Mixtures and Solutions

Contributor: April Stokes. Lesson ID: 10783

What is Outrageous Ooze? Ever create your own colloid? Why doesn't sandy water taste as good as lemonade? Watch videos, go scavenger hunting, and mix up stuff to learn about mixtures and solutions!

categories

Physical Science

subject
Science
learning style
Kinesthetic, Visual
personality style
Otter
Grade Level
Middle School (6-8)
Lesson Type
Dig Deeper

Lesson Plan - Get It!

Audio:

mixture question

Discuss the above questions with your teacher or parent.

Is the sugar 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?

The question above addresses 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, but no matter how much you mix, all the individual pieces are all 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 the 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 that would change a different physical property?

What sort of effect does creating mixtures and solutions have on the state of matter of various objects?

The water remains a liquid and the rock remains a 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 — and 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? Take a minute or two to talk about this with your parent or teacher.


Let's look at how a solution is formed.

When you put a sugar cube into water, the liquid water molecules are attracted to 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 molecules of the sugar and water are evenly mixed. When all the sugar has dissolved in this way, you have a solution of sugar water.

The water in the sugar-water is a solvent. A solvent is a substance that dissolves other materials. The sugar is a solute, the substance that is dissolved. 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) is a liquid-liquid-gas solution. Can you think of other liquid-liquid solutions?

It can be tricky to identify which is the solvent and which is 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 has the ability to break down the other is the solvent. The substance that seems to disappear — become part of the other substance — is the solvent.


Types of solutions and their various states

Solutions comes in all states of matter, and even mixtures of states of matter, as we 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 types of molten (melted or liquid) metal that are mixed together. 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, and 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 rate of dissolution. Both stirring and heating the solution increase the rate of collisions and decrease the amount of 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. In a solution, all the ingredients are uniformly distributed at a molecular level 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 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. Take a look at this article, Miscible Liquids: Definition & Examples, from Study.com, for a better understanding.


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.

So, did you come up with any ideas regarding the properties of non-Newtonian fluids yet?

How about a riddle: What do jellyfish, Dr. Seuss's infamous Oobleck, marshmallows, 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, we need to define ideal fluids. Let's take water, the universal solvent, as our 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 in the same way. In fact, 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 extremely common. Despite their odd physical properties, those same properties make them very useful products and materials. Foam, gel, glue, and clay are all examples of colloidal solutions. There are many colloidal solutions in food products like pudding, milk, butter, and jelly. 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 Great Picnic Mix Up: Crash Course Kids #19.1 to review some of the terms and concepts you just learned:

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