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:
Natural gas is a gaseous mixture.
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:
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:
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:
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 dm 3 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:
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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