Mixtures and Solutions

What are mixtures and solutions?

Mixtures and solutions contain multiple components, as opposed to pure substances, which contain one component.

Additionally, the substances in a mixture maintain their original form rather than undergoing a chemical reaction. Notice, that chemical reactions mix to form new substances as products. For example, water is a pure substance (not a mixture or a reaction). However, it can be combined with other chemicals to form a mixture, a solution, or a reaction.

Water can be combined with baking soda, and it will fizzle up, undergoing a chemical reaction to form carbon dioxide. Water can also be combined with lemon and sugar to make lemonade. Whereas the water and baking soda reacted to form a new substance, the mixture of water, lemon, and sugar maintain their individual chemical components. They do not react to create a new chemical; they just mix to make a tasty drink.

But is lemonade a mixture or a solution? Well actually, it is both! Let’s dive a little deeper to understand why.

The Meaning of Mixtures and Solutions

We discussed earlier how physical mixtures are made up of two or more substances that maintain their original chemical properties after being mixed. This means that no chemical reaction occurs between components in a mixture and no new substances are formed. The components do not interact strongly at the molecular level, they just exist together as a mixture.

There are two different types of mixtures: homogeneous mixtures and heterogeneous mixtures.

For example, lemonade is a homogeneous mixture because the lemon and sugar mix with the water to establish lemonade.

While lemonade is formed from three different substances, they mix and look like one.

However, if we add sand to water, it will not dissolve. They will still be mixed, but the sand component will be noticeably distinguishable from the water component.

A homogeneous mixture mixes fully to form a uniform mixture like air and juice. In contrast, a heterogeneous mixture mixes to form noticeably distinguishable parts like ice in a soda and vinaigrette salad dressing.

The different substances mix to constitute a uniform mixture that visually seems like just one thing.

However, the substances are not bonding together to undergo a chemical reaction, they are just dissolving to comprise a homogeneous solution.

In the lemonade example, the sugar and the lemon juice are both solutes (the components of the solution that are being dissolved), and the water is the solvent (the component of the solution that is dissolving the solutes).

The solute is usually the solid involved in the reaction. While the solvent is what the solute dissolves in, and together they form a solution.

Examples and Properties of Mixtures and Solutions

As we previously discussed, mixtures maintain their original chemical properties after being mixed.

Solutions also maintain their original chemical properties after being mixed, but they are always fully homogenous, containing solutes dissolved in solvents.

In reactions, however, substances are mixed and result in a change in chemical properties. Let’s consider some examples and their properties to make this clearer.

Heterogeneous Mixtures

For example, when we combine a mixture of water \(H_2O\) and benzene \(C_6H_6\), we can mix the substances together in one container, but they do not react.

We can pour them both into the same container and shake them up.

However, the benzene molecules will aggregate together, causing the organic solvent to separate from the aqueous water, and there will be a noticeable separation of the two layers in this heterogeneous mixture.

This works for both liquids and solids. For example, aluminum hydroxide \(AlOH\) is a solid that does not dissolve in water. We can mix \(AlOH\) and water in the same container and shake them up to mix them together. However, the heterogeneous mixture will have noticeably separate components of the solid and the liquid.

Homogeneous Mixtures

We can also create a homogeneous mixture of hexanes and ethyl acetate.

These two organic solvents will mix together to create one layer.

While they are not reacting to form a new product, they are mixed in a way that they appear to be one liquid.

However, this homogeneous mixture of hexanes and ethyl acetate does not count as a solution because it does not contain a solute, but rather two solvents.

Solutions

Solutions are homogeneous mixtures that contain solutes and solvents.

For example, if we mix table salt (sodium chloride, \(NaCl\)) in water, the \(NaCl\) will dissolve in the water. The \(NaCl\) would be the solute, and the water would be the solvent.

Although they are forming a homogeneous mixture, they are not reacting. The two components can be separated into their original, unchanged components using separation techniques discussed below.

Reactions

As opposed to mixtures and solutions, reactions result in a change in chemical properties after two substances are mixed.

For example, when we mix sodium \(Na\) and chlorine \(Cl\), the substances will undergo a reaction to form sodium chloride \(NaCl\), or table salt. A reaction has occurred between the \(Na\) and the \(Cl\) to form the product \(NaCl\), and the product cannot be separated into its original components with separation techniques.

Techniques for separating mixtures, solutions, and solution of mixtures

There are a variety of different techniques that can be used to separate mixtures and solutions depending on the constituents.

For example, filtration can be used to separate heterogeneous mixtures.

• A mixture of \(AlOH\) in water, which we determined to be heterogeneous since the \(AlOH\) does not dissolve, can be passed through a filter, causing the water to go through the filter and the \(AlOH\) to be caught by the filter, separating them from each other.

• Filtration cannot be used for solutions because they are uniform, and the entire solution will pass through the filter without separation.

Phase changes like evaporation can also be used to separate both mixtures, and solutions.

• For example, a mixture of sand and water can be boiled.

• This will cause the water to turn to vapor, and the sand will be left behind. This would work similarly for a solution of \(NaCl\) in water.

Distillation is a phase change separation technique similar to boiling, used only for solutions.

• With distillation, a volatile compound can be separated from solvent by heating the solution to vaporize it, and then cooling it to return it to its liquid state and collecting it separate from the other constituents.

• The compound will evaporate at a different temperature than the solvent and be collected separately.

Chromatography is another technique for separating solutions.

• In this technique, the solution moves through silica gel and is dissolved with different solvents. The solvents are used to carry the solutes through the silica gel, but the solutes move through the silica gel at different rates.

• This allows the solutes to come off the gel at different times to collect them separately and purify them from each other.

• The type of solvent used for the separation depends on polarity and is determined by thin-layer chromatography. This is a thin plate of silica gel that resembles column chromatography on a smaller scale.

All of this can be summarized as:

Figure 3: Common ways to separate mixtures shown. Daniela Lin, Study Smarter Originals.

The common ways we can separate mixtures and solutions include chromatography, filtration, distillation, and evaporation as shown in figure 3 above.