5-PS1-2: Weight Stays the Same When Matter Changes
Also called NGSS 5-PS1-2 · Topic: Structure and Properties of Matter · Physical Science
The Official Wording
Measure and graph quantities to provide evidence that regardless of the type of change that occurs when heating, cooling, or mixing substances, the total weight of matter is conserved.
Clarification statement: Examples of reactions or changes could include phase changes, dissolving, and mixing that form new substances.
Assessment boundary: Assessment does not include distinguishing mass and weight.
NGSS Lead States, 2013 · Read it on nextgenscience.org · checked October 15, 2026
What 5-PS1-2 Means in Plain English
Fifth graders weigh substances before and after a change, then graph the results. Whether they melt ice, freeze water, dissolve salt, or mix things that react, the total weight comes out the same, as long as nothing escapes or gets added.
Students use weight and do not have to tell mass and weight apart. The idea grows from 5-PS1-1, since particles that cannot be seen still count on the scale, and leads to MS-PS1-5, where atoms explain why the total holds.
5-PS1-2 I Can Statements
- I can weigh a substance before and after it changes.
- I can make a graph that compares weights before and after.
- I can explain why the total weight stays the same.
5-PS1-2 Worked Example
A weighing test that runs over two days with one capped water bottle and a kitchen scale.
Does water weigh more, less, or the same after it freezes and then thaws?
Set It Up: A small water bottle about half full with the cap screwed on tight, a digital kitchen scale, and the school freezer.
- Weigh the capped bottle of water and record the reading.Liquid: 268 g
- Freeze it overnight, wipe off the frost, and weigh it right away.Frozen: 268 g, ice level higher
- Leave it on the counter until all the ice melts, wipe it dry, and weigh it once more.Thawed: 268 g
- Make a bar graph with one bar for each weighing.Three bars, all the same height
The bottle weighed 268 g as liquid, as ice, and as liquid again, even though the ice took up more room.
Why it works: Freezing and melting rearrange the water particles but none are added or lost, so the total weight does not change.
Common Mistakes with 5-PS1-2
Expecting ice to weigh more
“The ice is bigger, so it must be heavier.”
Try this: Ice takes up a little more space than the same water, but the particles are the same ones, so the scale reading does not change.
Letting a gas escape
Fizzing tablets weighed in an open cup lose weight
Try this: When a gas forms, cap the container or stretch a balloon over it. The weight only drops when gas escapes into the room.
Weighing only part of the system
Salt water weighed after mixing, but only the water weighed before
Try this: Weigh every part before the change, including the container, and the same parts after. Compare the totals, not single pieces.
Where 5-PS1-2 Fits
Practice 5-PS1-2
Quick Check
- A cup of water and ice weighs 300 g. What will it weigh after the ice melts?
- You dissolve 20 g of salt in 200 g of water. What does the salt water weigh?
- An open cup of fizzing tablets in water weighs less afterward. Why?