided by the radius of the larger particles in a
powder, we would get the wrong answer because what we’ve been asked to find is how
many times smaller, not how many times bigger, the fine particles are compared to
the nanoparticles. So just be careful you get them the right way round.