Stirring and Dissolving Rate

name         Jen
status       student
grade        9-12
location     CT

Question -   How does stirring affect the rate sugar dissolves in water?
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Jen,

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.

Matt Voss
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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.

Vince Calder
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Any soluble crystal dissolves in water quickly until the water it touches
is saturated.
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
some more.
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
pot,
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
heated.
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.

Jim Swenson
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Jen,

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)
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