Stirring and Dissolving Rate
Date: December 2006
How does stirring affect the rate sugar dissolves in water?
An engineer would be slightly better suited to answer this question,
but I have worked with many engineers to transfer small chemical
reactions into large production facilities so I can probably do
justice here :) There are many of types of stirring, believe it or
not, but I will only describe two common forms--using a magnetic
stirbar and using mechanical/overhead stirring via a rotating paddle.
There are three main reasons why stirring could affect the rate of
dissolution. For our initial system, let's use a beaker with, let's
say, a half inch layer of sugar on the bottom and filled most of the
rest of the way with fresh, room temperature water. With no
stirring that layer of sugar is just going to sit on the bottom. If
you look very closely at the interface between liquid and solid, you
can actually see some of the sugar dissolving. There will be fluid
veins of different densities and will reflect light at slightly
different angles (see fluid dynamics for more information
http://en.wikipedia.org/wiki/Fluid_dynamics). Left alone, diffusion
alone is left to make the solution uniform and since sugar water is
denser than pure water, diffusion also has to overcome
gravity. Stirring increases the rate of diffusion and will increase
the rate of dissolution.
Another important factor is surface area. The top of a thick layer
of sugar has moderate to decent access to the water that is doing
the dissolving, but the bottom of that layer either does not have
access to water at all or quickly becomes saturated and no further
dissolution can occur. This is because the rate of diffusion in and
out of the solid sugar layer will be extremely slow. Stirring
drastically increases the available surface area of sugar to
water. Note that the total surface area of sugar at any point in
time is constant, it is the surface area that is exposed to fresh
water that is important.
The third thing is the type of stirring, which usually revolves
around the type of vessel being used. Beakers and round bottom
flasks commonly used in a chemical lab generally call for magnetic
stirbars. The stirbars are simply a small, usually cylindrical
piece of magnetic metal (like iron) coated in Teflon so that the
stirbar is inert and does not react with your experiment. These
stirbars sit at the bottom of the flask, which is placed upon a stir
plate. The stir plate is simply a rotating blade with magnets on
the ends, which in turn causes your stirbar to rotate and stir the
fluid. Since the stirbar is sitting on the bottom, stirring against
the surface of the vessel, sugar will get ground up by passing
between the stirbar and the vessel's surface. When the sugar gets
ground up, you increase the surface area to volume ratio of the
particles, which will greatly increase the rate of dissolution (due
to points one and two).
Let's also think of sugar sitting at the bottom of a 2L soda
bottle. The bottom of the bottle is not uniform and a stirbar would
be of little value here since there is no uniform surface for it to
effectively stir on. Here, a paddle attached to a shaft and
connected to a motor would be useful (as would shaking). The paddle
can rotate at different speeds, it can be different shapes and
sizes, and each of those things affects how well mixing occurs. On
small scale the blade is usually called a paddle, whereas on large
scale it is called a propeller. Stirring rates using motors are
generally measured in RPM, but it is important to note that at a
constant RPM, the longer a propeller is, the faster the tip of the
propeller is going. On industrial scale the tip of the propeller
can easily exceed 100 MPH. Depending on the shape of the propeller,
there can be a grinding effect similar to that of a magnetic
stirbar, but in general it is minimal and shouldn't be counted upon
to increase surface area to aid dissolution. There is a lot of
science behind mixing that I won't get into since your question was
aimed at dissolving rates instead of mixing, but I wanted to give
you a taste of how involve mixing can be.
Stirring increases the rate of solution of all substances, not just
sugar. This is because the dissolved molecules are carried away from
the solid by the stirring action. Although this is a general trend,
how effective stirring is quantitatively cannot be predicted by any
theory I am aware of.
Any soluble crystal dissolves in water quickly until the water it touches
Then it slows down and waits for this water to be moved away
and replaced with new under-saturated water, at which time it dissolves
So the dissolution rate tends to be proportional to the replacement rate,
which is similar to the stirring rate.
Sometimes it might be possible that when flames heat the bottom of the
sufficient convection occurs
to dissolve sugar in reasonable time without deliberate stirring.
If the heating is done very slowly, the sugar grains on the bottom
would make a sugar-rich solution layer near the bottom.
Sugar-water being lots heavier than pure water, it would resist rising when
Then it could take days or weeks to finish dissolving,
because the remaining sugar crystals will stop dissolving
until they feel some less-rich water around them.
Molecular diffusion from bottom to top gets the job done eventually,
but it can be very slow.
Stirring would be really essential then.
To eliminate this extreme rate uncertainty,
stirring is always part of the recipe.
There are two general types of controls that affect the formation of
solutions: thermodynamic and kinetic.
Thermodynamic factors involve properties of the solute and the
solvent. For example, because of the different type of interaction
that water makes with itself and the way oil interacts with
itself - the two do not dissolve with each other. On the other
hand, salt and water have similar interaction types and so they can
form solutions. If two molecules pass the thermodynamic factors,
that is, if the thermodynamic factors indicate that they can
dissolve with each other, then kinetic factors can take effect.
Kinetic factors control the *speed* on how two substances dissolve
into each other. For example, you may have noticed that a lump of
sugar takes longer to dissolve than fine granulated sugar. In
general, kinetic factors speed up the formation of a solution by
making the molecules touch each other more efficiently. The more the
molecules touch each other, the more they can interact, and form the
necessary intermolecular connections that are necessary to form solutions.
As such, stirring helps speed up the formation of solutions by
allowing the molecules to touch each other more often (by moving the
molecules around) and form the intermolecular connections necessary
to form solutions.
Greg (Roberto Gregorius)
Click here to return to the Chemistry Archives
Update: June 2012