Introduction: Make a Ballistic Bubble Machine

About: I'm a teacher (physics, grades 7 to 10), Maker and product designer. (Check out full of experiments, projects for Maker Education and kits for Bibberbeests!). Besides that, I write childre…

Time for an Instructable that makes a noise! This Ballistic Bubbles Machine produces soap-bubbles that make a very funny and sometimes very loud crack when ignited. The bubbles contain a mixture of two gasses: Oxygen and Hydrogen.

The mixture of these two gasses is called… oxyhydrogen. In Dutch (and in German too, I believe), the word for oxyhydrogen is the brilliant noun “knalgas”, which can be translated literally as “crack-gas” or "boom-gas". This easy and cheap to build machine shows how appropriate the word knalgas is…

Also, watching the video might show the fun of making and using this machine…

If you're having trouble viewing the video from here, have a look at it on YouTube...

There’s a lot of pretty cool science happening in the Ballistic Bubble Machine. In step 7, some hidden secrets are revealed by using Red Cabbage Magik. The final steps of this I’ble will be about the bubble-science, if you’re curious.

If you’re not curious at all about the science but want to make a totally cool machine, read on! Build the machine and have fun with it. New years eve is so close! When you got all the stuff listed, building will take you about 4 hours, I think. (Lucky you, it took me a hell of a lot longer to figure it all out! This project has been haunting me for a year!)



Grand prize: An Instructables 3 months pro-membership gift-code from me.
Ricalvarez is the lucky winner! Congratulations!

Somewhere in this I’ble is a hidden reference to another, rather famous Instructable. The hidden clue is on one of the pictures in one of the steps. The first person who posts the location of the picture with the hidden clue, the name of the instructable that is referred to and it’s author is the winner. Scoochmaroo and members of a certain family living in the northern part of The Netherlands are excluded from this contest…

Step 1: Safety and an Overview

Some people might want to build a machine like this. Building it is fun, and popping bubbles is hilarious, but, before you start...

Oxyhydrogen / knalgas is an extremely flammable mixture of gasses. The gas will explode immediately when ignited. This Ballistic Bubbles Machine produces soap-bubbles filled with oxyhydrogen on a surface of water. When ignited, the bubbles will explode with a crack, but without much energy. The fact that the bubbles are small makes this apparatus safe to use.

So, in a nutshell:
  • NEVER (and I mean Never) LIGHT THE GAS

Kids might enjoy the machine, but shouldn’t be allowed to play with it without an adult who is familiar with the machine and feels a responsibilty towards the kids being safe. Anyone operating the machine or being in close distance to it should wear safety glasses!


Understood? Sure? Did you see both pictures with this step?

Okay then, proceed to the next step and start building!

Step 2: Stuff and Tools

  • A small jar (about 0.2 Litres) with a wide cap that fits tightly. Jars for homemade marmelade are perfect. $2,-
  • Zinc-carbon batteries (not alkaline and not rechargeables!) for the electrodes. These are the cheapest batteries available. I bought two 4.5 Volt “flat batteries”, type 3R12 for $4,-. AA, C or D sized batteries are fine too, then look for code R6/UM3, R14/UM2 and R20/UM1 respectively. You’ll need 4 pillars. $2,- to $4,- at your local discount store.
  • One or two 9 Volt batteries to power the Bubble Generator. $5 to $10,-
  • One or two 9 Volt battery clips with longish leads. $1,-
  • A push button (capable of letting through 3 to 5 Ampere current). $1,- at a Radio Shack like store.
  • Silicone gel. $4,- at any hardware store.
  • Silicone grease (the greasy silicone stuff used in taps and valves) $2,- in the same hardware store.
  • Teflon tape (used for gas-proofing tubes)
  • 30 cm of tube, outer diameter 5-10 mm. I used tube from an infusion-bag (ask the pharmacist, I got it for free). Aquarium-tube is fine too. At the pet-shop, $2,-
  • A small porcelain or pottery bowl to hold the bubbles. Any low bowl with a wide gap is good. My bowl was meant to hold tapas, but it likes holding Banging Bubbles a lot better… $2,- at any warehouse / potteryshop.
  • A furnace-lighter or long matches.
  • A tray to mount the parts.
  • Water (ask your local ocean)
  • Salt, 1 tablespoon
  • Liquid soap for blowing bubbles
  • A small piece of stiff sheet, to make a custom washer (I used a piece of ABS plastic that happened to be in my way). Plywood works just as well.
  • Superglue, just one drop.
Total costs don’t need to exceed $20,-

