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You might be wondering why concrete and salt water are not new compounds when they are mixed together. The special trait of mixtures is that physical forces can still remove the basic parts. You can take the solid concrete and grind it up again. The crushed concrete can then be used as aggregate with new Portland cement. You might also sandblast concrete with decorative aggregate stones to reveal the stones trapped in the concrete. Salt water is even easier. All you have to do is boil the water off and the salt remains. It is as if you never mixed the two compounds. If the salt and water had reacted chemically, a new compound would have been created.

The thing to remember about mixtures is that you start with some pieces, combine them, and then you can do something to pull those pieces apart again. You wind up with the same molecules (in the same amounts) that you started with. The way you separate the molecules is as unique as the mixture. We have talked about grinding and boiling. If you have a mixture of salt and tiny pieces of iron (Fe), you can use a magnet to separate the iron from the mixture. Remember that gravity will help you separate both sand and oil from water if you wait a few minutes.

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Two classic examples of mixtures are concrete and salt water. If you live near the ocean, they surround you every day. Even if you're inland, you need to remember that your tap water also has many compounds inside, and they act the same way that salt does. Concrete is a mixture of lime (CaO), cement, water(H₂O), sand, and other ground-up rocks and solids. All of these ingredients are mixed together. Workers then pour the concrete into a mold and the concrete turns into a solid (as the cement solidifies) with the separate pieces inside.

While the cement hardening might be a chemical reaction, the rocks and gravel are held in place by physical forces. They are included in the mixture to increase the strength of concrete. The rocks and gravel are not chemically bonded to the cement. The gravel is also not evenly distributed. There are pieces of gravel here and there. You may have watched building construction before. They mix the concrete for hours to try and get all the little bits mixed evenly. Even with all that mixing, the concentrations of gravel still change from area to area.

Salt water is different. First, it's a liquid. Second, it's an ionic solution. Salt molecules separate into sodium (Na⁺) and chloride (Cl⁻) ions in the water.

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Let's finish up with a little information on emulsions. These special colloids (another type of mixture) have a mixture of oils and waters. Think about a bottle of salad dressing. Before you mix it, there are two separate layers of liquids. When you shake the bottle, you create an emulsion. As time passes, the oil and water will separate, because emulsions are mixtures.

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Amalgams are a special type of alloy. We like them because we think mercury (Hg) is a cool element. You might know mercury as "quicksilver" or the metal that is liquid at room temperature. Anyway, amalgams are alloys that combine mercury and other metals in the periodic table. The most obvious place you may have seen amalgams is in old dental work. The fillings in the mouths of your grandparents may have been amalgams. We already talked about mercury being a liquid at room temperature. That physical trait was an advantage when they made fillings. Let's say you have an amalgam of mercury and silver (Ag). When it is created, it is very soft. As time passes, the mercury leaves the amalgam and the silver remains. The silver left over is very hard. Voila! You have a filling!
NOTE: Never, ever, play with mercury! It is very poisonous. You shouldn't even touch it, because it will seep into your skin. Dentists don't usually use amalgams with mercury anymore, because some scientists think the mercury can get people sick. When there was extra mercury left in the fillings, it could seep into the blood stream. Most of you will never even have silver fillings. Many dentists use resin fillings, which are made up of plastic and very fine particles of glass.

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There are a few more words you might hear when people talk about mixtures. We can't cover all of them, but we'll give you a quick overview of the biggies. Alloys are basically a mixture of two or more metals. Don't forget that there are many elements on the periodic table. Elements like calcium (Ca) and potassium (K) are considered metals. Of course, there are also metals like silver (Ag) and gold (Au). You can also have alloys that include small amounts of non-metallic elements like carbon (C). Metals are the key thing to remember for alloys.

The main idea with alloys is that the combinations work better together than any of the metals do alone. Metallurgists (people who work with metals) sometimes add chromium (Cr) and/or nickel (Ni) to steel. While steel is already an alloy that is a very strong metal, the addition of small amounts of the other metals help steel resist rusting. Depending on what element is added, you could create Stainless Steel or Galvanized Steel. It's always about improving specific qualities of the original. Another good example of an alloy happens when metallurgists add carbon to steel. A tiny amount of carbon (a non-metallic element) makes steel stronger. These special carbon-steel alloys are used in armor plating and weapons.

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There are an infinite number of mixtures. Anything you can combine is a mixture. Think of everything you eat. Just think about how many cakes there are. Each of those cakes is made up of a different mixture of ingredients. Even the wood in your pencil is considered a mixture. There is the basic cellulose of the wood, but there are also thousands of other compounds in that pencil. Solutions are also mixtures, but all of the molecules are evenly spread out through the system. They are called homogenous mixtures.

If you put sand into a glass of water, it is considered to be a mixture. You can always tell a mixture, because each of the substances can be separated from the group in different physical ways. You can always get the sand out of the water by filtering the water away. If you were busy, you could just leave the sand and water mixture alone for a few minutes. Sometimes mixtures separate on their own. When you come back, you will find that all of the sand has sunk to the bottom. Gravity was helping you with the separation. Don't forget that a mixture can also be made of two liquids. Even something as simple as oil and water is a mixture.

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Mixtures are absolutely everywhere you look. Most things in nature are mixtures. Look at rocks, the ocean, or even the atmosphere. They are all mixtures, and mixtures are about physical properties, not chemical ones. That statement means the individual molecules enjoy being near each other, but their fundamental chemical structure does not change when they enter the mixture. If the chemical structure changed, it would be called a reaction.

When you see distilled water (H₂O), it's a pure substance. That means that there are only water molecules in the liquid. A mixture would be a glass of water with other things dissolved inside, maybe one of those powders you take if you get sick. Each of the substances in that glass keeps its own chemical properties. So, if you have some dissolved substances in water, you can boil off the water and still have those dissolved substances left over. If you have some salt (NaCl) in water and then boil off the water, the salt remains in the pan. The salt is left because it takes very high temperatures to meltsalt (even more to boil it).