One of my favourite chemistry demonstrations is the combustion of hydrogen in a balloon. It has all the qualities desirable in a flashy experiment, and the actual execution is exceptionally easy – the difficulty, however, comes down to obtaining the hydrogen gas if none is available. This procedure is meant to be conducted in a lab (e.g. at a high-school level) where hydrogen isn’t available (typically because of safety restrictions regarding storing hydrogen on the premises).
While there are many easy ways of generating small quantities of hydrogen gas (e.g. zinc in hydrochloric acid), filling a balloon requires two key aspects – firstly, a reasonably large quantity (several litres), and secondly a somewhat high pressure, which are not as commonly seen.
The procedure below involves mixing aluminum powder with a concentrated sodium hydroxide solution and bubbling the resulting gas through water to cool and purify, before capturing the gas in a balloon.
Warning: this reaction, in the form presented below, is not exactly friendly – proceed at your own risk
- Perform this reaction in a fume hood if possible
- The initial reaction produces a minimum of fumes, but excess hydrogen may be vented; additionally, there is a risk of the (hot, caustic) reaction product spilling and the balloon bursting/exploding.
- Ensure you use a Pyrex/Kimax flask
- The reaction is highly exothermic and a regular glass flask will shatter at the peak of the reaction
- Wear gloves
- Part of this procedure requires holding the flasks during the reaction – you will be maintaining the seal manually and there is a high chance that you will get hot, concentrated sodium hydroxide on your hands.
- Wear goggles (and possibly a lab coat)
- This reaction has a risk of explosion, and uses extremely caustic chemicals that will reach very high temperatures during the reaction.
- Avoid sources of static electricity (e.g. sweaters)
- The balloon, once filled, can explode unexpectedly, especially if exposed to small electrical discharges.
Note: this is a violent reaction – it takes a moment to get going, but accelerates rapidly – the products are extremely hot. If necessary, you can slow the reaction down by immersing the reaction flask in a container of cold water for the duration of the reaction.
Note: quantities are approximate.
- 100-125mL of concentrated (~18M) sodium hydroxide solution
- 2mL of aluminum powder (about 2cm in a standard test tube)
- 2 Erlenmeyer flasks (250mL each)
- 2 rubber stoppers (one with one hole, the second with two holes)
- 3 lengths of straight glass tubing (4cm, 8cm, and 10cm; should fit tightly into the stoppers)
- 15cm of rubber tubing (should fit tightly over the glass tubing)
- 150-200mL of cold (tap) water
- 1 balloon
Prepare the stoppers:
In all cases, below, the glass tubing should fit snugly into the stoppers – it must form an airtight seal.
- Insert the 4cm length of glass tubing (A) fully into the one-holed stopper so that it just reaches the bottom of the stopper, with about 2cm protruding from the top side of the stopper.
- Insert the 10cm glass tube (B) into the two-holed stopper, so that approximately 2cm protrudes from the top of the stopper and about 6cm extends from the bottom of the stopper.
- Insert the remaining, 8cm, glass tube (C) fully into the two-holed stopper, so that it just reaches the bottom of the stopper, with about 6cm protruding from the top side of the stopper.
- Connect the glass tube (A) from the one-holed stopper to the top of the glass tube (B) that protrudes from the bottom of the second stopper using the rubber tubing. Ensure that the rubber tubing is about 1.5cm over the glass tubing on each side, and is not loose.
Prepare the flasks:
- Fill one Erlenmeyer flask with about 100-125mL of concentrated sodium hydroxide solution
- I normally prepare my own solution with enough solid sodium hydroxide to cover the base of the flask – the dissolution of sodium hydroxide is extremely exothermic – the flask will be very hot when done.
- Fill the second flask with 150-200mL of cold water and firmly seal with the 2-holed stopper. The glass tube extending into the flask should be immersed in the water, but at least 1cm from the bottom. The other tube should not touch the water.
Note: the next steps must be done in quick succession – have all necessary materials within reach and be familiar with the steps before proceeding.
- Pour the aluminum powder into the flask containing the sodium hydroxide solution – the reaction begins slowly, but accelerates rapidly once it starts – it will bubble violently and get extremely hot.
- If you wish, you may immerse the base of the flask in (room temperature) water to slow the reaction – while this works well, it greatly slows the reaction, so I do not typically do this.
- You will have to hold the balloon in position for the duration of the reaction – it may be possible to attach a smaller stopper to the exposed glass tube, to hold the balloon in place, although, this is untested, and will not easily allow you to remove the balloon if it gets too full.
- At the same time (using the same hand), keep pressure on the stopper (of the flask containing water)
- The reaction will take around a minute to really get started, but should be able to fill the balloon in less than a minute after that.
- It is a good idea to stretch your balloon before starting (and, of course, ensure it doesn’t have any holes).
- This procedure may yield somewhat more hydrogen gas than required to fill the balloon (although, not significantly so) – remove your balloon when full, do not wait for the reaction to end.
- If the reaction is still going when the balloon is removed, try to maintain pressure on the stopper of the reactant flask (otherwise, the stopper will pop and splatter the caustic reagents).
- The reaction products are still quite caustic, but may contain solid particles, handle with care.
- If you end up with any liquid on your gloves or the balloon, it will be somewhat difficult to tie the balloon – keep paper towels nearby.
- Once the reaction is done, try to rinse the reaction flask before the contents dries on the sides, or it can be a bit of a challenge to clean.
- Explode the balloon as soon as possible – rubber balloons do not contain hydrogen well, and the gas will escape through the rubber comparatively quickly (in a few hours).
For the adventurous:
The best hydrogen balloons have two parts hydrogen to one part oxygen. You can add oxygen to your hydrogen balloon to improve the explosion (fill the balloon with hydrogen first, as the oxygen preparation is comparatively easy).
In brief, to prepare oxygen with sufficient pressure to fill the balloon, mix about 2mL of potassium chlorate with about 0.5mL of manganese dioxide in a test tube. Stopper the test tube with a one-holed stopper, and mount on a ring stand. Gently heat the test tube with a Bunsen burner. It is advisable to filter the gas through cold water (using the same procedure as above) to cool the oxygen. Fill the balloon from the exposed glass tube.