Chemistry - Q4 - Rate of Dissolution

Temperature

For solid and liquid solutes, a higher temperature increases the rate of dissolution and the solubility (the amount of solute that will dissolve). Here's an example. You make sun tea by putting tea bags and water in a glass jar and placing the jar in the hot sun. In a few hours you have iced tea. If you add sugar to the tea as soon as you bring it inside, you can dissolve more sugar in it than if you refrigerate the tea before adding sugar. For gases, the opposite is true. As the temperature increases, the rate of dissolution and the solubility of gas solutes actually decrease. Why does this happen? When you warm up a solvent, the molecules spread out, allowing the gas solute to escape because it's also moving faster. It's why your soda goes flat faster on a warm day; the carbon dioxide escapes more quickly. When the solvent and gas cool down, the molecules move closer together, and more collisions occur. As you have seen, more collisions increase the rate of dissolution.

Surface Area

It's much easier to digest your food if you chew it rather than swallowing it whole. Smaller pieces mean that more surfaces of the food (solutes) are exposed to the digestive juices in your stomach (solvents). In other words, big particles take longer to dissolve than smaller particles. Here's another example. Sugar cubes dissolve more slowly than loose sugar in a solvent, such as water. Crushing or grinding down the large sugar cube into small particles of loose sugar allows water (solvent) particles to contact more sugar (solute) particles. So, increasing surface area of the solute will increase the rate of dissolution in a solvent.

Temperature

Maybe you've noticed that sugar (solute) dissolves better in hot tea (solvent) than in iced tea (solvent). That's because the warmer the temperature of the solvent, the more rapidly the solute moves in it. When solute particles move faster, more collisions take place between solvent and solute particles. Dissolution depends on solute particles colliding with solvent particles, so the more collisions there are, the faster the solute dissolves.

Factors Affecting Rate of Dissolution Stirring

When a solvent interacts with a solute, sufficient kinetic energy must exist to bombard solute particles and break them apart. For solid and liquid solutes, agitation, such as stirring, speeds up the process of separating, interlocking, and rearranging the particles. At home, we can stir a mixture with a spoon or electric mixer. In a chemistry laboratory, there are a couple of choices. We can use a stirring rod to stir by hand. But when stirring needs to be constant, we use magnetic stirbars. Stirbars are small magnets, as small as a pea or as large as a peanut shell. We place the stirbars at the bottom of a beaker or flask, which is chemical glassware that contains a mixture. The glassware is similar to a cup or pitcher. We put the flask on a magnetic plate to start the stirring. For gas solutes, however, stirring is actually counterproductive. It allows more gas to escape from the mixture because moving it around creates more spaces for the gas to leak. Remember, dissolution depends on solute particles colliding into solvent particles. In making a solution of gas in liquid, stirring decreases collisions because gas solute molecules escape during agitation, much like steam escapes from the bubbles at the top of a pot of boiling water.

Rate of Dissolution Stirring

You can make iced tea by dissolving a powdered mix in a pitcher of water. The powder is the solute and water is the solvent. When you pour the powder into the water, the solution begins to form. Sometimes there are clumps in the water. Stirring the solution with a spoon increases the rate at which the powder (solute) dissolves in the water (solvent). The rate increases because there are more collisions between the solute and the solvent. You can see the powder crystals swirl around in the water while you stir until they dissolve. The end result is a solution: all parts of the iced-tea mixture look the same.

Summary

You have seen three ways to influence the rate of dissolution: stirring (agitation), temperature, and surface area. How these factors affect the rate of dissolution of solids and liquids are sometimes different for gases. So if you want to speed up a dissolution, it's important to know these factors and their effects on the rate of dissolution. Here are some real-life examples of how you affect the rate of dissolution: Chewing your food before swallowing increases the surface area of food particles, allowing your stomach to digest the food faster. Stirring the sugar into tea increases the chances of the sugar dissolving more quickly. Refrigerating a soda keeps it from going flat because a colder temperature decreases the chances that the carbon dioxide gas will escape. Monitoring the temperature in a fish tank ensures that enough oxygen gas is dissolved in the water for the fish to breathe.

Introduction

You studied the parts of a solution and how a solution is formed. You explored the dissolution process. Here are some important points about solutions: A solution is formed when a solute is dissolved in a solvent. The dissolving process is called solubility. For a solute to dissolve in a solvent, the forces that attract the molecules of a solute and solvent to each other must be greater than the forces that keep the solute together and the forces that keep the solvent together. The Golden Rule of Solubility is "like dissolves like." This means nonpolar solutes dissolve in nonpolar substances, and polar solutes dissolve in polar substances. Polar and ionic substances mix easily with one another.

Surface Area

You've seen that your stomach digests food more easily when you chew it rather than swallowing it whole. Smaller pieces mean more surface area is exposed to the digestive juices. As we increase the surface area of a solute, more sides touch the solvent. Dissolving takes place on the surface of the solute, so exposing more surface area increases the rate. In other words, more solute molecules mean more potential collisions and quicker re-bonding with the solvent molecules around them. In the chemistry laboratory, we often use a mortar and pestle (see the picture) to create more surface area on a solute. We put the solute in the bowl, which is the mortar. We use the pestle to pulverize, or grind up, the solute into a fine powder. The mortar and pestle are an internationally recognized symbol of a pharmacy. That's because pharmacists use them often in their work.