In early chemistry classes, students are introduced to the classification of matter. They discover that matter is divided into pure substances and mixtures. Pure substances are further divided into elements and compounds, while mixtures are split into homogeneous and heterogeneous mixtures.

Another term for homogeneous mixture is “solution,” a substance that exists in one visible phase and is uniform throughout. Students have, in fact, been exposed to solutions all their lives, from the sugary drinks they enjoy to the air they breathe. They are likely familiar with the terms “dilute” and “concentrated” and have possibly experienced adding more salt to their foods to enhance its flavor.

In this article, we’ll explore some important concepts related to solutions and share tips for how you can teach this subject to your students.

Solutions are a PHYSICAL Change

Many students believe the formation of solutions involves a chemical change, which is incorrect. Although it looks like two separate substances are becoming one, the reality is that this mixture can still be separated based on their physical properties. Ask your students what happens when they boil a pot of salt water. Some may recognize that after all the water evaporates, the salt would be left behind.

Solute vs. Solvent

In a solution, there is always a solute and a solvent. Simply put, the solute is the substance present in a smaller quantity and the solvent is the substance in a larger quantity. We say the solute dissolves in the solvent and the solvent dissolves the solute.

Solutions Exist in Various Forms

The most commonly known type of solution is a solid in a liquid, but solutions can exist in all other phases.

• Gas in gas solution (e.g. oxygen, nitrogen, and carbon dioxide forming air)
• Solid in solid solution (e.g. zinc and copper forming brass)
• Gas in solid solution (e.g. carbon dioxide in water forming soda.)
• Liquid in liquid solution (e.g: ethanol in water forming alcoholic beverages)
• Liquid in solid solution (e.g. mercury in silver forming amalgams in dental fillings.)

Solubility Describes How Well a Solute Dissolves in a Solvent

A substance is considered soluble if more than 1.0 g of the solute can dissolve in 100 mL of the solvent. It is slightly soluble if between 0.1 g and 1.0 g of the solute can dissolve in 100 mL of the solvent. A substance is deemed insoluble if less than 0.1 g of the solute can dissolve in 100 mL of the solvent.

At a certain temperature, most solutions have a solubility value that expresses how much of the solute can dissolve in 100mL of the solvent. For example, at 20°C, 36.0 g of NaCl can dissolve in 100 mL of water.

The ability of a solvent to dissolve a solute depends on the forces of attraction between compounds. If the solute-solute attraction is greater than the solute-solvent attraction, the solute is insoluble. However, if the solute-solute attraction is weaker than the solute-solvent attraction, the solvent is strong enough to break apart the solute, making it soluble.

How Temperature Affects Solubility

• Solid solutes in liquid solvents: Higher temperatures INCREASE solubility. This is because higher temperatures increase the kinetic energy of the particles, causing solute particles to move further apart. Solvent particles also gain energy, colliding with more force, which helps disrupt the solute-solute attraction.

• Liquid solutes in liquid solvents: Higher temperatures have LITTLE EFFECT on solubility. Since the solute and solvent particles are already quite far apart in the liquid state, adding energy doesn’t significantly increase the distance between particles.

• Gas solutes in liquid solvents: Higher temperatures DECREASE solubility. With greater kinetic energy, gas particles are more likely to escape the liquid phase. Ask your students whether soda from the fridge or soda left at room temperature tastes fizzier.

Here are some topics to help you get started on teaching solutions and solubility to your chemistry students. Stay tuned for future articles with more ideas on the rest of the unit.