Investigate matter, volume and changes of state

Learning goal: Use observable evidence that matter has mass and occupies space, compare solids, liquids and gases, and investigate changes caused by heating or cooling. Matter is anything that has mass and takes up space. A stone, a drop of water and the air inside a cup are all matter. Something does not need to be visible to occupy space or have mass.

Mass measures how much matter is present in an object. Volume measures the space occupied. Two objects with similar visible size can have different masses, such as equal-size pieces of foam and wood. Different materials can hold different amounts of matter in the same space; foam has many air-filled spaces. Compare them with an appropriate balance rather than concluding that equal volume guarantees equal mass. Use consistent units, such as grams for mass and milliliters for liquid volume, or a nonstandard unit such as paper clips balanced against the object, and record both the measured property and the tool.

Air occupies space. In a teacher demonstration, hold an empty-looking clear cup upside down and lower it straight into a basin of water. The trapped air keeps water from filling the entire cup. Tilting the cup raises one edge of its rim. Air escapes as bubbles from that raised edge while water enters, until only the air above the level of that edge is left. The cup was not truly empty. This demonstrates space occupied by air; a separate suitable mass measurement would be needed to demonstrate its mass directly.

Matter still exists when it is broken into pieces too small to see. Stir a sugar cube into a cup of water until no grains are visible, even with a hand lens. Measured on a balance, the cup of water and the sugar cube together before stirring have the same mass as the sugar water afterward, as closely as the balance can show. The sugar is now in pieces too small to see, but its matter has not disappeared. Do not taste laboratory samples.

A solid usually retains its shape and volume unless a force changes it. A liquid flows and takes the shape of the part of its container that it occupies, while keeping the same volume when it is poured and none is spilled or left behind. A gas spreads through the available container space. Solids, liquids and gases all remain matter; shape differences do not separate matter from nonmatter.

Gases can be compressed into a smaller volume more easily than liquids and most solids. With teacher-controlled needle-free plastic syringes, compare gentle pressure on trapped air with gentle pressure on trapped water. Do not force a sealed tool or point it at a person. The air space can become smaller; the water volume changes very little under gentle pressure. This is an observable comparison, not an introduction to atomic structure.

Heating can melt a solid into a liquid. Cooling liquid water enough freezes it into solid ice. Heating liquid water makes it evaporate faster, and strong heating makes it boil; either way it becomes water vapor, an invisible gas. Cooling water vapor can produce liquid droplets. The mist you can see above hot water and the droplets on a cold surface are tiny drops of liquid water, not the gas itself.

Materials besides water can change state. A small sealed sample of butter can soften and melt in a teacher-prepared warm-water bath, then become more solid when cooled. Keep the sample separate from food to be eaten. Record what changes in flow and shape. Not every visible change is a change of state: cutting paper makes smaller solid pieces, and bending a solid does not automatically make it liquid.

Worked interpretation: Water poured from a wide vessel into a narrow one rises to a greater height, but the amount poured can remain the same. Compare marked volume measurements before and after rather than using height alone. The liquid changes shape to fit the container while retaining its volume if none is lost.

Worked investigation plan: Put equal small ice pieces in matching dishes, one in a warmer location and one in a cooler location. Record the conditions and observe at equal intervals. Predict faster melting in the warmer condition, but record the actual observations rather than filling in the prediction as data. This compares a state change under two conditions.

Guided performance: Observe the trapped-air demonstration, compare solid and liquid shapes in containers, and measure the masses of equal-size sample objects. Conduct one supervised heating or cooling investigation with drawings before, during and after. Use warm rather than dangerously hot materials appropriate for the classroom.

Independent evidence: Explain what the air demonstration shows and what it does not measure. Classify unfamiliar ordinary samples using several properties, and explain a melting, freezing or condensation example. If an invisible gas is described as “nothing,” return to the space it occupies and the observable changes when it moves.