Weighing Before and After

Step 1: Something to Notice

Watch first. The explanation comes later.

Watch the idea first — 42 seconds. Then read on, and try it yourself in the next step.

A sealed jar reads 100 g. Something inside it dissolves, melts, and fizzes into gas. The balance still reads 100 g.

Can a change ever make matter appear or disappear?

The tablet fizzes away. The balance never moves: 100 g before and after.

Step 2: Find Out

Run every change sealed. Then take the lid off and run them again.

Step 1 — Predict

A gas is released inside a sealed jar. What does the balance do?

Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.

Step 3: So Here Is Why

Now the explanation, after you have seen it happen.

Matter is conserved: the particles are rearranged, never made and never destroyed. Weigh a sealed container before and after any change inside it and the reading is identical. That rule has never yet been found to fail.

Conserved The total stays the same, whatever happens.

You have to seal the container

In an open jar a gas can leave, and the balance drops — because matter went out, not because it stopped existing.

Gases have mass

The air in a classroom weighs about as much as a large dog. Gas is not nothing.

A dropping reading is a leak

If the mass falls, ask what got out. That question always has an answer.

mass How much stuff a thing is made of.

The number refuses to move

Weigh a sealed container before and after a change inside it and the mass is identical. Dissolving, melting, freezing and even burning in a sealed vessel all leave the total unchanged.

Why the container must be sealed

Burning wood in the open seems to lose mass because gases escape. Seal it, and the mass holds. Apparent losses are almost always something leaving that nobody weighed.

Step 4: A Common Mistake

Lots of people think

“A gas weighs nothing, so making a gas makes the mass go down.”

Step 5: The Experiment That Settled It

The same idea somewhere new.

In the 1770s Antoine Lavoisier burned things inside sealed glass vessels and weighed them. Everyone expected burning to destroy matter — things visibly shrink to ash. Sealed, the mass never changed. That single stubborn measurement is the foundation of modern chemistry.

Weighed sealed

Step 6: Think It Through

Practice makes it stick.

The Open Jar

Problem 1 of 2

Sealed the jar reads 100 g; with the lid off after a gas is released it reads 91 g. How much gas escaped?

grams

The Burning Log

Problem 2 of 2

A log burns to a small pile of ash that weighs far less. Was matter destroyed?

Before and After

1 of 4

What does conserved mean?

2 of 4

In a sealed jar, what does the balance read after the ice melts?

grams

3 of 4

Do gases have mass?

4 of 4

The balance reading drops. What should you ask?

Step 7: Quick Check

Show what you know.

Question 1 of 1

A sealed jar weighs the same before and after every change. What does that show?

Step 8: Explain It in Writing

Claim, evidence, then reasoning.

The question

You ran every change with the jar sealed, and then ran them all again with the lid off. Write an argument answering the question: when a change makes the balance reading drop, has matter been destroyed?

Evidence you can use

Sealed, all four changes left 24 particles inside and the balance on 100 g.
With the lid off and a gas released, the balance dropped from 100 g to 91 g.
With the lid off, 15 particles were left inside the jar and 9 were drawn above it.
With the lid off, dissolving and melting still read 100 g. Only the gas change moved the number.
The lid was the only thing I changed between the two sets of runs.
Lavoisier burned things inside sealed glass and the mass never changed.
A log burned in the open leaves only a small pile of ash.

Words to usemassconservedsealedgasescapedthe same

ClaimOne sentence that answers the question.EvidenceWhat you saw or measured that supports it.ReasoningWhy that evidence supports your claim. This is the part most people leave out.

Fill in all three boxes. The reasoning box is the one that matters most.

What You Found Out

Matter is conserved: changes never create or destroy it.
You must seal the container, or a gas can leave.
Gases have mass.
A falling reading means something got out, not that matter vanished.