5. HOMOGENEOUS MIXTURES OR SOLUTIONS

Talking book

Some mixtures are not easy to recognise because we can't see where each substance is. If we mix sugar and water, we know the sugar is there because we taste it, but we can't see it. This is a homogeneous mixture.

In this example, we say that the sugar has dissolved in the water, which is why it is called a homogeneous mixture or solution. In a solution, the particles of all the substances are mixed together so well that it's impossible to distinguish them. All solutions have two components:

The solvent is the main component in a solution.
The solute is the other substance or substances in a solution, found in smaller quantities.

The solvent and the solute can be found in any state of aggregation. The solvent is most often a liquid, usually water, in which case we talk about an aqueous solution. Here are some examples:

Solute	Solvent	Example
Solid	Liquid	Water with sugar
Liquid	Liquid	Water with alcohol
Gas	Liquid	Fizzy drinks

Although, as we said, solvents are often liquids, there are also solvents that are not liquid (they can be solid or gas). For example:

Air and natural gas. All the components are gaseous.
Alloys: solutions formed by two or more chemical elements of which one is a metal, such as bronze, steel or brass. All the components are solids.
Weblink 3: Solute vs. solvent

Brass is an alloy, where all the components are solid.

Key concepts

In a solution, the solvent is the substance found in greater quantity. The other substance is the solute.

5.1. Calculating concentrations

Talking book

To work with solutions, we need to know the proportion of the solute and solvent, that is, the concentration.

The concentration of a solution indicates the quantity of the solute in a given quantity of a solvent or of a solution.

5.1.1. Percent composition (by mass)

Talking book

There are many ways to express a concentration but the easiest and most commonly-used one is the percent composition (by mass).

The percent composition of a solute in a solution is the mass of solute found in 100 units of the mass of the solution. If we use grams as the units of mass:

estilo tamaño 14px Percent espacio by espacio mass espacio paréntesis izquierdo solute paréntesis derecho igual fracción numerador mass espacio of espacio the espacio solute espacio paréntesis izquierdo normal g paréntesis derecho entre denominador mass espacio of espacio the espacio solution espacio paréntesis izquierdo normal g paréntesis derecho fin fracción multiplicación en cruz 100 fin estilo

It's not necessary to work in grams. You just have to make sure to use the same units of mass in the numerator and denominator.

The result will not have units because it is a percentage.

Talking book

To make a 925 sterling silver ring, a jeweller uses 15.73g of pure silver and 1.27g of copper. Calculate the percent composition of the solute in the alloy.

First, we have to work out what the solute is and what the solvent is in the solution (alloy):

Solute → copper (in a lower proportion)
Solvent → silver (in a higher proportion)

Next, we calculate the mass of the solution from the data:

m (solute) = 1.27g
m (solvent) = 15.73g
m (solution) = m (solute) + m (solvent) = 1.27g + 15.73g = 17g

Finally, we substitute our values in the equation for the percent composition of the solute:

$porcentaje espacio by espacio mass igual fracción numerador mass espacio of espacio the espacio solute espacio paréntesis izquierdo normal g paréntesis derecho entre denominador mass espacio of espacio the espacio solution espacio paréntesis izquierdo normal g paréntesis derecho fin fracción multiplicación en cruz 100 igual igual fracción numerador 1.27 espacio normal g entre denominador 17 espacio normal g fin fracción multiplicación en cruz 100 casi igual a 7.5 porcentaje$

Therefore, 925 Sterling Silver always contains 92.5% of pure silver and 7.5% of another metal, usually copper, as in this case.

5.1.2. Mass concentration

Talking book

Another common way to express a concentration relates the amount of solute to the volume of the solution.

The mass concentration (g/L) of a solute in a solution indicates the mass of the solute (in grams) that is dissolved in every litre of the solution:

normal g dividido por normal L igual fracción numerador mass espacio os espacio the espacio solute espacio paréntesis izquierdo normal g paréntesis derecho entre denominador volume espacio of espacio the espacio solution espacio paréntesis izquierdo normal L paréntesis derecho fin fracción

Remember the relationship that exists between units of capacity and volume: if we make a cube of 1 dm and we fill it up to the top with liquid, the volume of the liquid in the cube is 1 L. So:

1 dm³ is equivalent to 1 L

Talking book

. A student has to prepare an iodine alcoholic solution by dissolving 15g of iodine in alcohol to obtain a solution with a volume of 250mL. Calculate the mass concentration of the final solution.

