Learn what dissolution is and the factors that affect the rate of dissolution, such as temperature, surface area, polarity and pressure. Learn what miscible and immiscible mean in regards to liquids.

Dissolution

When a solute in a solvent forms a solution, it is called dissolution. A solute is the primary substance that is dissolved in a liquid called the solvent. So, what this means is that it is when something dissolves in something else.

There are many factors that affect the rate at which a solute will dissolve. The general rule is that ‘like dissolves like.’ This means that a polar substance will dissolve in another polar substance – and non-polar in non-polar. Solid substances with greater surface areas dissolve faster than solid substances with smaller surface areas. In general, solids dissolve faster with increased temperature. The solubility of gas depends on pressure and temperature.

To summarize:

  • Like dissolves like
  • Greater surface area increases dissolvability
  • Temperature increases dissolvability
  • Enthalpy and entropy affect dissolvability of salts

Like Dissolves Like

You know that oil and water don’t mix, but do you know why? It’s because water is polar, and oil is non-polar. Non-polar molecules have mostly only intermolecular forces holding them together, along with the London forces. The temporary dipoles that form in non-polar molecules allow them to dissolve easily in other non-polar substances.

Non-polar substances dissolve in other non-polar substances, but not in polar ones. The opposite is also true. Polar substances dissolve in other polar substances, but not in non-polar.

The polar molecules have a charge on them. These charges help pull molecules toward each other and help them dissolve because the positive charge will be attracted to the negative charge of other polar molecules.

Liquids can dissolve in other liquids. When a liquid dissolves easily in another liquid, such as ethanol and water, they are said to be miscible. When a liquid does not dissolve easily in another liquid, such as oil and water, it is said to be immiscible. In general, a polar molecule will dissolve in solvent made of other polar molecules, but it will not dissolve in a solvent made of non-polar molecules.

Greater Surface Area

Greater surface areas of solids help them dissolve faster. The only place that dissolution can take place is at the surface of a solid. Therefore, it stands to reason that the greater surface area available, the faster the substance will dissolve.

Dissolution only takes place on the surface of a solid.
Dissolution Surface of Solid

You can try this at home. Put a sugar cube in a glass of water. How quickly does it dissolve? (No cheating – no stirring or breaking up the cubes.) Now, put a spoonful of loose sugar in a glass of water. How fast does it dissolve? The loose sugar will dissolve faster because the solvent (the water) can get to every surface of the sugar crystal molecule. In the sugar cube, it can only reach the sugar crystal molecules on the outside of the cube, not the inside.

I mentioned no stirring of the solutions as you were trying to dissolve them. As you know, mixing a solution will help solutes dissolve. This is because it creates more chances for the solute to interact with the solvent, essentially speeding up the interactions, which helps things mix and dissolve faster.

Temperature

In general, solubility increases with temperature. When you increase the temperature of a solvent, you increase the kinetic energy (or energy of movement) of the molecules, and this greater energy helps dissolve more of the solute molecules.

Think about tea. When you drink tea in the winter, it’s usually hot tea. Sugar added to hot tea dissolves easily and rapidly. In the summer, you usually drink iced tea. What happens when you add your sugar to iced tea? It usually falls to the bottom of the glass and just sits there. You have to stir it up and make it dissolve, and even then there is usually some left in the bottom of the glass. That’s because the solution is cold, and solutes don’t dissolve as easily in cold solutions as they do in hot ones.

Gas

Gas solubility depends on pressure and temperature.
Gas Solubility Depends on Pressure

It might seem a little weird to think of, but gas can dissolve in liquid. Think of soda. Soda is carbonated because CO2 gas has been dissolved in the liquid. When the soda bottle is unopened, you see few bubbles, but as soon as it is opened and the pressure on the liquid decreases you see a lot of foam and thousands of bubbles. When you opened that bottle, you may have seen a little bit of gas escape. This is because the gas is coming out of the liquid and is no longer dissolved in it.

While this example shows you that the solubility of gas depends on pressure, it also depends on temperature. A freshly opened warm soda will not be as carbonated as a freshly opened cold soda. Warm soda tastes flat because there isn’t as much CO2 dissolved in it as there is in cold soda. This is because the increased temperature means there is increased movement of the molecules in the solution, and this allows the gas to escape from the liquid molecules easier.

Lesson Summary

Dissolution is when a solute dissolves in a solvent to form a solution. Many factors influence the rate of dissolution. These include:

  • Like dissolves like
  • Surface area
  • Temperature
  • Pressure

Learning Outcomes

After you’ve finished this lesson, you’ll be able to:

  • Point out what happens during dissolution and describe factors that affect its rate
  • Relate the reason that like dissolves like
  • Note the differences between miscible and immiscible
  • Summarize the way in which temperature and surface area increase solubility
  • Visualize the effects of temperature and pressure on gas solubility