Fifth Grade · Science · week 3

Conservation of matter (weigh it!)

Measure the mass of substances before and after mixing or dissolving to show that the total amount of matter is conserved.

Here is one of the biggest ideas in science: matter cannot just appear or disappear. When you mix, dissolve, or even burn something, all the original matter is still around somewhere; it might just look different or be spread out. This is called conservation of matter, and the best way to prove it is with a scale.

If you weigh a cup of water, then weigh the salt you are about to add, and add the two masses together, that total should match the weight of the salty water afterward, as long as nothing escaped as a gas or spilled. Weighing before and after is how scientists check that matter was conserved, not lost.

📋 For the teacher
Teach it (10–15 min):
  • Use a real kitchen or postal scale; the exercise only convinces if the numbers are real, not just read from a page.
  • Always have them predict the total mass before mixing, then check. The predict-then-check pattern is the heart of this lesson.
  • If a gas escapes, like from a fizzy tablet in water, the mass reading will legitimately go down; use that as a teaching moment, not an error.
  • Tie back to week 2: dissolved salt looks gone but conservation of matter, and the scale, proves it is not.
Talk about it:
  • Why do we weigh before mixing, not just after?
  • If the total mass matches your prediction, what has that proven?
  • What would explain a measured mass that is lower than predicted?

Mastery looks like: They can predict a total mass before mixing two or more substances and confirm the prediction with a real scale.

Common stumbles: Forgetting to include the mass of the container; assuming dissolved substances lose mass because they become invisible.

Homework for this lessonType it, write it with a stylus, or print it for pencil & paper.open →
Records attendance + 15 min of Science for this learner.
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