1 - Introduction to Solutions SCH3U – Solutions and Solubility

Introduction to Solutions

Solution is a homogenous mixture (the same throughout) of two or more substances

Solvent is the substance that is present in larger quantity Solute is the substance that is present in smaller quality (dissolved in solvent)

Chemicals are often dissolved in solvents to better facilitate reactions Molecules or ions must physically collide in order to react Being dissolved in a solvent is like having “maximum surface area” An unsaturated solution can still dissolve more solute

A saturated solution that cannot dissolve more solute

A super saturated solution has dissolved more solute than it should have Solutions can be “anything” dissolved in “anything else”

Example: oxygen gas dissolved in nitrogen gas (air)

Example: tin dissolved in copper (bronze)

Making a solution is a physical change, not a chemical reaction, and it does not alter the solute chemically

This process can usually be reversed

Water – The Universal Solvent

You know that water is a bend, polar molecule

Water is excellent at surrounding a solute

and forming attractions with it

With water, we call this hydration

The ions or molecules are completely

surrounded by water molecules so they become

part of the solution

In order for something to dissolve, the solute must be capable of being attracted to the solvent

Some ionic compounds will not dissolve in water (to be discussed later) For molecular compounds, we have the Golden Rule: “like dissolves like”: Polar substances dissolve in polar solvents

Non-polar substances dissolve in non-polar solvents

Size is a factor (especially for non-polar substances)

Solubility

Solubility is the amount of solute that will dissolve in a given quantity of solvent

Example: oil is insoluble in water but sugar is soluble

Which is more soluble in water – NH₃ or CH₄?

NH₃— Difference in polarity. Since H20 is polar, more polar substances will dissolve better.

Which is more soluble in water – CH₃OH or C₆H₁₃OH?

CH₃OH – Molecule size. Smaller molecules dissolve better than larger molecules.

*On a test it will be about either polarity or size, not both.

Ionic Compounds in Solution

Example: Show the dissociation of Na₂CO₃ in water.

We will use the subscript (aq) to show that they are ions that are dissolved in water This process is also reversible – the solid will reform if you dry it out

Electrolytes

When ionic compounds dissociate in water, we called them electrolytes which when present allow the solution to conduct electricity

Strong electrolytes conduct current efficiently

Weak electrolytes conduct only a small current due to a small amount of ions present Non-electrolytes - no current flows, only molecules present.