  • Cutting pliers
  • Small hacksaw
  • Stripping pliers
  • Multimeter (for checking and debugging)
  • Soldering iron and solder
  • Third hand
  • Powerdrill and drillbits 4 mm and 5 mm (4 mm is the diameter of the electrodes from the zinc-coal batteries, 5 mm is the size of the outer diam. of the tube)
  • Marker
  • Sharp knife or fretsaw
  • Cutting mat

Step 3: Make the Electrodes

The heart of the Ballistic Bubble Machine consists of 4 electrodes, The elctrodes are the carbon cores in carbon-zinc batteries.

Getting the carbon rods out of the batteries can be very easy, or a bit less easy, depending on the type of battery you have. You need four electrodes.

As you will find out, the carbon rods are brittle and break easily. So treat them gently.

The easy way (pics 1 through 6):
  1. Take a 4,5 Volt "flat battery" (see pic 1).
  2. With a small saw, cut off the plastic cap (where the poles of the battery stick through) and remove it (Pic 2 and 3).
  3. Inside the plasic case, you'll find three zinc-carbon cells. They're connected with copper wire. Cut the wires and pull out the three cells.
  4. With a fretsaw, carefully saw loose the very top of the cells. See pic 4.
  5. You can now gently pull out the carbon core from the zinc-carbon cell (pic 5). If you're not gently enough, the carbon rod can break. See pic 6.
  6. With cutting pliers, remove the zinc rim around the plastic cap on the rod's tip. Leave the plastic cap on top of the rod.
The less easy way (pics 6 through 8):
  1. Take a AA (or C or D) sized zinc-carbon battery
  2. With cutting pliers, cut loose the metal directly below the plus-pole of the battery (pic 6).
  3. In the pile-batteries, the core is stuck into the shell. You'll have to break away the core's surrounding with the cutting pliers (pics 7 and 8). Not so easy, but it can be done.

Repeat this until you have four carbon electrodes.

Test the electrodes. With crocodile clamps, connect one electrode to the plus-side of a battery, and an other electrode to the minus-side of the same battery. Put the electrodes in salted water, but don't let them touch each other. You should see bubbles forming against the electrodes: Hydrogen and oxygen!

More about electrodes...
When choosing material for electrodes, you cannot use any conducting material you want, unfortunately. The ideal material for this use is platinum, and platinum electrodes are commonly used in labs. BUT... platinum is a very rare and valuable metal, and a set of lab-quality platinum electrodes will set you back at least $80,-. On eBay platinum electrodes that can be mounted on skin are sold for about $5 a set, but I haven't figured out a way to use them in an electrolyte (that is, in the salted water around the electrodes in the jar).

Second best choice for electrodes is Graphite / carbon, which is cheap and commonly available. Pencil-leads work fine as electrodes, but the cores of zinc-carbon batteries are much better suited for the job (the are made to act as an electrode, unlike the pencil leads).

Step 4: Preparing the Jar's Cap.

The jar's cap is the vital part of the Ballistic Bubble Machine. It has two main functions:
  1. It holds the four electrodes and the tube
  2. It seals the jar, so that the gas coming from the electrodes can escape the jar ONLY via the tube.
The sealing of the jar is very important. Oxyhydrogen is a very volatile gas and can escape through the smallest of slits. Sealing the tube and electrodes (with silicone gel) is vital to the working of the Ballistic Bubble Machine.

See picture 1 for a close-up on the lid.

Preparing the cap (pics 2, 3 and 4):
  • Lay out the holes for the electrodes and the tube. I made a lay-out in Illustrator and stickes that on the cap.
  • With a nail or center-punch, mark the centers of the holes.
  • Drill the holes in the cap. Use drills that match the size of the tube and the electrodes as close as possible. I used a 4 mm drill for the electrode-holes, and a 4.8 mm drill for the tube-hole.

