Unit 6: Solubility and Redox Reactions

Section 1: The Facts About Solutions

* Introduction
* Terms Used In Defining Solutions
* Defining Solutions
* Likes Dissolve Likes
* Pressure
* Temperature

Introduction

Solutions, in chemistry, are very important. This is why it is important to
know what the composition of a solution is. When we talk about solutions, we
normally talk about the concentration of the solutions, but there are other
ways to describe a solution. Also sometimes, we need to understand how to
increase or decrease the solubility of a substance. In this section, we will
discuss the solutions and different ways to effect the solubility of
substances.

Terms Used In Defining Solutions

Before we start defining solutions, we need to define a few terms, and we
must understand how solutions are made. First, solutions are made of solute
(the substance being dissolved) and solvent (the dissolving medium.) In most
cases the solute is a solid, and the solvent is a liquid (but remember, this
isn't true all of the time.)

Defining Solutions

There are 4 ways that we will discuss here to define solutions. These ways
are very straightforward, so I will save you the irritation of reading
paragraphs and just give you the formulas.

* Mass Percent = (grams of solute / grams of solution) x 100

* Mole Fraction of Compound A = (moles of A / moles of A + moles of B)

* Molality = (moles of solute / kilograms of solvent)

* Molarity = (moles of solute / liters of solution)

 

Likes Dissolve Likes

Likes dissolve likes is the basic rule that we follow to figure out what
solvent is necessary to dissolve a specific solute. For instance, water is a
good solvent for NaCl, because NaCl is polar and water is polar. Water is
not a good solvent for SO3, because water is polar and SO3 is nonpolar. So
basically what I am saying is ... polar solvents dissolve polar solutes ...
nonpolar solvents dissolve nonpolar solutes ... likes dissolve likes.

Pressure

Pressure does not play much of a role in the solubility of a solid into a
liquid, but it does play a big role in the solubility of a gas. For
instance, soda is a carbonated drink, and it contains CO2 (that is why it
bubbles). There is a mathematical representation between pressure and
concentration. It is called Henry's Law, and it states that the partial
pressure of the gas (P) is directly proportional to the concentration of the
solution (C) or in terms of the variables ...

P = k C
where k is the constant for that particular gas

Temperature

Temperature does play a role in the solubility of things, but there is no
set rule for predicting the solubility change by changing the temperature.
Some people think that as temperature increases, solubility increases, but
this in not always the case. Most of the time this is true, but sometimes
substances dissolve more slowly. An example of a substances that dissolves
more slowly when heated is sodium sulfate.