Mixtures and Solutions

Lesson Plan - Get It!

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• What happens to sugar in water?
• Is it gone? If so, why?

• What would happen if you put sand in a glass of water, stirred, and waited for five minutes?
• Would you be able to separate the sand from the water?

These questions address the difference between a mixture and a solution.

You first need to start with pure substances, like water and salt. A pure substance has a definite and constant composition.

A pure substance can be either an element or a compound, but the composition of a pure substance will never vary. A water molecule will always have two hydrogen atoms and one oxygen atom.

Mixtures maintain the physical properties of each of the pure substances. You can create a mixture of sand and water, and after a few minutes, you can separate the sand from the water.

In a mixture like sand and water, when the particles settle to the bottom or are too big to dissolve, you have what is called a suspension.

Think of suspension mixtures as salads. You can put different vegetables or fruits together in a bowl. Still, no matter how much you mix, all the individual pieces are suspended there, holding each other up, not one dissolving into another. You can pick apart the individual elements that make up that salad.

A salad is also a good example of a heterogeneous mixture, a mixture made up of different substances that remain physically separate.

A solution is a mixture in which the particles of a different substance are evenly distributed and are too small to see with the naked eye. A solution appears to be a single substance.

The sugar and water make a solution. The sugar crystals dissolve in the water, changing the physical properties of the liquid. The water now takes on the sweet taste of sugar! The same physical change (taste) would occur with salt or lemon juice.

Solutions are considered homogeneous mixtures because the molecules of each substance are equally distributed and have become one new solution.

• Can you think of another substance that can be added to water to change a different physical property?

• How do mixtures and solutions affect the state of matter in various objects?
• The water remains liquid, and the rock remains solid, but what about the salt and sugar when they become part of the water?

Solutions can be separated; it's just a bit more tricky than separating a mixture.

For example, you can separate the sugar from the water by boiling — thus evaporating — the water. You could also use filtration, magnets, other forms of heat, freezing, and various other means of producing a physical change.

One More Fun Fact

• Did you know that there is a fourth state of matter, called a colloidal or non-Newtonian fluid, that comes from combining two substances?
• Based on the name non-Newtonian, what qualities do you think this type of matter possesses?

How Solutions Form

When you put a sugar cube into the water, the liquid water molecules attract the solid sugar molecules. Sugar molecules at the surface of the cube pull free. They move about the water molecules and fill the spaces between them.

After a while, all the sugar and water molecules are evenly mixed. You have a sugar-water solution when all the sugar has dissolved in this way.

The water in the sugar-water is a solvent. A solvent is a substance that dissolves other materials. Sugar is a solute, the dissolved substance.

All solutions have solvents and solutes. In this instance, the solution is made of a solid dissolving in a liquid. But solutions can be made from liquid solutes as well.

For example, flavored syrup and carbonated water (commonly known as soda) are liquid-liquid-gas solutions.

• Can you think of other liquid-liquid solutions?

It can be tricky to identify the solvent and the solute but think of the word dissolve. Dissolve and solvent both have the letter v. A solvent has the power to dissolve the solute.

Whichever substance can break down, the other is the solvent. The substance that seems to disappear — become part of the other substance — is the solvent.

Solution Types and Their Various States

Solutions come in all states of matter, and even mixtures of states of matter, as you saw with the soda example. Solutions can be solid-liquid, liquid-liquid, gas-liquid, solid-solid, solid-gas, and gas-gas mixtures.

Air is an example of a gas-gas solution. Gasoline is an example of liquid-liquid, and carbonated beverages are examples of liquid-gas solutions.

Some solutions even change states, like metal alloys. Alloys begin with two mixed types of molten (melted or liquid) metal. When they cool and re-harden, the resulting substance looks uniform, but it now has the combined properties of both metals.

Many coins are made of metal alloys. Steel is also an example of a solid-in-solid mixture.

The particles of a substance move faster when they are at a higher temperature. (Check out the Elephango lesson found under Additional Resources in the right-hand sidebar for more about how heat affects matter!)

The solution process depends on contact between particles of the solvent and particles of the solute; any action that increases the number of collisions between the solvent and solute particles tends to increase the dissolution rate.

Stirring and heating the solution increases the collision rate and decreases the time it takes for the solid solute to dissolve.

To sum up, when one substance is dissolved into another, a solution is formed, unlike the situation when the compounds are insoluble, like sand in water.

All the ingredients are uniformly distributed at a molecular level in a solution, and no residue remains. If you mix lemon juice and water, it becomes lemon water, a whole new solution.

A solvent-solute mixture consists of a single phase (state of matter) with all solute molecules occurring as solvates (solvent-solute complexes), as opposed to separate continuous phases (different states of matter, like a rock in the water), like in suspensions and other types of non-solution mixtures.

The ability of one compound to be dissolved in another is known as solubility. If this occurs in all proportions, it is called miscible.

More About Solutions

• Why do some things become solutions while others become mixtures? And what's in-between?

Solutions can be further divided into two groups: true solutions and colloidal solutions.

In true solutions, the dissolved particles are so tiny, they cannot be seen with a microscope! When salt is dissolved in water, it creates a true solution.

The particles in colloidal solutions are a little bigger, big enough that they can be seen using some microscopes. But they are not as big as the particles that are in suspensions.

• Did you come up with any ideas regarding the properties of non-Newtonian fluids yet?

How about a riddle: What do jellyfish and quicksand have in common?

They are all colloidal solutions. Colloidal solutions have very interesting physical properties. This is mostly because they are non-Newtonian fluids.

Non-Newtonian fluids break the rules of ideal fluids described by Isaac Newton in the 1700s.

To understand this, you need to define ideal fluids. Take water, the universal solvent, as an example.

Imagine you're at a swimming pool on a hot summer day. If you decide to cannonball, the water will move out of your way, responding to the force of your body and creating a big splash. If, on the other hand, you choose to do a graceful dive and slide into the water, it will move, but not as quickly.

Ideal fluids move out of the way when you exert force against them, and the greater the force, the faster they move.

However, non-Newtonian fluids, like colloidal solutions, do not act similarly. They act in almost the opposite fashion. A fast, hard force will cause a colloidal solution to act like a solid, but a slow, even force will cause the colloidal material to flow like a liquid.

Colloidal solutions are widespread. Despite their odd physical properties, those properties make their beneficial products and materials. Examples of colloidal solutions are foam, gel, glue, and clay.

Many colloidal solutions in food products like pudding, milk, butter, and jelly exist. Building materials like cement, stucco, plaster, and paint are colloidal solutions. Even our bodies and other living organisms are made of colloidal solutions.

Just wait until you see what these solutions can do!

But first, watch the video below to review some of the terms and concepts you just learned.

Move on to the Got It? section to practice all that you learned.