Homogeneous and Heterogeneous Aqueous Systems

Key Questions What is the difference between a solute and a solvent? What happens in the solution process? Why are all ionic compounds electrolytes? How do you write a formula for a hydrate? What is the difference between a suspension and a solution? What distinguishes a colloid from a suspension and a solution?

Vocabulary Aqueous solution Solvent Solute Solvation Electrolyte Nonelectrolyte Strong electrolyte Weak electrolyte Hydrate Suspension Colloid Tyndall Effect Brownian Motion Emulsion

Solvents and Solutes Water dissolves so many substances that it comes in contact with, that chances are the water you encounter is not pure water! An aqueous solution is water that contains dissolved substances – A solvent dissolves the solute. – The solute becomes dispersed in the solvent

Dissolving in water Substances the dissolve readily in water include: ionic compounds and polar covalent molecules. Why?

Close your eyes for a minute… Picture yourself as a particle of table salt. You are dropped in water and stirred. What is happening to you?

The solution process Water molecules are in constant motion When solutes are placed in water, the water molecules collide with the solute, help to break up the solute and the polar ends of the water molecule then attract the ions. This is known as solvation.

Electroyltes and Nonelectrolytes Electrolyte: a compound that conducts electric current when it is in aqueous solutions (or molten state) – all ionic compounds are electrolytes. Why? Nonelectrolyte: compound that does not conduct electricity

Strong vs. Weak Electrolytes Strong electrolytes: – Nearly completely dissolves – Strong conductor of electricity Weak Electrolytes: – Does not 100% dissolve – Weak conductor

hydrates When you let some solutions sit out, some of the water will evaporate. Other water molecules will be taken up as part of the crystal lattice structure. This is a hydrate. Use the “dot” as part of the compound name to include the water. Ex: CuSO 4  5H 2 O

How to find the percent of water in a hydrate?

Efflorescent hydrates: – Water escapes easily or “effloresce” due to hydrate’s vapor pressure being greater than water alone Hygroscopic hydrates – Water is “pulled into” the compound from the atmosphere and absorbed Deliquescent compounds: – Water is pulled in so much that the compound forms a solution

Heterogeneous Aqueous Systems

Suspensions Heterogeneous systems are not solutions Suspension: particles of suspensions are much larger and do not stay suspended indefinitely (will settle out) – Ex: clay with water

Colloids Heterogeneous mixtures containing particles that spread throughout the medium that are larger than solutions but smaller than suspensions These do not settle out over time (like suspensions) and can go through filter paper)

Tyndall Effect

The scattering of visible light as it passes through a colloid is the Tyndall Effect. Colloids and suspensions

Brownian Motion Rapid collisions of the dispersion medium with the colloid particles, which do not allow the colloid particles to settle out.

Emulsions A colloidal dispersion of a liquid in a liquid Emulsifying agent is essential for the formation of an emulsion and for maintaining the emulsion’s stability