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students have to have developed some clear expectations about materials and how they should behave. These are exactly the kinds of expectations that they have been developing in grades 3-5, as they are learning to measure both weight and volume and coming to understand that matter has weight and takes up space.

There are a large number of situations in which this basic data pattern (of volume change but weight conservation) can be readily observed by students. Some involve solids, some involve liquids, some involve gases, and still others involve a change of state. In the course of teaching, students should be exposed to all these situations. For starters, however, consider one phenomenon that research has shown to be especially intriguing and puzzling for middle school students and how it can be used to invite initial debate and discussion about whether matter is fundamentally particulate or continuous (Snir, Smith, and Raz, 2003).

The phenomenon involves mixing two equal volumes of water and alcohol, which are both colorless liquids. If you mix a given volume of water (say 50 ml) with a given volume of alcohol (also 50 ml), the resultant mixture of water and alcohol is only about 96 ml, not 100 ml, which is what students would have expected. Students immediately suspect that some liquid has been lost in the transfer. To rule out this possibility, it can be shown that there has been no loss of material: the weight of the mixture is equal to the weight of the two component parts. In addition, to allow students to more fully study the mixing itself, the two liquids can each be colored (with different food coloring) so students can watch more clearly what happens as they mix. Just as before, they can collect data showing that the total weight, but not the total volume of the system has been conserved. They can also see that if the (blue) colored water is mixed with the (red) colored alcohol, the two liquids intermingle and intermix, turning a uniform purple throughout. A number of provocative questions can be raised about this simple demonstration, including:

• How can two (continuous) liquids intermix?

• Why is the volume of the mixture less than the sum of the volumes of its parts?

• Why is the weight of the mixture equal to the sum of the weights of its parts?

Students are very intrigued (and surprised) by this demonstration, and in searching for possible explanations, they can be asked: What might matter be like at a very tiny scale (much too small to directly observe), in order for this to be? Students can consider a number of alternative models of the situation, based on different assumptions about what matter is like at such a small scale. For example: Would it be continuous all the way down (i.e., no

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