Desert Samples: Predict the Scale

The Desert Field Station

A research team at a desert field station practices weighing samples before taking them indoors. Team member Imani uses clean, dry sand and smooth pebbles supplied by the station. She asks what a scale will show after the materials are mixed. She uses a digital scale that measures weight in grams and checks that the empty scale reads zero before each measurement.

For each mixing trial, Imani weighs an empty jar with its lid, then weighs the sand and pebbles separately. She adds both materials to the jar and closes the lid. She weighs the whole closed jar before mixing. Next, she gently turns the jar several times and weighs it again. The same jar, lid, sand, and pebbles stay in each trial. Nothing is added or removed during mixing.

Her records show these weights:
Trial A: jar and lid, 20 g; sand, 30 g; pebbles, 50 g; whole closed jar after mixing, 100 g.
Trial B: jar and lid, 40 g; sand, 40 g; pebbles, 60 g; whole closed jar after mixing, 140 g.
Trial C: jar and lid, 40 g; sand, 60 g; pebbles, 80 g; whole closed jar after mixing, 180 g.

The team also compares two identical wide-mouth bottles holding equal amounts of water. Each bottle sits on its own tray. One bottle stays capped. The other stays open, with its cap on its tray. Each measurement includes the bottle, water, tray, and cap. Both setups spend three hours side by side in the same sunny spot. No water spills, and no equipment is removed. An adult handles the bottles after they stand in the sun.

Water can change from liquid into water vapor, a gas that can mix with the surrounding air. After three hours, the capped bottle has small drops on the inside of its walls. The graph later on this sheet shows the bottle measurements. The team keeps another sealed bag of damp sand for a shape-changing test.