The Solubility of Potassium Nitrate

The Solubility of Potassium Nitrate

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The Solubility of Potassium Nitrate


To investigate how the solubility of Potassium Nitrate is affected by

Background Knowledge

Potassium Nitrate (KNO3) is an ionic compound. The strong ionic bonds
hold the compound in an ionic lattice which gives KNO3 its crystalline
structure. These ionic bonds also have other properties which will
affect my investigation, I must be aware of these properties for
greater accuracy in my method. The ionic bonds give KNO3 high melting
and boiling temperatures. In the case of KNO3, ionic bonds are
present, which are strong and hard to break under room temperature; I
believe that this may have an impact on the solubility of KNO3 at low
temperatures, where there is very little energy present to break these

Particles move faster and collide with a greater energy output. A
greater proportion of these particles now have enough energy to react.
Therefore there is a greater chance of a collision between KNO3 and
water molecules resulting in a successful reaction.


My hypothesis is that the temperature of the water affects the
solubility of Potassium Nitrate. As I have already addressed this, as
you increase the temperature, you also increase the kinetic energy of
each particle; thus increasing the chance of a successful reaction.
This means more potassium nitrate will be broken down and dissolved in
the water as the temperature increases. I believe that the solubility
of the KNO3 will increase at a proportional rate to the increase in
the temperature of the water.

[IMAGE]When the KNO3 dissolves, it can be classed as a chemical
reaction. It follows therefore that in order for the KNO3 to dissolve,
an activation energy barrier must be overcome. Activation energy is
the energy required to kick-start a chemical reaction. If the
activation energy barrier is not reached (i.e. if the particles do not
have enough energy on collision to react) then the reaction will not
proceed and the KNO3 cannot dissolve in the water.

In order for KNO3 to be dissolved, it must come into contact with

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water molecules. The collision between the water molecules and KNO3
generates energy which, if strong enough allows the reaction to take
place causing the KNO3 to dissolve.

What follows is the Maxwell-Boltzman distribution for molecular

(See Diagram) On the x-axis, we have the 'energy of molecules' and on
the y-axis we have the 'number of particles'. The activation energy is
marked from the point 'Ea'. The area under the curve to the right of
'Ea' represents the number of particles with enough activation energy
to dissolve in the water. With a lower temperature, the curve is
shifted to the left. As a result, there are very few particles with
enough activation energy to dissolve should the water and the KNO3
particles collide. At a higher temperature, the curve is shifted to
the right. As a result, there are a greater number of particles with
enough activation energy to dissolve in the occasion of the water and
KNO3 particles colliding. Therefore we can see that increasing the
temperature, increases the general energy of each particle in the form
of kinetic energy; the

Text Box: In warmer water the ionic bonds are opened up and let water particles get into the gaps causing more Potassium Nitrate to be dissolved, unlike the cold water.



Cold Water Hot Water

Independent Variables

- The volume of water used

- The time taken to allow the KNO3 to dissolve

- The duration of which I spend shaking the test tube

- The mass of crystalline KNO3 used.

In this experiment, I must use an excess of KNO3salt. An excess is
required to ensure that all of the salt dissolves to form a solution
at the saturation point.

To ensure that fair testing is maintained, I must ensure that only one
independent variable is tested at any one time. In this experiment, I
will be altering the independent variable of the temperature of the
solution; all other variables must be kept constant to the highest
degree of accuracy available in the lab.


* Test tube rack

* Five test tubes

* Five Plunges

* Potassium Nitrate

* Spatula

* Thermometer

* Stop Watch

* Scale (Electric)

* Water Bath (at 40 oC, 60 oC and 80 oC)

* Ice (for 4 oC)

* Room at 20 oC

* Water


1. Collect all the required Apparatus.

2. Fill up the five test tubes with 10ml of water, which are at five
different temperatures.

3. Weigh the Potassium Nitrate without the containers lid on.

4. Add one spoonful of potassium nitrate to the fist test tube,
which has the lowest water temperature (4oC).

5. Place a plunge on the test tube and shake for 1 minute.

6. Check if the solution has become saturated this is where no more
salt can be dissolved in the water and crystals form at the bottom
of the test tube. If no crystals appear repeat steps 4 to 5until
the solution becomes solute.

7. Once the solution is saturated, re-weigh the potassium nitrate
and work out the difference. By working out the difference this
shows how much potassium nitrate was dissolved (Starting weight
take away end weight equals difference).

Results Table

My Results


Weight Before

Weight After

Amount of Salt Dissolved

4 oC (10 oC)




20 oC




40 oC




60 oC




80 oC (85 oC)




Class Results






























































My Graph and table shows that the temperature of water does affect the
solubility of potassium nitrate, as I said in my prediction, which was
higher, the temperature, the more potassium nitrate will be dissolved.
The main pattern in my graph was that the higher the water temperature
is the more potassium nitrate is dissolved, At 80 oC more potassium
nitrate dissolves because the warm water particles spread out letting
the potassium ions getting into the gaps, then it did at 4 oC, when
the KNO3 atoms came into contact with each other, there wasn't enough
thermal energy, which could be generated into kinetic energy to
provide enough Activation energy for the KNO3 to be dissolved. In my
experiment there were two sets of anomalous results, I do not know the
reasons for these results they could be caused by human error or when
the KNO3 was added to the water it decreased the water temperature
giving me incorrect results.


The results that I acquired from this experiment are the kind of
results, which I expected. In my opinion I think that the results,
which I got where quite accurate, but there were some inaccurate, and
I have highlighted these in my results table. The results which were
anomalous which were compared to others have many reasons behind them.
One of these reasons is that it is a scientific fact that potassium
nitrate reduces the temperature of water when added so to solve this
problem we could have kept an eye on the temperature more. Also we
could have taken the temperature of the water while we were adding the
potassium and when we were shaking the tubes we could have used a
machine so the rigorousness of the shakes is almost the same.

Another reason for this was that as we would be adding potassium
nitrate to the water the temperature would be decreasing, the reason
for this is as I have written before, the reaction needs Activation
energy and this energy is taken away from the warm water particles as
heat energy and then, when the particles collide with KNO3 ions this
is then converted into kinetic energy so if the energy is strong
enough allows the reaction to take place causing the KNO3 to dissolve.

I think I should have repeated this experiment at least three times
making sure that the results I gather are accurate or I could have
worked out an average, or I could have repeated the experiment the
results which were anomalous. I think I did get a suitable range
because these are the results I predicted and to prove this I could
plot them on my scatter diagram and they would fit in with the
correlation of the points. There is a particular pattern in my graph
because as I increase the water temperature the more potassium nitrate
is being dissolved, so the line of best fit in my graph is a curve.
There is only one of a possible many variables of the original
question that I could investigate, if given time I would do the
experiment the other way round, not testing the solubility of
potassium nitrate in water, but how water evaporates in a solution of
potassium nitrate and water.
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