Factors Affecting Solubility

Solubility can be very useful in helping to purify products. Cooling down a solution until a solid solute comes out is called “crashing out.” When the solute assumes the form of crystals, this process is referred to as crystallization. The crystals that form are pure products, and can be very pretty!

There are several factors affecting solubility, including temperature and pressure. In the following experiments, you'll learn whether each factor has a positive or negative effect on the solubility of different compounds.

Objective

Explore the concept of solubility.

Will heating or cooling increase solubility? What about pressure?

Materials

• 4 small, sealed bottles of soda
• Labeling tape
• Jar or drinking glass
• Olive oil
• Water
• Small cooking pot
• Stove
• Wooden skewer
• Table sugar

Procedure

Gases

1. Label each of the small, sealed soda bottles with a number. Two will be refrigerated overnight, and two will not.
2. Open one of the bottles that will be refrigerated and one of the bottles that will be left out and let each sit with the cap off for a few minutes. What does this do? Record your observations.
3. Place one of the sealed bottles and one of the open bottles in the refrigerator to allow them to cool overnight.
4. Place the last two bottles somewhere warm, like a sunny window, and leave them there overnight.
5. The next day, when the two refrigerated bottles are sufficiently cooled and the other two are room temperature or warmer, set the bottles out side by side and record any observations.
6. It’s time to shake them up! Shake each bottle about 20 times. For each of the two bottles that you opened, plug the mouth with your thumb as you shake.
7. Don’t let the gas escape just yet! Observe what happens to the bubbles in the container. Record your observations.
8. If you want to make a mess (which can sometimes be fun), take the bottles over to the sink and open them (carefully!). Record your observations.

Liquids

1. Set out a jar or drinking glass.
2. Pour small amounts of olive oil and water into the glass at the same time until the jar or glass is half full. Record your observations.
3. Let the mixture sit for a few minutes. Does anything change?

Solids

1. Fill a small cooking pot with water and set it on the stove.
2. Before turning the stove on, add sugar to the water until the added sugar stops dissolving and you’re left with a nice pile of solid sugar at the bottom. What does this do?
3. Turn on the stove to a low setting. You want to heat the solution slowly so you can make observations.
4. Stir occasionally. What happens as the water gets heated? Record your observations.
5. Once all of the sugar is dissolved, turn off the heat.
6. Set the wooden skewer in the solution and let the liquid cool slowly. Don’t stir! If the pot has a cover, go ahead and cover it. What does covering the pot do? Why is this helpful?
7. Uncover the pot after an hour or two. What has happened?

Results

Gases: The cold, sealed container will have the most dissolved gas, CO₂. When shaken, its liquid will react more than the liquids in the other containers. When opened, it will demonstrate the most violent escape of gas bubbles. The opened, warm container will have the least amount of dissolved gas (and cause the least mess).

Liquids: Water and oil do not mix, and will form two layers in the cup or jar.

Solids: Heating the water will allow more sugar to dissolve. When cooling, the sugar will crash out of the solution and form solid crystals, many of which will likely be on the wooden skewer.

Why?

Gases:  Carbon dioxide, or CO₂, is the gas that makes soda fizzy. Sodas are filled and capped under pressure, which increases the amount of CO₂ that can dissolve in the soda (a solution that’s primary water). The sound that is made when opening a sealed soda bottle or can is the sound of the pressurized gas escaping. In addition, dissolved CO₂ turns back into a gas when the pressure drops, which explains the bubbles you see rising to the surface.

CO₂ is more soluble in water when the water is cold, so the sealed, cold soda will have the most dissolved CO_2. This explains why it has the most bubbles and creates the most mess (and the best sound!) when shaken and opened. In the warm sodas, CO₂ cannot dissolve in solution as well, and many of the gas bubbles escape to the top of the soda. The opened warm soda is also unpressurized, which allowed some of the gas to escape when the soda was opened.

Liquids:  Water and oil are immiscible, meaning that they do not mix. This means are the insoluble in each other. The water and oil will be in two separate phases, which will be easy to see in two different layers.

Solids: Adding sugar to the water until solids appear at the bottom of the pot creates a supersaturated solution. Heating the water allows more of that solid sugar to dissolve. This is because energy added to the system in the form of heat makes the heated molecules in the system move around faster. This allows more sugar molecules to dissolve, because they come into contact with the moving water molecules at a greater frequency. As the water is cooled, the energy of the system decreases and the sugar molecules begin to find each other again. Cooling slowly without stirring allows molecules to align with each other and produce crystals. Sometimes, if cooled slowly enough, you can grow crystals much bigger than the ones you originally put in the water.