Chemistry for kids – How to separate water into hydrogen and oxygen using electrolysis

 

Electrolysis of water for kids

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
  • water
  • battery (we used 6V, a bit like this one)
  • 2 pairs of crocodile clips
  • waterproof tape
Electrolysis of water for kids
Electrolysis apparatus

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.

What happens

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?

Water is a covalent molecule (H20) held together by shared electrons in covalent bonds.

During electrolysis, the molecules are reduced at the cathode to to hydrogen gas, and oxidised at the anode to oxygen gas.

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.)

Twice as much hydrogen as oxygen is produced, reflecting the molecular composition of water.

Electrolysis of Water
Credit – J Squish

 

Here’s a fairly easy-to-follow explanation of the electrolysis of water.

If you’re looking for a more detailed explanation, see Wikipedia.

{Thank you so much, Sarah, for pointing out my earlier misunderstanding and for making this post more accurate!}

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.

Further investigations

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)?

Electrolysis of water for kids
Changes as a result of electrolysis

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!

Electrolysis of water for kids
Bleached filter paper
Electrolysis of water for kids
After electrolysis our solution was slightly acidic

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.

Electrolysis of water for kids
Unharmed £10 note

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.

Electrolysis of water for kids
Making elephant toothpaste

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

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58 thoughts on “Chemistry for kids – How to separate water into hydrogen and oxygen using electrolysis

  1. Wow, this is really excellent, so much extension from a seemingly straightforward experiment! We did the very first part of the electrolysis, with slight variation from yours, and didn’t think to test the gas! I really like how you approach science!

    1. Thank you so much, Hwee – you are very kind! I do love it when science unfolds like this did. It feels so easy when one is naturally caught up in the excitement!

  2. You really are making science fun. These are subjects I figured were too mature/boring for my younger student but this gives me ideas on how to introduce it. I’m visiting from Weekly Wrap Up hop.

    1. Nita, I did wonder if this would go over my kids’ heads but it was something I decided I wanted to do anyway, so any learning on their part was a bonus! Perhaps because of that, they go caught up in the fun and learned quite a bit too!

    1. Thank you, Carol. It was one of our favourites so far. As soon as I get hold of a giant bag of M&Ms we’ll be doing your recent science activities – they’ll complement this well!

  3. Lucinda,

    Your enthusiasm for science experiments is evident. I bet you and your kids have so much fun together. Do you enjoy science as much as your children? It certainly seems that way which is probably why your science demonstrations are so successful and mine aren’t!

    I do like the look of that elephant toothpaste. Science provides lots of opportunities for interesting photos!

    1. Sue,

      I think I probably do enjoy science at least as much as my children! Once I get around to doing it, that is… It’s so much easier to gather maths books, or pencils and paper to write stories, than it is to set up equipment for science. But once we’ve started, momentum definitely carries us forward. I remember being very inspired by your posts about Charlotte’s interest in chemistry.

      It’s funny you mention the elephant toothpaste photos. I enjoyed taking those so much that after we’d finished the experiment, I put the camera memory stick in my pocket so I could look at the photos on my Mac during the children’s swimming lesson. But… when I went to get the memory stick out, it was gone. The only place I could think I might have dropped it was when we’d walked the dog before swimming. So after swimming I went back to the woods … and found the memory stick buried in mud, right where I’d parked the car! The photos on it have been pinned hundreds of times, and they’re near the top of a google image search for “elephant toothpaste.” C suggested I should give all our photos a spell in mud!

  4. I love the way you do science. I need you to teach my guys their science. I keep procrastinating this term because Native Americans are so much more interesting!

    1. Elephant’s toothpaste is so much fun, isn’t it, Jessy? I must get some more hydrogen peroxide in so we can do it again – it’s such a great science standby!

    1. Thank you so much – you are very sweet!

      Brave… foolish …?! I suspect a lot of the science I do with my kids goes over their heads, but they definitely get caught up in my excitement – which can’t be a bad thing, I hope! 😀

  5. This is a good post…you light up some new idea in kids mind ,i am thinking how we can make H2 as a fuel for vehicles and industries….

      1. Ok,Welcome,fine i would like to join your group regarding this,i completed my M.sc food science and technology..and now working in an MNC as QA supervisor come analyst..

          1. Yup ,sure its interesting ..me searching jobs in European countries..i heard that there are lots of vacancies… what about your carrier and family..I think you and your group trying to give knowledge to kids..am I right?

    1. Hi Danielle,
      This tied in for us with our “oxygen pancakes” activity, in which we looked at the composition of a water molecule and hydrogen and oxygen atoms. (Using food, which always goes down well round here!)

  6. Hello Ms.? or Mr.?

    My Name is Daniel and I was wondering how do create oxygen with electrolysis without obtaining chlorine or and black pieces. Pls inform. I put a 9 volt battery in water and i had the same results. Pls help thanks.

    1. Hi Daniel, When we used baking soda (bicarbonate of soda) as our electrolyte we didn’t produce chlorine or black pieces. We only got them when we performed electrolysis on a saturated salt solution. If you follow the procedure outlined at the start of this post you should be able to do the same.
      I hope this helps. Let me know how you get on.

  7. Hello all..
    This is Jewel..I have food technology background.Now I am working in Qatar,from my knowledge what I would like to share you all is ..water is the elixir of life…like our food washing,car washing…water can remove the poisonous material inside our body. The waste removal in our body is in mainly three ways
    1.by Sweating
    2.by urine
    3.by fecal
    so if we drink more water we can remove maximum waste from our body,while summer time the water mainly goes through the urine and while cold its mainly by urine.
    So by god gift we can do wonders with water,may be that’s why god given water more to earth.
    Like fuel burning mechanism,oxygen should need for each process..so by water/oxygen/and good thinking we can do wonders…

    1. Hi Jesse,
      I’m so pleased your electrolysis worked!
      What did you use as your electrodes, and as your electrolyte?
      The greenish foam may have been a product of a reaction involving one of them?

  8. Hi, I think this experiment is great, but I just want to point out that your explanation of the electrolysis of water is incorrect. You cited Wikipedia, but you have misinterpreted the information presented there. Water is NOT an ionic substance and the hydrogens and oxygen don’t simply pull apart and collect at the different electrodes. Water is a covalent molecule held together by shared electrons in the covalent chemical bonds. During electrolysis, the molecules are reduced at the cathode to hydrogen gas and oxidized at the anode to oxygen gas. That’s two different reactions going on, not one single splitting. Compounds like table salt are ionic. It dissociate into positive sodium and negative chloride when it dissolves. I recommend that you revisit your Wiki reference and revise your explanation or at least delete it. Try this resource for an easier to understand explanation: http://www.nmsea.org/Curriculum/7_12/electrolysis/electrolysis.htm

    1. Hi Sarah, Thank you so much for taking the time to leave your comment, I appreciate it. I’m going to have a good look at that link and will update the post asap!

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