8.7 Summary & Keep the Curiosity Alive

Key Points

  • A mixture is two or more substances mixed together; the components keep their properties and don’t react chemically. Mixtures can be uniform (air, seawater) or non-uniform (salad, sand & water).
  • Alloys (stainless steel, brass, bronze) are uniform mixtures of metals.
  • A pure substance has only one kind of particle and can’t be split by physical means. It is either an element or a compound.
  • An element cannot be broken down into anything simpler; it is made of identical atoms (gold, oxygen). Elements are metals, non-metals, or in-between metalloids.
  • A compound forms when elements combine chemically in a fixed ratio (water = 2:1, salt = 1:1); its properties are completely new, and it can only be separated chemically.
  • The iron + sulfur test shows the difference: mixed = a mixture (magnet separates the iron); heated = the compound iron sulfide (new properties, not magnetic).
  • Minerals are natural solids, mostly compounds (a few are native elements). Light, heat, electricity and thoughts are not matter.

A Closing Challenge: Keep the Curiosity Alive

Think It Through

  1. Big sort. Classify as element, compound or mixture: carbon dioxide, sand, seawater, magnesium oxide, muddy water, aluminium, gold, oxygen, rust, iron sulfide, glucose, air, water, nitrogen, sodium chloride, sulfur, hydrogen, baking soda. Then list which are pure substances.
  2. A + B → C. A and B can’t be broken down chemically. What are A, B and C? Which has a fixed composition?
  3. Why so different? Water is made of hydrogen (a fuel) and oxygen (supports burning), yet water puts out fire. Explain.
  4. Two reactions. Iron rusts in moist air; magnesium burns in oxygen. Classify every substance involved as element, compound or mixture.
  5. Both at once? Can a substance be an element and a compound at the same time? Why or why not?

That completes our look at how matter is organised. Next, we zoom in on one especially useful kind of uniform mixture — the solution — to see how substances dissolve, how much will dissolve, and how strong a solution can get.