LET’S TRY ONE
A sealed bag contains 100 g of water and 10 g of sugar. The sugar dissolves, and nothing enters or leaves. What is the total mass of these contents?
110 g
100 + 10 = 110 g; dissolving does not remove the sugar's matter.
Learning steps & teaching tips
Account for all matter, including gas, when interpreting a stated closed-system mass measurement.
- Identify what is inside the measured system.
- Ask whether any material crosses its boundary.
- Use the before-and-after measurements without treating invisible gas as missing matter.
Try it together
Sketch a sealed bag containing water and sugar. Label the measured boundary and explain why dissolving changes appearance without removing matter.
Check the system boundary and include every material inside it before comparing masses.