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Solutions Concepts

Key Concepts

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Types of Solutions

If we consider solutions made up of only two components, known as binary solutions, we can see that there are 9 possible combinations:

solvent is a gas solvent is a liquid solvent is a solid
1. one gas dissolved in another gas

2.a liquid dissolved in a gas

3.a solid dissolved in a gas

4. a gas dissolved in a liquid

5.one liquid dissolved in another liquid

6. a solid dissolved in a liquid

7. a gas dissolved in a solid

8. a liquid dissolved in a solid

9. one solid dissolved in another solid

The table below gives an example of each type of solution.

Examples of Binary Solutions
  Type of Solution Solute Solvent Example
Gaseous
Solutions
gas dissolved in gas gas gas oxygen dissolved in nitrogen
liquid dissolved in gas liquid gas chloroform dissolved in nitrogen
solid dissolved in gas solid gas dry ice dissolved in nitrogen
Liquid
Solutions
gas dissolved in liquid gas liquid carbon dioxide dissolved in water
liquid dissolved in liquid liquid liquid alcohol dissolved in water
solid dissolved in liquid solid liquid sugar dissolved in water
Solid
Solutions
gas dissolved in solid gas solid hydrogen dissolved in palladium
liquid dissolved in solid liquid solid mercury dissolved in gold
solid dissolved in solid
(see Binary Alloys)
solid solid copper dissolved in nickel

The most common binary solutions you will use in your chemistry course will be aqueous solutions.

An aqueous solution is one in which water is the solvent.
Water is a liquid at room temperature and pressure, so the formula for liquid water used as a solvent is H2O(l).

The solute, the substance that dissolves in the liquid water solvent may be a solid, liquid or gas.
The state of the solute is indicated by placing an s for solid, or a l for liquid, or a g for gas in round brackets, (), after the chemical formula for the solute:

In a chemical equation, an aqueous solution is indicated by placing aq in round brackets, (aq), after the formula of the solute.

Examples of aqueous solutions are shown in the table below:

Example Solute Solvent Solution
sodium chloride dissolved in water sodium chloride
NaCl(s)
water
H2O(l)
aqueous sodium chloride solution
NaCl(aq)
ethanol dissolved in water ethanol
C2H5OH(l)
water
H2O(l)
aqueous ethanol solution
C2H5OH(aq)
ammonia dissolved in water ammonia
NH3(g)
water
H2O(l)
aqueous ammonia solution
NH3(aq)

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Solvation: Dissolving a Solute in a Solvent

Dissolving a solute in a solvent can be thought of a two step process:

  1. attractive forces between the solute particles are broken
  2. solvent particles are attracted to, and completely surround, each solute particle

The process in which solute particles are completely surrounded by solvent particles is known as solvation.
Note that when water is the solvent, the process of solvation is usually referred to as hydration.

If we pour a small amount of liquid ethanol, C2H5OH(l), into a large amount of liquid water, H2O(l), the attractive forces between the ethanol molecules are broken and each ethanol molecule is attracted to water molecules so that water molecules completely surround each ethanol molecule. Because the molecules making up the solution are in constant motion, the ethanol molecules become evenly disperserd through the solution resulting in an homogenous aqueous solution.
Note that there has NOT been a chemical change, ethanol molecules are still ethanol molecules and water molecules are still water molecules, there is no rearrangement of atoms to form a new chemical substance.

We could represent a small amount of ethanol dissolving in excess water as:

word equation: ethanol liquid + water liquid aqueous ethanol solution
chemical equation: C2H5OH(l) + H2O(l) C2H5OH(aq)

When chemists write a chemical equation to show the process of dissolving a small amount of solute in excess water, the water molecules are often omitted from the equation because the presence of water molecules as the solvent is implied when an aqueous solution is formed.

word equation: ethanol liquid aqueous ethanol solution
chemical equation: C2H5OH(l) C2H5OH(aq)

However, the states of matter for the solute and the solution MUST be shown.

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Concentration of Solutions

There are two main ways to express the concentration of a solution:

Note that in either case the amount of solute is always expressed first.

1. Concentration of a solution is often expressed in terms of the amount of solute present in the total amount of solution.
The amount of solute and/or solution can be expressed in terms of mass, volume, or number of particles.

