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Important Vocabulary
Solute: any substance in a solution other than the solvent.
Aqueous Solution: a solution where water is the solvent.
rate of dissolution: a measure of how quickly a solute dissolves in a solvent.
temperature: the measure of the average kinetic energy of the particles in a material.
dissolution: the process of dissolving a solute in a solvent.
surface area: the amount of area covered by the surface of an object.
agitation: to disturb or to stir
Lesson 1.4 Effects of Dissolution
Lesson 1.4 Effects of Dissolution
The Rate of Dissolution in Aqueous Solutions
You might recall that a solution is made of a combination of solvent and solute. Each of these holds its own properties before they are combined. After they are combined they hold new properties. For example, sugar is a substance in the solid form. This is then combined with a greater quantity of water, the solvent, in a liquid form. You might ask why water is considered our solvent. Recall that a solute is any substance in a solution other than the solvent. Therefore, water is our solvent. Once combined, the aqueous solution, a solution where water is the solvent, is in liquid form and the sugar is no longer in solid form, as it has dissolved. But what determines how quickly the solute dissolves? Let's take a look at three factors that affect the rate of dissolution, a measure of how quickly a solute dissolves in a solvent.
Solutions and Temperature
Why is it that when we make gelatin, the directions instruct us to use hot or boiling water? If you have ever decided to forgo the directions to speed things up and used only cold water instead, what happened? The reason the instructions say to use hot water first before using cold water is that hot water helps the powder mix dissolve first. Cold water is added later to lower the concentration of the gelatin.
Temperature and Dissolution
One key component of dissolution is temperature, the measure of the average kinetic energy of the particles in a material. Dissolution is the process of dissolving a solute in a solvent. As the temperature increases, this allows a solute to dissolve faster, resulting in an increase in the rate of dissolution.
As temperature rises, the kinetic energy rises as well. The increase in kinetic energy causes an increase in the motion of the solvent molecules. Increased motion leads to more collisions of solvent and solute molecules with greater force and frequency. This ends up increasing the rate of dissolution. While solid solutes dissolve at a much higher rate with an increase in temperature thanks to an increase in kinetic energy, gases do not.
Solutions and Surface Area
When thinking about how a solute and solvent respond to temperature, it all comes
down to the molecular level. If the temperature increases, the solvent molecules can
break apart the solute molecules more efficiently. An increase in temperature is not the
only way to increase the rate of dissolution. An increase in the surface area of the solute
allows for more of the solute to come into contact with the solvent at the same time; in
turn, this will result in an increased rate of dissolution. Surface area is the amount of
area covered by the surface of an object.
For example, on a salt cube, the surface area would only account for the sides of the cube and not what is on the inside. If that salt cube were placed in water, our solvent, it would sink to the bottom of the cup and the water would start to dissolve the outside of the cube. Once that was dissolved, it would dissolve the next layer and so on until the whole cube was dissolved. Think of how you lick a lollipop. You cannot get to the center until you have licked the outside away first. This is how a salt cube dissolves.
Expand Surface Area
Now crush the salt so it looks like the salt granules that are shaken into our food. The surface area has increased. No longer are the sides of the salt cube the only exposed surfaces. Now every piece of the crushed cube is opened. The salt granules are added to the water, and every little piece starts to dissolve on its own. The granules do not have to wait until the outside layer is gone because there is no longer an outside layer. Now that there is no longer the wait time to get to every layer, the rate at which the salt is dissolving has increased.
However, while the surface area of a solute can change the rate of dissolution, it cannot change the amount of solute being dissolved. This has to do with the solubility of the solute. There is a maximum amount of solute that a solvent can hold. If it exceeds that amount, it does not matter how much the temperature is increased or how small the solute is crushed; it will not dissolve.
Solutions and Agitation
Dissolving a solute into a solvent happens all around us each day: when we mix toothpaste and water to brush our teeth, when we make coffee or tea in the morning, or when you feel sick and put an antacid tablet in water to drink to help settle your stomach.
Typically, time is a factor when we are doing these tasks; we want to finish quickly to move on to the next thing. Not all the solutions in our everyday lives would be good if heated, so temperature is not always an option, and sometimes crushing up our solute adds time. So, what does that leave us with to speed up the rate of dissolution? Agitation! Agitation means to disturb or to stir. Agitation could mean stirring, whisking, shaking, spinning, rotating, or any other type of motion similar to these.
When a solution is stirred, the movement allows the solute molecules to come in contact with a greater amount of the solvent molecules at a faster rate than if the solution was not stirred. The faster the solute and solvent molecules collide, the higher the rate of dissolution. For example, pools use chlorine, which comes in different forms like a tablet or powder, to kill bacteria that form over time for several reasons. There are various ways that the chlorine is distributed into the pool. Tablets can be placed into the pool skimmer, where water brings in debris that has landed in the pool and pushes particles of the chlorine back out. Or the tablets could be placed in a floating chlorine dispenser that moves around the pool. With both distribution methods, the water is moving over the chlorine, taking the particles to different places in the pool. The more the chlorine is stirred up, the faster it will dissolve.
Chlorine can also be used in powder form. Unlike the tablets, the powder is distributed by shaking or pouring the powder into the water. Once chlorine powder is dispersed into a pool, the blowers stir up the water and chlorine solution.
Molecular Level
Taking a closer look at the molecular level, there are a few things happening to the chlorine once it is in the pool. The movement of the water that is caused by the blowers spreads out the chlorine molecules giving the solute a larger surface area that comes into contact with the solvent (water). The agitation also allows for the contact with the water to be continuously changing so new water molecules are touching each chlorine molecule.
If there was no motion or agitation, the powder would stay at the top, clumping together before eventually dissolving as it sinks. This process would take a lot more time and decrease the time available to swim. Agitation changes the rate at which the solute is dissolved, but it does not change the amount that is dissolved.
The rate of dissolution can be changed by each factor on its own or a combination of the factors. When making instant hot cocoa, the solute is already in powder form, so it has a larger surface area than if it were in a cube. Before the mix is poured in, the water is heated, so the increased kinetic energy helps the solvent molecules separate the solute molecules more efficiently. The last step after the mix is poured into the hot water is to stir the mix, spreading out the solute molecules again, creating more points of contact. The agitation also increases the kinetic energy of the solvent, causing the temperature to rise as well.
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