Place the tube (pics 5, 6 and 7):
  • Cut a piece of 30 cm from the tube.
  • Give the tube a flange. I cut out a circle (diam. 15 mm) from a sheet of ABS, and drilled a 4.8 mm hole in it. The flange is glued to the tube using superglue.
  • Put a nice layer of silicone kit onto the flange, and stick the other end of the tube through the appropriate hole in the cap.
  • Pull the tube through the cap, and push the flange firmly against the inner side of the cap. The silicone gel should bulge between the cap and the flange.
  • With a wet finger, smear the gel neatly against the rim of the flange and the cap.

Place the electrodes (Pics 8, 9 and 10) (very similar to placing the tube):
The electrodes from the batteries have a ready made flange, which make them very easy to mount.
  • Put a nice layer of silicone gel on the flanges, around the electrode.
  • Stick the electrodes through the cap (remember, the electrodes must go INTO the jar)
  • Push the flange firmly against the lid.
  • With a wet finger, smear the silicone neatly against the flanges' rim and the cap.
Note: If you're using a metal cap, it is possible that the electrodes will be shortcutted. To prevent this, wrap a piece of ducttape around the electrode close to the flange. See also the comment in pic 9.

Step 5: Connecting Electrodes and Power

The four electrodes will be connected in pairs. Two electrodes are connected to the plus-side of the power source, and two electrodes connected to the minus-side.

To wire it all up, I only used two 9 Volt battery connectors. The leads were long enough for all the wire I needed. Picture 1 is an overall picture of this step.

Pictures 2, 3 and 4:
The electrodes forming an even pair should be facing eachother (being on opposite sides of the jar's lid). The soldered battery-connectors and the push-button are on picture 4.

Pictures 3 and 5:
The leads between two electrodes and from the electrodes to a battery-pole are soldered inline. How to solder two leads inline can be found in this cool instructable.

Pictures 6, 7 and 8:
Soldering the leads to the electrodes' caps is a bit tricky! The heat makes the plastic caps melt a little, causing the carbon rods to slide. I set the rods straight again by re-heating the soldered joint and pushing the carbon rods straight again. Hold them straight for a few seconds after removing the soldering iron, so the plastic can cool down and clot. Fixate the lid with the electrodes in a third hand or clamp to do this.

Pictures 9 and 10:
As an extra, I mounted the push-button in a piece of plastic or plywood, and glued the battery-connectors on both sides of the push-button.

Picture 11:
With a little play-dough / synthetic clay, cover the caps of the electrodes. Just to protect the soldered joints against bumps.

Step 6: Finish Up!

You're almost done! What's left is adding an electrolyte and sealing and closing the jar with the lid. Here you go:

First, wind three to four layers of teflon-tape around the rim of the jar.

Adding an electrolyte (the stuff through which the current is flowing between the electrodes):
The electrolyte is tap-water with a little salt. I added one tablespoon of salt to 0,5 Litre of tapwater.
Fill the jar until 1 cm (0.4") under the rim.

Sealing and closing the jar:
Spread a nice layer of silicone grease inside the lid, against the screw-helix and into the corner.

Now close the jar. Do this by placing the lid, with electrodes and all, on the jar. Then hold the lid, and turn the jar to close it.

Push two 9 Volt batteries into the connectors, and put the tube's end in a narrow, tall glass. Fill the glass with a mixture of soap and water, and you're ready to rock and roll.

Have fun!

Step 7: Red Cabbage Magik

By adding some cooking water of red cabbage in the electrolyte, you van add a mysterious effect to the Ballistic Bubble Machine.

Red Cabbage water is a bit of a magic vegetable. It turns light red in an acid environment, and blueish / greenish in a alkalic environment.

