[1]
Purpose
To have students consider how the total weight of the water and alcohol compares with the total volume
Description
Conduct Part 3 of Student Experience 1 again (mixing water and rubbing alcohol), this time adding a step in which students weigh each graduated cylinder with the liquid in it. Have students record these weights and predict the total weight when the two liquids are poured together. Combine the two liquids, having a student confirm the decrease in volume. Then, weigh the combined liquid (and the empty graduated cylinder), showing that the weight was conserved (i.e., is the same as the sum of the two weights).
Questions to Ask Students
- Why do you think the total volume was less than the sum of the two individual liquids but the combined weight was the same as the sum of the individual weights?
Student Thinking
- When water and alcohol are combined, no particles are destroyed. As a result, matter (and therefore weight) is conserved, even though the combined volume is less than the sum of the two. Students may have some misconceptions about this, including:
- Some may believe that the decrease in volume also results in a decrease in weight.
- Some may believe that particles “disappeared” or were destroyed when the volume decreased; this thinking will be challenged by the constant weight of the liquids.