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5th Grade Science Lesson

Where Did the Mass Go? Exploring Conservation of Mass in Chemical Reactions

Shared by A STEM Educator

5th GradeNGSS Phenomenon-Based5-PS1-2, 5-PS1-445 min
Learning Objective

Students will plan and carry out investigations to gather evidence that matter is conserved during chemical reactions, including gas-producing reactions, and construct explanations using the Law of Conservation of Mass (5-PS1-2).

Lesson Overview

Students investigate the puzzling observation that a baking soda and vinegar reaction appears to lose mass, then design closed-system experiments to discover that mass is always conserved when gas is captured.

Materials

  • Digital or balance scale (accurate to 0.1 g)
  • Baking soda (sodium bicarbonate), small amounts per group
  • White vinegar
  • Resealable zip-lock bags (quart size)
  • Small plastic cups (to hold baking soda inside the bag)
  • Graduated cylinders or measuring spoons
  • Permanent markers for labeling
  • Student investigation handout
  • Sealed plastic bottles with tight lids (for extension)
  • Compost scenario cards (printed)
  • Science notebooks or recording sheets
  • Optional: effervescent antacid tablets and water in sealed bags

Scaffolded Task Progression

1Task 1
Observe and Record (Floor): Your teacher just mixed baking soda and vinegar in an open cup on a scale. The mass dropped from 85.4 g to 83.1 g. In your science notebook, answer:
(a) What do you observe happening during the reaction?(b) What is the difference in mass?(c) Write a hypothesis: where did the 'missing' mass go? [ Interactive graph / diagram — available in the full lesson ]
2Task 2
Sealed Bag Investigation (Developing): With your group, measure and record the mass of: (1) a zip-lock bag, (2) a small cup with 5 g of baking soda placed inside the bag, and (3) 20 mL of vinegar in a second small cup inside the bag. Seal the bag completely. Record total mass. Now tip the bag so the vinegar mixes with the baking soda — without opening the bag. Wait 2 minutes, then measure total mass again. Record both readings in the data table.

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