As in the previous example exercise, we first have to identify the solute and the solvent:

Solute → iodine (in a smaller quantity)
Solvent → alcohol (in a greater quantity)

Next, given the grams of the solute (15g), we have to calculate the volume of the solution in litres:

m (solute) = 15g
V (solution) = 250mL = 0.25L

Finally, we substitute these values into the mass concentration equation:

$normal g dividido por normal L igual fracción numerador mass espacio of espacio the espacio solute espacio paréntesis izquierdo normal g paréntesis derecho entre denominador volume espacio of espacio the espacio solution espacio paréntesis izquierdo normal L paréntesis derecho fin fracción igual fracción numerador 15 espacio normal g entre denominador 0.25 espacio normal L fin fracción igual 60 espacio normal g dividido por normal L$

Therefore, the solution will have a concentration of 60g/L.

Key concepts

The mass concentration of a solution gives the quantity of solute in a certain quantity of a solution.
There are different ways to express the concentration: in percent composition (by mass) or in g/L.

5.2. Preparing solutions

Talking book

Physiological saline solution is made with sodium chloride (NaCl, otherwise known as salt) in water with 0.9% mass and is used a lot in hospitals. Here's how to prepare 100mL.

**Calculate the mass of the solute** (NaCl) that we need: to do this, we just have to remember that 0.9% solute means that for every 100 g of (saline) solution, there is 0.9g of NaCl.

**Weigh the NaCl** (0.9g) with a digital scale, using a beaker.

**Add a little distilled water** to the beaker (in this case about 20mL is sufficient). Stir well with a glass rod until it dissolves completely.

With a funnel, **pour the solution** you have just obtained into a graduated flask of the volume you need (100mL). **Rinse the beaker** a few times to get out all the remains of the NaCl.

**Add water to the flask up to the mark.** We use a dropper to reach the exact mark of volume required so that we don't go over. **Put a top** on the graduated flask and **shake** the contents well.

Key concepts

In the process of making a solution, the particles of the solute spread out among the particles of the solvent.

5.3. Solutions and the Kinetic Particle Theory

Talking book

When we mix two substances to make a solution, the solute particles leave their original position and get distributed among the particles of the solvent; that way, the particles of the solute move in to occupy positions that were previously occupied by solvent particles.

Weblink 4: Kinetic Particle Theory

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Activity 15

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Activity 16

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Activity 17

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Activity 18

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Activity 19

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Activity 24

Read the label on a bottle of mineral water. What solutes does it contain?

Name two non-liquid solutions that are not mentioned in your book.

Find out about the mixture of gases that make up natural gas.

Is the silver that jewellers use a pure substance or a mixture? What about 1, 2 and 5 cent coins? Use the Internet to find out.

Calculate the mass of acetic acid in 10 g of a commercial vinegar with a label indicating a percent composition of 6%.

Acetic acid is the .

Water is the .

Mass of acetic acid = g

Calculate the mass concentration in g/L of a solution that has 7 g of a pure substance in half a litre of water.

Mass concentration = g/L

In how many litres of water would we have to dissolve 100 g of salt to obtain a solution with a mass concentration of 5 g/L?

In L of water.

What is the mass concentration of a solution with 12 g of potassium chloride and 300 cm³ of water?

Mass concentration = g/L

What is the percent composition of sugar in water if it contains 30 g of solute in 600 g of water?

% of sugar

We know that the mass concentration of sodium chloride in a solution is 8 %. How many grams of sodium chloride is dissolved in 75 g of the solution?

mass of NaI = g

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5. HOMOGENEOUS MIXTURES OR SOLUTIONS

5.1. Calculating concentrations

5.1.1. Percent composition (by mass)

5.1.2. Mass concentration

5.2. Preparing solutions

5.3. Solutions and the Kinetic Particle Theory

Analyse

Apply

Activity 15

Activity 16

Activity 17

Activity 18

Activity 19

Activity 20

Activity 21

Activity 22

Activity 23

Activity 24