Red Cabbage water can be obtained easy (pics 1, 2 and 3)
  1. Buy a red cabbage.
    Cut two handfuls of red cabbage in small pieces.
    Boil the cabbage for a minute or two in 0,5 Litres of water.
    Separate the cabbage from the cooking water. 
    Trash the red cabbage (yuk)
    Keep the deep purple water.
  2. Buy a jar of pre-cooked red cabbage. 
    Separate the cabbage from the water in the jar.
    Trash the cabbage (yuk).
    Keep the not-so-deep purple water.
Prepare the Bubble Machine with red cabbage water:
Fill the jar of the bubble machine with clear water and some red cabbage water (about 1/3 fresh red cabbage water 2/3 plain water).
The mixture should look transparent blue, like pic 4).
Add a pinch of salt in the mixture (half a teaspoon).

Put the lid with the electrodes and tube back on the jar, and let the current do it's magic...
(See pics 5 through 10)

More about this red cabbage science in step 9, later...

Step 8: Other Ways to Build It...

What's happening inside the jar is called electrolysis of water: Electric energy from the batteries is used to break down water-molecules into oxygen (O2) and hydrogen (H2) molecules.

In the picture I put down the correct naming of the vital parts of a electrolysis set-up.

For some parts of the set-up are alternatives:
  • For the powersource: Just one 9 Volt battery will work too. A "powerbrick" (power adapter) capable of providing 2 amperes and 9 to 24 Volts should work too.
  • For the electrolyte: Salt isn't the best way to make a water-based electrolyte, but the easiest and the cheapest. A disadvantage of salt is the forming of chlorine at the positive electrode, which is a nasty gas. That's why you should use as little salt as possible to make the water conduct. What works best is a H2SO4 solution (sulfuric acid) or a Na2SO4 solution. Baking soda dissolved in warm water works too.
  • The electrodes. Ah, the electrodes! I tried many different things before I got to the carbon rods. Stainless steel, aluminium (foil), gold plated headphone jacks, pencil-graphite, copper and iron wire, it tried them all. The carbon rods give the best result by far. Second best are the pencil leads, mentioned in step 1. But they can't compete with the carbon electrodes, really!

Step 9: What's Going On?

In the heart of the machine, an electric current splits water molecules (H2O) into the two elements that they are build of: Hydrogen (H) and Oxygen (O). These elements will form H2 and O2 molecules, which escape from the water in bubbles.
The funny thing is, that when the hydrogen and oxygen-bubbles are put together again, they won’t re-unite into water-molecules all by themselves. They must be given a little ‘push’ to do that. When 'pushed' (for instance with a spark or a flame), oxygen and hydrogen will bond again as water-molecules, with a loud crack and some heat as a thank you.

Rocket fuel
The heat that is produced when hydrogen and oxygen are combined to water, makes rockets launch. The large clouds that appear when rockets launch, are made of pure and plain water. The large tanks that are mounted under the space shuttle contain hydrogen and oxygen seperately. So, somewhere at Nasa, there must be a BIG Ballistic Bubble Machine.

Q's and A's, from the top down
There's a lot to tell about what's happening when cracking knalgas-bubbles. The questions and answers below are a sort of cascade: The answer to the first question gives rise to another question, and so on...

Q: Where does the crack come from, when the bubbles are lighted?

A: What happens during the crack is an extermely fast reaction between one oxygen (O2) molecule and two hydrogen (H2) molecules. During this reaction, two water-molecules (H2O) and energy are formed. The energy that “emerges” during the reaction causes the crack you hear.

In shorthand, the reaction looks like this:
1 O2 + 2H2 → 2 H2O + energy

In a picture, it looks like pic numer 3.

Q: Why is the reaction so extremely fast, then?

A: The reaction can happen so fast, because in the gasbubbles there is exactly one oxygen molecule present for every two hydrogen-molecules. So after the reaction, there will be no oxygen or hydrogen left. Such a mixture of gasses is called a stoichiometric mixture. In dutch, it's called an "explosive mixture".

Q: So where does this "explosive mixture" of hydrogen and oxygen come from?

A: The oxygen and hydrogen come from the water inside the jar. Water is a "compound" of two atoms hydrogen and one atom oxygen: H2O. In the jar, the water-molecules are decomposed into hydrogen and oxygen. There will be twice as much hydrogen as oxygen, because water contains two hydrogen-atoms and only one oxygen-atom.

Both hydrogen and oxygen are gasses at "normal" temperatures, so they form bubbles of gas that escape the jar through the tube.

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