The Sealed Beaker and the Missing 40 Grams
On Monday, a fifth-grade class set up two identical 250 mL beakers. Each contained 150 g of dry soil and 40 g of water, for a total starting mass of 190 g. Beaker A was sealed tightly with plastic wrap. Beaker B was left open. Both sat on the same windowsill at 22 °C. On Friday, students used the same balance. Beaker A still read 190 g, and drops of water clung to the underside of its wrap. Beaker B read 150 g, and its soil looked dry at the surface. A student said, 'The water in Beaker B was destroyed. That is why it weighs less.' Another student said, 'It evaporated, so the matter is gone.' The teacher asked the class to use the evidence to decide what happened and whether matter was created or destroyed.
1. Use the mass data to explain where the water in Beaker B went and why Beaker A stayed at 190 g.
Read the worked answer
The 40 g of water in Beaker B changed from liquid water to water vapor and escaped into the air. The beaker's mass dropped by exactly 40 g, from 190 g to 150 g, which matches the water that left. In Beaker A, the same evaporation happened, but the plastic wrap trapped the vapor. Some vapor condensed on the cooler wrap as droplets. Because no matter crossed the wrap, the total mass stayed 190 g. This supports conservation of matter: water molecules moved to a new location and changed state, but they were not destroyed. The invisible gas in the sealed beaker still has mass and takes up space.
2. Suppose the plastic wrap is removed from Beaker A and the beaker is left on the windowsill for three more days. Predict the mass and explain your reasoning.
Read the worked answer
The mass should decrease from 190 g toward about 150 g. Once the wrap is removed, water vapor can escape into the classroom air instead of being trapped. The 150 g of dry soil will remain, so the final mass should not drop below 150 g unless soil is lost. The exact value depends on temperature, humidity, and how much water is still in the soil after three days. If the air is very humid, evaporation may be slower. The prediction follows from conservation of matter: the water is not destroyed, but it leaves the beaker and mixes with the air, so the balance no longer measures it.
3. Evaluate the claim that evaporated water is no longer matter. Use evidence from the investigation.
Read the worked answer
The claim is incorrect. Water vapor is matter, even though it is invisible. In Beaker A, water evaporated and then condensed as droplets on the plastic wrap. Those droplets show that the water was still present as a gas inside the sealed space. The mass of Beaker A did not change, which means the water vapor contributed mass even though students could not see it. If the vapor from Beaker B could be captured, its mass would be recovered. Evaporation changes the state and location of water molecules, not their existence. Matter is conserved in a closed system.