What happens when you mix water and alcohol?

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Navigating this Pathway

[stextbox id = “info”] This pathway poses a question to students: What happens when you mix water and alcohol? We recommend having students write the question in their science notebook and/or displaying this driving question prominently in the classroom throughout the pathway. Students begin the pathway by making predictions about and then observing the total volume of 50 mL of water when combined with another 50 mL of water and then 50 mL of alcohol and 50 mL of alcohol (both have a total volume of 100 mL). Next, students make predictions about the total volume of 50 mL of water and 50 mL of alcohol, and find that once combined, the volume is less than 100 mL and are challenged to explain why. Students also consider the weights of the individual and combined liquids in comparison with the volumes. The students then draw pictures of the particles in the alcohol and water using their “super strong glasses,” more powerful than the strongest microscope. In order to help students understand what is happening with the water and alcohol particles in relation to the particle model, the teacher does a demonstration using marbles and sand, and then students engage with a computer simulation. Finally, the students revisit their super strong glasses drawings and revise using their emerging understanding of the particle nature of matter.
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Connections to Other Pathways

[stextbox id = “info”] This pathway does not depend on students having done any other pathways first. However, this pathway does introduce students to the idea of empty space between particles, which they can use to explain phenomena in the other pathways.
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Teacher Content Background

[stextbox id = “info”] All matter is made of particles that are too small to see, that are in constant random motion, and that have empty space between them. “Empty space” is literally empty; that is, nothing at all is between the particles that make up matter. The empty space is most noticeable in gases, which can contract and expand noticeably because of the empty space (think of an inflated balloon shrinking when the temperature does down), but is apparent in other states as well.

All the particles in a substance are the same size, but the particles of a different substance will have a different size (larger or smaller). Because the particle sizes and shapes of different substances are different, the particles of one substance may fit in between the particles of another substance, similar to the way small pebbles can fill the space between larger rocks. This is what happens when water and alcohol are mixed (the water and alcohol particles fit between one another); the resulting volume when they are together is less than the sum of the two separately, but not by much.

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Student Thinking

[stextbox id = “info”] From mathematics and everyday experience, students expect 2 plus 2 to equal 4, and 50 plus 50 to equal 100. They will expect that 50 mL of water and 50 mL of alcohol will combine to make 100 mL of liquid.
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Student Experiences

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