Water as Solvent

Learning Objectives

  • The comparison of solution processes in water for molecular substances and ionic compounds

  • The importance of the solvent properties of water in selected biological, domestic or industrial contexts

Summary Notes

Solutes and solvents are components that come together to form a solution.

When a solid, liquid or gas dissolves in water, an aqueous solution is formed.

Solution produced is called a homogenous solution in which the original solute and solvent cannot be distinguished once mixed.

When dissolving things, there are 3 things that need to happen:

  1. Solute particles are separated from each other

  2. Solvent particles are separated from each other

  3. Solute and solvent particles attract each other

Solute-solvent interaction must be stronger than the solute-solute and solvent-solvent interaction for a solution to form.

The rule for whether a solution will dissolve or not is like dissolves like

  • Polar solute dissolve in polar solutions

  • Non-polar solutes dissolve in non-polar solutions

Substances that dissolve readily in water have ionic or polar covalent bonding. They fall into 3 groups:

  1. Polar covalent molecular compounds that form hydrogen bonds with water

  2. Polar covalent molecular compounds that ionise

  3. Ionic compounds that dissociate

Molecular compounds

There are 2 ways that molecular compounds can dissolve in water:

  1. They form hydrogen bonds with water

  2. They dissolve in water by ionising to form hydrated ions (ionisation reaction)

Forming hydrogen bonds with water to form an aqueous solution from a liquid solution of ethanol

Forming hydrated ions via an ionisation reaction

Ionic compounds

Ionic compound dissolves in water separating into its ions. This is called a dissociation reaction.

Ion-dipole attractions between ions formed and polar water molecules.

This intermolecular bonding is stronger than even hydrogen bonding because the charge of any ion is much greater than the charge of a dipole moment.

Forming aqueous solution of sodium and chloride ions from a sample of NaCl solid

Videos to help build your understanding

Explaining Dissolving Molecular Substances in Water

Explaining Dissolving Ionic Compounds in Water

How to Write Dissociation vs Ionisation Reactions

Practice Material

Indicate with a ✔ if the following pairs will result in a dissolved homogenous solution

  1. Methane and water

  2. Ethanol and water

  3. Ethanol and hexane

  4. Sodium chloride and water

  5. Octane and water

Answers

  1. Methane and water

  2. Ethanol and water ✔

  3. Ethanol and hexane

  4. Sodium chloride and water ✔

  5. Octane and water

Explaining Solubility Practice

Teacher's Tips:

You should be able to clearly explain the process of dissolution while utilising key words in your answer such as 'intermolecular forces', 'polar' or 'non-polar', 'hydrogen bonding' etc

In order to determine the reaction that a solute undergoes, you need to first be able to identify if the chemical is a polar covalent molecule or an ionic compound. Then you need to determine if hydrogen bonds are formed or ionisation occurs.

You should be able to differentiate between a dissociation and ionisation reaction.