The Balloon That Inflated Itself

It started because Jonah wanted to know if he could blow up a balloon without using his mouth.

He'd been at a birthday party the day before and had watched someone's dad inflate about thirty balloons in a row and then sit down heavily on the sofa and not move for some time.

"There must be a better way," Jonah said. He was seven and had strong opinions about efficiency.

"What if the balloon inflated itself," I said.

He looked at me. "How."

I got out the baking soda and the vinegar.

The Setup

We found a plastic bottle — an empty water bottle, about 500ml — and a balloon from the drawer where balloons live alongside rubber bands and dead batteries and a mystery key. I told Jonah we needed two things inside the balloon-bottle system that very much wanted to meet each other, and that when they did, they were going to make a gas.

"What gas?"

"What do you think comes out of vinegar and baking soda when they mix?"

He didn't know. I told him to smell the vinegar. He did, and recoiled slightly.

"Carbon dioxide," I said. "Same stuff as in fizzy drinks. Same stuff that made the raisins dance."

He remembered the raisins. This got his attention.

I poured about three tablespoons of vinegar into the bottle. Then we spooned two teaspoons of baking soda into the balloon itself, tipping it in carefully, tapping it down to the bottom so it sat in a little white lump at the end.

"Now," I said, "we stretch the neck of the balloon over the bottle without letting the baking soda fall in yet."

This took a couple of attempts. He was impatient. The baking soda fell in once before we were ready and it fizzed immediately and the balloon barely twitched and Jonah said "it didn't work" with the certainty of someone who has given something one try.

We reset. Got the balloon on properly this time, the baking soda still sitting safely in the bulge at the end.

"Now," I said, "lift the balloon up."

He tipped it. The baking soda slid down into the bottle.

The fizzing was immediate and loud, a proper aggressive hiss, the kind that sounds like something is very unhappy about something. And the balloon, stretched over the bottle neck, started to fill. Slowly, then faster, puffing outward, rising, becoming round.

Jonah put it down in surprise.

The balloon kept inflating for a moment, then settled. Slightly lopsided, a bit wrinkled at the neck, not the sort of balloon you'd hand to a child at a party. But inflated. Definitely inflated.

He picked it up and held it carefully, like evidence.

"I did do it without my mouth," he said.

The Science Bit (Which Happened Naturally, I Promise)

"Okay," I said. "What just happened in there?"

"They mixed and made bubbles."

"Made gas. Carbon dioxide gas. And gas needs space, it pushes outward in all directions. Inside the bottle, where does it go?"

He looked at the balloon. "Up into the balloon?"

"It's the only way out. So it pushes in and the balloon stretches to make room." I took it from him and held it up. "This is carbon dioxide. All of it."

He poked it. "So there's no air in here?"

"Not much. Mostly CO2."

He thought about this for a moment. "Is it heavier than air?"

I had not expected that question. Carbon dioxide is, in fact, slightly heavier than regular air, which is why it settles in low places, why it can be poured like a liquid if you're careful. I told him this. He looked at the balloon with new suspicion.

"So it wants to fall down."

"It does rather."

He held the balloon out and let go.

It dropped faster than he expected. Faster than a normal air balloon. He picked it up and dropped it again. Then a third time. Then he took it to the kitchen doorway and dropped it from as high as he could reach, stretching onto his toes.

"It's definitely heavier," he said, with the confidence of someone who has now run three trials.

The Testing Phase

He wanted to know: would more baking soda make a bigger balloon? We tried. Yes, up to a point. There's only so much vinegar to react with, so you hit a limit fairly quickly. Did it matter if the vinegar was cold? We tried it straight from the fridge. The fizzing was slower, the balloon smaller. He found this deeply unfair. Could you do it with just water instead of vinegar?

"What do you think?" I said.

He looked at the baking soda. "I don't think water does anything to it."

"Try it."

He tried it. Nothing. The baking soda sat in the water looking inert and slightly smug.

"So they have to actually react," he said. "It's not just the mixing."

That is a useful distinction for a seven-year-old to land on by himself.

He also wanted to know if you could inflate two balloons off one bottle, which you cannot, but working out why kept him occupied for a good fifteen minutes. The vinegar runs out, or the baking soda does, or both. Once the reaction stops, it stops. There's no restart.

"It's a one-time thing," he said, a little mournfully.

"Most good reactions are."

The Next Day

He told his mum that carbon dioxide is heavier than air and that you can prove it by dropping a balloon. She looked at me over his head with an expression I've come to recognise.

"We were doing physics," I said.

He also spent some time that evening investigating whether other combinations made gas. He tried salt and vinegar (a small fizz, not much), sugar and vinegar (nothing), and cornflour and vinegar, which produced a paste that was a different sort of discovery entirely and required some intervention.

The cornflour incident did not make it onto the fridge. But the balloon did, tied to the door handle, where it hung for three days slowly going soft. Jonah checked on it each morning.

"The CO2 is leaking out," he told me on day two. "And getting replaced by air."

I hadn't told him that. He'd worked it out himself from watching it happen.

Your Turn

You need a plastic bottle, a balloon, white vinegar, and bicarbonate of soda. Three tablespoons of vinegar in the bottle. Two teaspoons of bicarb in the balloon. Stretch the balloon over the bottle neck carefully, this is the fiddly bit and worth taking your time over, then tip the baking soda in and watch.

Once you've done the basic version, try cold vinegar versus room temperature and see if you notice a difference. Try more bicarb and see whether the balloon gets bigger or just reaches the same point.

Then drop the balloon and see what you think.

If someone asks whether you could blow up a hundred balloons this way for a party, the answer is technically yes. You would need a great deal of vinegar. It would smell absolutely terrible. And it would be completely worth trying.