Composition of Mixture

Mixture Definition

Before we discuss the different kinds of mixtures, we should first define what a mixture is.

We see different kinds of mixtures every day, from salads to saltwater, and even the air we breathe are all mixtures! There are two types of mixtures: homogeneous and heterogeneous. First, we will be explaining homogeneous mixtures.

Homogeneous Mixtures

A great example of a homogeneous mixture is saltwater. When the salt is dissolved, it is dispersed evenly throughout the water. The amount of salt that is dissolved in the water can change, but the distribution is the same.

Here's a diagram to illustrate:

Saltwater is a homogeneous mixture independent of the concentration of salt. StudySmarter Original

Even though each mixture has a different concentration of salt, the distribution is the same. If you cut each sample in half, the composition within each would be the same for both halves.

Heterogeneous mixtures

Now, let's talk about heterogeneous mixtures.

Heterogeneous samples are composed of two or more phases. A great example of a heterogeneous mixture is chocolate chip cookies. The distribution of chocolate chips is essentially random.

Consider chocolate cookies as an example, the two phases are the cookie and the chocolate chips. The cookie and chocolate chips individually are considered homogeneous since they are uniform in their ingredients (ex. the cookie has an even mixture of flour, egg, sugar, etc.). However, when they are combined, they form a heterogeneous mixture.

Separation of Mixtures

Unlike chemical compounds, mixtures can be separated using non-chemical means. The separation methods rely on the different physical attributes of each component, such as density, boiling point, and size of particles.

There are 4 common ways that chemists use to separate mixtures:

1. Evaporation

2. Distillation

3. Filtration

4. Separation through a separation funnel

First up, we have evaporation. This technique is typically used to separate a liquid solvent (dissolving substance) and a solid solute (dissolved substance). The solvent is boiled off, and the solute is left behind in its solid state.

Next is distillation. This process focuses on the different volatilities (likelihood to evaporate) of the different liquids in the mixture. Below is what a distillation apparatus looks like:

Distillation experiment. Wikimedia commons.

The mixture is placed in the round-bottom flask and heated. The more volatile liquid will evaporate first, then be condensed and collected as a liquid.

The third method is filtration, which is used to separate an (undissolved) solid from a liquid.

Filtration set up. Wikimedia commons.

A filter paper is placed inside a funnel over a flask. The solid will be caught by the filter paper, and the liquid will pass through it into the flask.

Lastly, we have separation using a separation funnel. This method utilizes the different densities of the liquids in a mixture.

A separation funnel. Wikimedia commons

The liquid mixture is poured into the funnel. The (closed) funnel is shaken and inverted until the liquids have been separated into phases, as you'll see above. The stopcock (handle) is turned to open the funnel to let the lower phase out

Composition of Mixtures: Examples

When looking at a mixture, it's common to want to know its composition. This is especially important in the pharmaceutical industry because we always want to know the percentage of a drug within a pill.

When calculating composition, we are utilizing percent composition.

Let's walk through an example of how to determine the composition of a given substance in a mixture.

Let's do one more example:

Gas Mixtures and Partial Pressure

While we calculated the composition of a gas mixture in the previous example, you normally would be measuring gases by their pressure or molar amount. Instead of using percent composition, we use the mole fraction.

Here's how we relate these two variables, PV = nRT, using the ideal gas law:

$$\frac{P_{A}}{P_{tot}}=\frac{\frac{n_{A}R}{T}}{\frac{n_{tot}R}{T}}=\frac{n_A}{n_{tot}}=\chi_A$$ Rearranging this we get: $$P_A=\chi_AP_{tot}$$ Now that we have our formulas let's work on a problem.