How to teach the law of conservation of mass

Why this matters

A student who watches the open cup lose 2 grams can decide that the fizz used up some of the matter. On a lab sheet, nobody answers that idea until the sheet is graded, and the student who already knows a gas escaped gets nothing harder to think about. The teacher has to wait for quiz day to find out who thought what.

The law of conservation of mass

The law of conservation of mass says that matter is not created or destroyed when substances change. In a chemical reaction the atoms rearrange into new substances, so the total mass after the reaction equals the total before. When a reaction makes a gas in an open cup, the gas escapes into the air and the scale reads less, so mass looks lost. In a closed bottle the gas stays inside and the reading stays the same. The NGSS asks fifth graders to measure and graph quantities as evidence that the total weight of matter is conserved when substances are heated, cooled or mixed (NGSS 5-PS1-2).

In the Flint activity, each student weighs baking soda and vinegar before and after they react, once in an open cup and once in a capped bottle, and sends Sparky four photos of the scale. Sparky reads the numbers, graphs them and asks about that student's own readings. A lab sheet can't answer a student who writes that the fizz used up matter. Sparky sends that student back to their capped-bottle readings and asks what the cap did.

Nia's fizzing bottle lab

Nia is an elementary student who watches baking soda volcano videos on YouTube. She can read a kitchen scale, but she thinks gas weighs nothing, so she expects some matter to disappear in the fizz.

Every student in the class does the same experiment at home or in class, with an adult helping. They weigh 1 teaspoon of baking soda and 1/4 cup of vinegar on a kitchen scale before and after the two react: once in an open cup, and once in a 2-liter bottle with the cap screwed on. Each student sends Sparky four photos of the scale. Sparky sets the experiment in each student's interest, and its next message depends on what that student just said.

Sparky uses Nia's interestSparky answers what Nia just said

N

Nia

Interest: Baking soda volcano videosStarting point: Reads a scale, thinks gas weighs nothing

Sparky

Hi, scientist! Today you'll weigh a fizzy reaction and find out where the weight goes. An adult should help, and everyone wears goggles. First, tell me: what do you love to do or watch? A sport, a show, cooking, anything!

Nia

i like watching volcano videos on youtube, the ones where they use baking soda and vinegar

Sparky

No way, that's perfect, because today you get to be the scientist running your own mini volcano experiment! And here's the cool twist: you're going to weigh the reaction to find out where the fizz goes. Does the weight disappear? Does it stay? Send me all four photos when you're done and we'll dig into the data!

Nia

ok i did it with my mom! here are my 4 pictures. the first 2 are the open cup and the last 2 are the bottle with the cap

Sparky

Awesome work, scientist! Here's what I can read from your photos. Can you confirm these numbers look right? Open cup before: 90 g. Open cup after: 88 g. Capped bottle before: 113 g. Capped bottle after: 113 g.

Nia

yes thats right

Sparky

Here's your bar chart! Take a good look at it. Then tell me: what happened to the weight in the open cup? What about in the capped bottle?

Nia

the open cup went down 2 grams. i think the fizz used up some of the matter

Sparky

Ooh, interesting idea! But let's check it with your capped bottle. The same fizzing happened in the capped bottle too. Look back at your photos: what did your scale say before and after in the capped bottle? What's different about the bottle compared to the open cup? Think about what the cap does!

Nia

the bottle was 113 before and 113 after. the cap kept the bubbles inside so they couldnt get out. the bubbles are a gas

Sparky

Yes! You've got it! The cap kept the gas trapped inside, and the weight stayed the same. That's a huge clue. So here's the harder question: the open cup lost 2 grams. The fizzing made a gas there too, but the cup was open. So where did those 2 grams go?

Nia

they floated out of the cup into the air in my kitchen. so the gas weighs something

Sparky

YES! That is exactly right. The gas (it's called carbon dioxide) floated out of the open cup and into your kitchen air. And since the scale read less after, that means the gas has weight! Now for the big final question. Why does the total weight stay the same when nothing can get out?

Nia

because the stuff just changes from baking soda and vinegar into gas and liquid. none of it goes away so if it cant get out the scale stays the same

Sparky

You just explained conservation of mass like a real scientist. Matter doesn't disappear. It just changes form. Those volcano videos you love? Same thing happening: matter changing form, never disappearing.

Said the fizz used up matter, then explained where the 2 grams went

Nia is an example student, her four scale photos are drawings, and Sparky's replies are shortened.

What Flint did for Nia

• Sparky set the lab up as Nia's own mini volcano after she said she watches volcano videos.
• Sparky read the scale in each of Nia's four photos and asked her to confirm the numbers before using them.
• Sparky made a bar chart of Nia's readings, so the open cup's 2-gram drop sat next to the capped bottle's two equal bars.
• When Nia said the fizz used up matter, Sparky sent her back to her own capped-bottle photos and asked what the cap did, instead of giving the answer.
• Once Nia said the bubbles were a gas, Sparky asked the harder question: where did the open cup's 2 grams go?

A teacher can't do this by hand for 30 students at once. Sparky does it in every student's chat.

The activity page shows who still thinks matter disappeared

While students work, Needs attention lists each student who is stuck and the message that showed it. Once three or more students submit, activity analytics sums up what the class understood and what to practice next.

Set up the activity for free

The fizzing bottle lab is in Flint's public library, with the experiment steps, Sparky's guidelines and the greeting already written. Open it, duplicate it into a free Flint account, and assign it to a class. Every student gets their own chat with Sparky, and the activity page fills in as they work.

Each student needs a kitchen scale that reads in grams, a cup, a small dish, a 2-liter plastic bottle with its cap, a tissue, 1 teaspoon of baking soda and 1/4 cup of vinegar. The lab uses a screw cap instead of a balloon over the bottle. A balloon filling with gas pushes air aside, and that air holds the bottle up a little, so the scale reads 1 to 2 grams lower and the closed bottle looks like it lost weight too.

For more science activities, see Flint for science.