When a solute is not completely dissolved in a solvent it forms?

Key Concepts

⚛ A homogenous mixture is usually called a solution.

⚛ A substance is said to be soluble if it dissolves in a solvent to form a solution.

⚛ A substance is said to be insoluble if it does not dissolve in a solvent to form a solution.

⚛ The substance that dissolves in a solvent to form a solution is referred to as the solute.

⚛ A solution is formed when solvent particles completely surround solute particles which are then uniformly dispersed.

· When a solute dissolves in water, the solution is referred to as an aqueous solution.

⚛ The amount of solute present in the solution is referred to as the concentration of the solution.

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

· A dilute solution has less solute present in the solution than a concentrated solution.

⚛ 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 a solute in a given solvent refers to the maximum amount of solute that can be dissolved in that solvent at a given temperature.

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Types of Solutions

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

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:

⚛ solid solute, place (s) after the formula for the solute:

for example NaCl(s)

⚛ liquid solute, place (l) after the formula for the solute:

for example C2H5OH(l)

⚛ gaseous solute, place (g) after the formula for the solute:

for example NH3(g)

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:

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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 dispersed 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:

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.

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Concentration of Solutions

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

• quantity of solute present per quantity of solution
• quantity of solute present per quantity of solvent

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

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

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

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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:

• The more dilute solution is the one which contains fewer solute particles in the solution
• The more concentrated solution is the one which contains more solute particles in the solution

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 quantity 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 quantity of the solvent at a specified temperature.

The solubility of copper sulfate in water at 25°C is known to be 14 grams per 100 grams of water (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 25°C. Imagine adding 7 grams of solid copper sulfate to 100 g of water in a beaker at 25°C. 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

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

Number of copper sulfate particles dissolved in 100 g of water
= 6 × 4.4 × 1021 = 2.6 × 1022

Solution is unsaturated because, if more copper sulfate is added to the solution some (if not all) of it will dissolve.

Saturated Solution (a)

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

Number of copper sulfate particles dissolved in 100 g of water
= 12 × 4.4 × 1021 = 5.3 × 1022

Solution is saturated because if any more copper sulfate is added to the solution it will NOT dissolve.

Saturated Solution (b)

O = 4.4 × 1021 copper sulfate particles Black box represents the volume of 100 grams of water at 25°C

Number of copper sulfate particles dissolved in 100 g of water
= 12 × 4.4 × 1021 = 5.3 × 1022

Solution is saturated because some copper sulfate has not dissolved in the solution.

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Sample Question: Introduction to Solutions

Which solution is more dilute?

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