In grades 3-5, the core concepts in atomic-molecular theory become more sophisticated. Some of the core concepts important to develop in these grades include understanding that:

• Objects are made of matter that takes up space and has weight.

• Solids, liquids, and gases are forms of matter and share these general properties.

• There can be invisible pieces of matter (that is, too small to see with the naked eye).

• Matter continues to exist when broken into pieces too tiny to be visible.

• Matter and mass are conserved across a range of transformations, including melting, freezing, and dissolving.

Although these concise statements summarize key aspects of the science, they do not reflect the ways in which students express understanding of atomic-molecular theory. In fact, the student who simply memorizes or repeats these statements verbatim may very well understand little about the actual science behind them. Students should be able to describe these concepts in their own words in order to show their understanding, as the goal is for students to understand the core concepts behind the words.

Students in grades 3-5 continue to engage in a wide variety of scientific practices. They pose questions, make predictions, design and conduct investigations, represent and interpret data, design models, and make arguments that support conclusions. Furthermore, the scientific practices of older elementary school children become more complex in several ways. No longer reliant on mere sensory measures, and having established a theory of measure, they can now engage in more complicated forms of measuring and graphical representation. Thus, students build on their understanding of area to explore the volume of rectangular solids, develop greater precision in measurement through more general understanding of fractional units, and construct graphs that show the relation between volume and mass instead of displaying each property separately.

We’ll see several of these practices at work as we look at a third-grade classroom that is investigating the properties of air.

Reggie Figueroa’s third graders were carrying out a scientific investigation that involved weighing air.⁷ In the previous weeks, they had weighed and measured different kinds of objects and materials, had predicted which objects would be heavier, and had graphed their results. Now the children were investigating whether or not air could be weighed. Some of the students were sure that air couldn’t be weighed because “you can’t weigh something that’s nothing.” Others disagreed and thought that air was definitely something.

One student, Jeremiah, reminded the others about the time each one of them had measured their own lung capacity by blowing through a tube into an upside-down jar full of water which was immersed in a fish tank. Pointing to a wall graph that showed lung capacity, height, and resting pulse rate for each student, he reminded them that he’d