Close-up Look at Change

This activity is about classification of matter into pure substances and mixtures. It provides background information for the interactive activity below.

Among other things, students learn that solutions are not always liquid: homogeneous mixtures can also take gaseous or solid forms. (In the case of a solid solution, like a metal alloy, the different chemicals typically become mixed in the liquid phase and later harden into a solid.)

First show the diagram’s various stages that show how different categories of matter fit into one another. In this diagram, more specific categories are nested inside of more general ones.

Paraphrase:

Pretty much all the materials around us, and even in us, fit into one of two categories:

• Pure Substances (also called chemicals)
• Mixtures (combine multiple chemicals with chemcial reaction)

Within pure substances category there are:

• Elemental Substances (made of just one kind of atom)
• Compounds (combine multiple atoms in definite proportions)

Within the mixture category there are:

• Homogeneous Mixtures (also called "solutions")
• Heterogeneous Mixtures (unevenly mixed)

Paraphrase:

To practice classifying various familiar objects, we will play a Turn and Talk sorting game during which all teams will be shown an object. Then your team will discuss which of the categories corresponds to the object. Some of the answers will be surprising, so between each slide we’ll read a brief explanation about WHY the item fits into its category.

Teachers can choose whether or not to keep score.

Answers:

1. A gold bar is made of just one thing: the element gold.
2. Bronze is a homogeneous mixture of copper and tin. It’s a mixture (not a compound) because the proportions of copper and tin aren’t exactly the same in all bronze. Mixtures of metals are called alloys.
3. Steel is an alloy made by mixing a little carbon, and sometimes other elements, into iron. The term alloy usually means a mixture of metal, but adding carbon to iron is also considered an alloy, even though carbon is not a metal.
4. Sometimes gases can be dissolved in liquids. For example, carbonated drinks have carbon dioxide dissolved in water, making a homogeneous mixture.
5. Water is a chemical substance that contains only molecules made of two hydrogen atoms and one oxygen atom, so it's a compound.
6. Table salt crystals are made of sodium and chlorine atoms in a one-to-one ratio. That is, there are always equal numbers of the two kinds of atoms. The scientific name for this salt is sodium chloride, which is a compound.
7. Oil and vinegar don’t dissolve into one another, so they always remain unevenly mixed. It's a heterogeneous mixture.
8. In a smoothie, you can see small pieces of chopped up fruit mixed into the liquid. It's a heterogeneous mixture.
9. Concrete is made by mixing coarse material like broken stone, gravel, or sand into cement. It's a heterogeneous mixture.
10. Diamond is made of just one element: carbon. Each carbon atom in diamond is connected to four other carbon atoms, in a crystal that extends on and on. There are other forms of pure carbon where the atoms are bonded differently, notably charcoal and graphite.

Students drag the labels to their correct positions on the diagram using the words at the top of this page.

Answers shown on the right.

Open new window for student version.

​