PS-3.5 Explain the effects of temperature, particle size, and agitation on the rate at which a solid dissolves in a liquid
Temperature (effect on particle motion)
Particle size (effect on surface area)
Agitation (effect on solute/solvent collisions)
Kinetic energy is the energy of moving objects. The Kinetic Theory of Matter states that mater is composed ( made of) particles called atoms. These particles are in constant motion, and the motion of the particles is proportional to the heat energy they contain. In simple terms, the hotter the atoms are, the faster they move, and the less heat they contain, the slower they move. The diagram below illustrates this concept.
Beaker "A" contains atoms that are not moving ( A very unlikely situation!). The matter in this beaker is said to be at "Absolute Zero", the temperature at which all particle motion stops ( -273.15oC or 0Kelvin). Obviously, in beaker "A" all of the heat has been removed from the sample. In beakers B, C, and D, the energy content of the matter in the beaker increases from left to right. The matter in beaker "B" has less energy than the matter in beaker "C", and the matter in beaker "D" has the most heat energy, because the atoms or molecules have the greatest amount of kinetic energy. So, add heat to a substance and you can increase its temperature, increase its temperature and you increase the kinetic energy of its atoms or molecules.
Substances dissolve when particles of a solvent bump against particles of a solute, thus separating the solute particles from each other and allowing them to go into solution between the solvent particles. This diagram shows moving solvent particles hitting a crystal of solute and kicking the solute particles into solution.
Though there are several ways to increase the rate at which a solute dissolves in a solvent, they all involve somehow increasing the number of particle collisions. One way to increase the number of particle collisions is to break or crush the solute into very small particles.
Particle size plays an important roll in the process of dissolution. Decreasing the size of solute particles increases surface area, thus increasing the number of solute / solvent particle collisions, and this increases the rate of dissolution. An increase in solute surface area means faster dissolution. The diagram below illustrates that decreasing particle size increases the total surface area of the solute particles that can come in contact with the solvent.
Another way to increase the rate of dissolution is to increase the kinetic energy of the solvent by increasing the temperature. Increasing the temperature of the solvent makes the molecules or atoms of the solvent move faster. Faster solvent molecules hit solute particles more often. This has the effect of increasing the rate of dissolution.
A third way to increase the rate of dissolution is to increase the kinetic energy of the solute particles through agitation. Agitation is just a fancy word for stirring and shaking the solution. Many laboratories use automated shakers or stirrers to dissolve stubborn solutes. Polyvinyl Alcohol ( PVA) crystals are extremely slow to dissolve in water. Even with automated stirring it may take 24hours to dissolve a few grams of PVA in warm water.
SUMMARY: If you want to dissolve something faster:
A. Add more kinetic energy by warming up the solvent. This increases the number of particle collisions and speeds up dissolving.
B. Crush the solute into smaller particles. This increases the number of particle collisions by increase surface area and this speeds up dissolving
C. Add more kinetic energy by agitating the solution. This increases the number of particle collisions.