18 Sep 2014 25 Comments
We’ve all been told that water is made up of hydrogen and oxygen. But how do we really know that? Can this wet substance that quenches our thirst and cools our bodies on hot summer days really be made up of two gases?
We tried to separate water into oxygen and hydrogen using electrolysis. We managed it after a series of experiments that left us with even more questions than we had before we started. Which isn’t necessarily a bad thing – curiosity is a great learning state! (See the mysterious case of the missing oxygen, below.)
You can benefit from our mistakes and perform electrolysis the quick way. Here’s how to split water into hydrogen and oxygen using electrolysis. Afterwards I’ll tell you about what we did first, which produced a different gas entirely.
How to separate water into hydrogen and oxygen
What you need
- glass or plastic tub
- 2 elastic bands
- 2 test tubes (with lids if possible)
- bicarb of soda (1 tbsp)
- graphite pencil leads
- battery (we used 6V, a bit like this one)
- 2 pairs of crocodile clips
- waterproof tape
What you do
See this video for detailed set-up instructions – the elastic band arrangement keeps the test tubes in place perfectly.
If you can’t watch the video, here’s the gist of it: Connect one end of each crocodile clip to a piece of graphite, and the other to the battery. Secure the graphite ends to the bottom of the tub with the graphite sticking up, and place an inverted test tube over each piece of graphite (held in place by the elastic bands). Dissolve the bicarb of soda in the water and fill the tub. Finally, remove each test tube, fill it with the water, and carefully replace it over the graphite. Any gases collected during the electrolysis will replace the water in the tubes, so make sure there are no air bubbles.
Bubbles of gas quickly start to form at each electrode. More gas collects at the negative electrode (cathode) than at the positive (anode).
How to test your gases
When you’ve collected plenty of gas at each electrode, carefully put the lids on your test tubes (while they’re still underwater).
To test for hydrogen
We hypothesised that the gas at our (negative) cathode was (positively charged) hydrogen. Hydrogen is explosive. It won’t wreck your house in these quantities, but it will make a cool popping noise in the presence of a lighted splinter of wood. You can hear it in the video below.
To test for oxygen
We test for oxygen with a glowing splint. If enough oxygen is present, the splint rekindles. The gas we collected at our anode gave a brief glow which confirmed it to be oxygen, but after the excitement of the popping hydrogen, we were a bit disappointed. We produced much more oxygen later using a different method – see below for a video of our relighting splint.
How does electrolysis work?
During electrolysis, we pass an electric current through an ionic substance to break it down into simpler substances. Pure water doesn’t conduct electricity, so we need to add an electrolyte, like bicarbonate of soda. (You wouldn’t believe the number of websites that tell you to use salt. We tried it, and collected a completely different gas. More on that later.)
Hydrogen is positively charged in the H₂0 molecule, so it collects at the negative electrode. Oxygen is negatively charged, so it collects at the positively charged electrode.
Twice as much hydrogen as oxygen is produced, reflecting the molecular composition of water.
Wikipedia has an excellent detailed explanation of the electrolysis of water.
The mysterious case of the missing oxygen
(Or, what happens when you use salt as an electrolyte.)
Before we successfully split water into hydrogen and oxygen using the method above, we tried adding salt to help our water conduct electricity. And not just a pinch of salt. I decided that if a little salt would help a bit, then a lot of salt would be even better. (It works for crystals, after all.)
We set up our electrolysis using the same apparatus as above but this time with a saturated salt solution. And there we sat, eagerly looking for our bubbles of hydrogen and oxygen.
What happened? Well, plenty at our cathode. Gas quickly began to fill the test tube. We tested it and discovered it was hydrogen. And at the positive electrode? Not one single bubble of gas! What had happened to the oxygen from our water molecules?
I did a bit of research overnight.
It seems that during the electrolysis of sodium chloride (salt) solution, sodium chloride breaks down at the positive electrode to form chlorine gas and sodium hydroxide solution. (Click the link for a more detailed explanation.) Chlorine dissolves easily in water, so won’t collect as a gas until the solution is saturated and can absorb no more chlorine.
So if our positive electrode was busy attracting chlorine, and hydrogen was collecting at the cathode … what had happened to the oxygen? Or to the sodium from our sodium chloride (NaCl), for that matter? According to the chemists, the sodium and oxygen combine to make sodium hydroxide solution. Further investigation was called for.
We’d left our apparatus set up – disconnected from the battery – overnight. We decided to examine it for clues.
What changes had taken place as a result of electrolysis?
Our salt solution had turned a brownish colour. Was this dissolved chlorine? Broken down graphite? Corroded crocodile clip (which had been attached to the anode)?
Filtering the solution.
Some of our positive electrode (anode) broke down, leaving black bits in the solution. We use graphite in electrolysis because it is an inert (non-reactive) metal, but perhaps the large amounts of chlorine we produced had caused it to react? We filtered the brown solution to see if any insoluble bits remained. They didn’t. But we did notice some white spots on the filter paper – the chlorine produced at our positive electrode must have bleached the paper!
Testing the pH of the solution
We hypothesised that the solution would be slightly alkali due to the sodium hydroxide. But when we tested it, we found the opposite. It was slightly acidic – like chlorine. We guessed this meant the solution must contain more chlorine than hydroxide.
More fun with oxygen
I’m going slightly off topic here, but I promised to say how we created enough oxygen to successfully test for it. We got the idea from going to The Magic of Oxygen show at the Royal Institution. I’d love to share with you one of the demonstrations we saw there.
The presenters asked me if they could borrow a £10 note from me – and then they set fire to it! Here’s a video of my flaming money.
Not long afterwards the scientists returned my £10 note – completely undamaged. The trick was the scientists first soaked the money in alcohol. The alcohol burning in oxygen produces heat, light, carbon dioxide and water. The temperature the alcohol burns at is too low to evaporate the water, so the water protects the note from burning.
The Magic of Oxygen scientists also demonstrated how to make “elephant toothpaste” by breaking down hydrogen peroxide. We remembered how we once made our own elephant toothpaste. When we got home we decided to make elephant toothpaste again, and use a glowing splint to test for oxygen gas.
When you place a glowing splint into oxygen, the splint re-lights.
Why this is my favourite way to do homeschool science
As you can tell, this was not the the kind of homeschool science demonstration where mum knows exactly what’s going to happen and why. I studied chemistry until I was sixteen – nearly thirty years ago! I didn’t know the answers to many of the questions generated by these experiments.
But not knowing what would happen made me curious and inspired to learn more, and the children were definitely caught up in my excitement. And I’m glad we made the “mistake” of using salt as an electrolyte first, because if we hadn’t we would have missed out on some very cool science!
Have you done any fun science recently?
Have you ever investigated a case of missing oxygen?
I’m appreciatively linking up here:
Weekly Wrap-Up – Weird Unsocialized Homeschoolers
Collage Friday – Homegrown Learners
The Home Ed Link Up #16 – Adventures in Home Education
Science Sunday – Adventures in Mommydom
Finishing Strong – Starts at Eight
The Hip Homeschool Hop – Hip Homeschool Moms