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. Putting Together and Breaking Apart
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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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