Concentration: amount of solute present in solution
concentration term amount of solute as amount of solution as concentration units
percent by mass
% w/w or % m/m
mass mass g/100 g
mass/volume percentage
w/v % or m/v %
mass volume g/100 mL
volumes/volume percentage
v/v %
volume volume mL/100 mL
parts per million
ppm
mass mass mg kg-1, μg g-1
mass volume mg L-1, μg mL-1
mole percent
X
moles moles %
molarity
M
moles volume mol L-1
molar, M

2. Concentration of a solution can also be expressed as the amount of solute dissolved per unit of solvent.

Concentration: amount of solute per amount of solvent
concentration term amount of solute as amount of solvent as concentration units
weight ratio percentage
mass mass g/100 g solvent
molality
moles mass mol kg-1
molal , m

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Dilute and Concentrated Solutions

The terms dilute and concentrated refer to the relative concentrations of two or more solutions.

When comparing two or more binary solutions made up of the same solute and solvent:

Dilute Solution Concentrated Solution
O = 1022 solute particles
Black box represents a volume of 1 L of solution
O = 1022 solute particles
Black box represents a volume of 1 L of solution
                    O                  
                                  O    
      O                                
                          O            
          O                            
                                      O
        O           O                  
                O                 O    
  O                       O            
              O                   O    
        O                 O            
                O                     O
No. of solute particles = 6 × 1022 in 1 L of solution No. of solute particles = 12 × 1022 in 1 L of solution
No. of solute particles = 1.2 × 1023 in 1 L of solution
Fewer solute particles in 1 L of solution More solute particles in 1 L of solution
Solution is more dilute. Solution is more concentrated.

If the solute is coloured, when it dissolves in the solvent the resulting solution will be coloured.
The deeper the colour, the more concentrated the solution is.

For example, copper sulfate is a blue colour.
When copper sulfate dissolves in water, the blue copper(II) ions are evenly distributed throughout the solution so that the solution appears to be a uniform blue colour.
The more blue copper(II) ions there are present in solution, the more concentrated the solution is, and the deeper the blue colour.
The fewer blue copper(II) ions there are present in the solution, the less concentrated the solution is, and the lighter the blue colour.

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Saturated and Unsaturated Solutions

A solution is said to be unsaturated if more solute could be dissolved in a given amount of the solvent at a specified temperature.

A solution is said to be saturated when no more of the solute can be dissolved in a given amount of the solvent at a specified temperature.

The solubility of copper sulfate in water at 25oC is known to be 14 g/100 g of water.
This means that a maximum of 14 grams of copper sulfate can be dissolved in 100 grams of water at 25oC.
Imagine adding 7 grams of solid copper sulfate to 100 g of water in a beaker at 25oC. All the copper sulfate would dissolve and the solution would appear uniformly blue.
We can add another 7 grams of solid copper sulfate to this solution and all of this copper sulfate will also dissolve resulting in a solution with an even deeper blue colour.
But, if we add another 7 grams of copper sulfate to this solution, the copper sulfate will not dissolve because the solution has become saturated in copper sulfate. The colour of the solution will not get any darker and the excess solid copper sulfate will sink to the bottom of the beaker.

Unsaturated Solution Saturated Solution Saturated Solution
7 grams of copper sulfate ≈ 2.7 x 1022 copper sulfate particles
O = 4.4 × 1021 copper sulfate particles
Black box represents the volume of 100 grams of water at 25°C
14 grams of copper sulfate ≈ 5.3 × 1022 copper sulfate particles
O = 4.4 × 1021 copper sulfate particles
Black box represents the volume of 100 grams of water at 25°C
O = 4.4 × 1021 copper sulfate particles
Black box represents the volume of 100 grams of water at 25°C
  = undissolved copper sulfate particles
                    O                  
                                  O    
      O                                
                          O            
          O                            
                                      O
        O           O                  
                O                 O    
  O                       O            
              O                   O    
        O                 O            
                O                     O
<
      O                 O              
                  O               O    
      O                 O              
                O                      
  O                 O         O        
          O       O O O             O  
    O           O O O O O              
No. copper sulfate particles dissolved in 100 g of water
= 6 × 4.4 × 1021 = 2.6 × 1022
No. copper sulfate particles dissolved in 100 g of water
= 12 × 4.4 × 1021 = 5.3 × 1022
No. copper sulfate particles dissolved in 100 g of water
= 12 × 4.4 × 1021 = 5.3 × 1022
Solution is unsaturated because, if more copper sulfate is added to the solution some (if not all) of it will dissolve. Solution is saturated because if any more copper sulfate is added to the solution it will NOT dissolve. Solution is saturated because some copper sulfate has not dissolved in the solution.

Quick Question 1

Which solution is more dilute?
(a) 5 g of KNO3(s) dissolved in 1 L of water.

(b) 24 g of KNO3(s) dissolved in 1 L of water.