Mixtures, Solutions and Concentration

Look around. Almost everything you see is made of more than one kind of matter mixed together. The salt water in the sea, the air you breathe, the lemonade in your glass, even granite rock — all of these are made of two or more substances combined. But not all combinations behave the same way. Some are blended so evenly you cannot see the parts; others are obviously chunky and easy to separate. Today you will learn how scientists sort matter into pure substances, mixtures, and solutions, and what concentration really means.

Pure Substance — Just One Kind

A pure substance is matter made of only ONE kind of stuff. A bar of pure gold is only gold atoms. A tank of oxygen gas is only oxygen molecules. Distilled water is only H2O molecules — no salt, no minerals. Pure substances have a fixed melting point, a fixed boiling point, and predictable properties, because every particle inside is the same. There are two types: elements (one kind of atom, like copper) and compounds (one kind of molecule, like pure water).

Mixture — Two or More Kinds, Not Blended

A mixture is two or more substances physically combined, but NOT chemically joined. The parts keep their own properties, and you can usually separate them again. Mix sand and iron filings — the iron is still magnetic, the sand is still sand, and a magnet pulls them apart. Pour oil on water — they sit in separate layers. Granite is a mixture of three minerals you can actually see. Mixtures often look uneven (called heterogeneous) — different spots have different stuff in them.

• Sand and pebbles — separate by sieving.
• Iron filings and sand — separate with a magnet.
• Oil and water — separate by letting them settle.
• Fruit salad, granite, dirt — chunky, you can see the parts.

Solution — A Special Kind of Mixture

A solution is a special mixture where one substance dissolves so evenly into another that you cannot see separate parts. Salt water looks like just water — but taste it and the salt is everywhere. The substance that gets dissolved is called the SOLUTE (the salt). The substance that does the dissolving is called the SOLVENT (the water — usually the larger amount). Solutions are uniform (called homogeneous), which means every drop has the same mix in it.

• Salt water = salt (solute) + water (solvent).
• Sugar in tea = sugar (solute) + tea (solvent).
• Air = oxygen and other gases (solutes) + nitrogen (solvent).
• Lemonade = sugar and lemon juice (solutes) + water (solvent).

Concentration — How Much Is Dissolved?

Concentration tells you HOW MUCH solute is dissolved in a set amount of solvent. A DILUTE solution has only a little solute — weak tea, lightly salty water. A CONCENTRATED solution has a lot of solute — syrupy lemonade, ocean water. Add more water to a concentrated solution and you DILUTE it. Boil water away and the solution becomes more concentrated, because the same amount of solute is now in less solvent. Concentration is why some drinks taste strong and others taste weak.