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

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.

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)?

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!

Did you know that scientists didn’t used to believe in oxygen? Oxygen in the air helps things to burn. But chemists used to think that anything that could be burned contained a mysterious element called phlogiston.

The element that weighed less than zero

Scientists thought that the red hot glow of a burning metal was evidence of phlogiston escaping. They even decided that, because metal weighs more after burning, phlogiston must weigh less than zero! (We now know that the extra weight comes from oxide that forms on metal when it’s heated.)

More phlogiston nonsense

Joseph Priestly (1733-1804) was the first scientist to trap oxygen – but he didn’t realise what he’d done. The phlogisticians thought that when they placed a burning candle under a glass, it gave off phlogiston until the air in the glass was completely saturated with phlogiston.

So when a candle burned even more brightly in the “air” Priestly collected, he reasoned that the air must not contain any phlogiston at all. He  called his oxygen sample “dephlogisticated air”!

Of course, this was exactly backwards. From The Mystery of the Periodic Table:

“The air Priestly thought was full of phlogiston was actually emptied of oxygen. The air he thought was entirely emptied of phlogiston, was actually full of oxygen.”

Goodbye phlogiston, hello oxygen

It was French scientist Antoine Lavoisier (1743-1794) who finally sorted things out and put phlogiston in its rightful place (the history books). How did he do it?

Lavoisier wanted to find out what really happened when a metal was heated. Was something removed from the metal and released into the air (as the phlogisticians believed), or was the reverse true – was something removed from the air and drawn into the metal?

He had the genius idea of measuring the volume of gas in his apparatus before and after the metal was heated.  The result? Lavoisier found that when he heated metal, the volume of air around it decreased. Some of the air had combined with the metal!

Next, Lavoisier heated the specks that had formed on the metal and measured how much gas they gave off. Of course, it was the exact same amount as had left the air and gone into the metal previously.

Lavoisier had proved that neither phlogiston nor dephlogisticated air were real. He renamed dephlogisticated air, “oxygen”.

(Water + phlogiston) + (Water − phlogiston) = Water?

Even before Lavoisier’s breakthrough, scientists had begun to figure out that water was a combination of two separate things.

Henry Cavendish (1731-1810) had found that two parts of what he called”inflammable air” [hydrogen] combined with one part of “dephlogisticated air” [oxygen] made water.

But Cavendish’s inability to see beyond phlogiston got him in a bit of a pickle.

“He thought that inflammable air (hydrogen) was actually water plus phlogiston, and that dephlogisticated air (oxygen) was actually water minus phlogiston. What happens when you add water-plus-phlogiston to water-minus-phlogiston? The plus and minus phlogistons ‘cancel’ each other out, and you are left with only water!”

 The Mystery of the Periodic Table

Thankfully Lavoisier – debunker of phlogiston – was able to put things in order. He made water by sparking oxygen with some of Cavendish’s “inflammable air”.

Now that he had proved that phlogiston didn’t exist, Lavoisier realised that inflammable air must also be an element itself. He named this gas, “hydrogen” (Greek, for water-generator).

Atomic Pancakes

The French rewarded Lavoisier for his services to science by chopping off his head. (They were a bit guillotine-crazy back then.) We decided to honour the great scientist by making atomic pancakes.

You need

• Pancake batter
• White chocolate chip “protons”
• Dark chocolate chips “neutrons”
• Small sweets e.g. M&Ms (all the same colour) – “electrons”
• Chocolate sauce (and a toothpick for spreading it into “orbits”)

(We actually used red and green grapes as protons and neutrons, but we struggled to fit them all into the nucleus of our oxygen atom.)

{See full instructions here.}

First, we made two small hydrogen pancakes.

Each hydrogen atom has one proton at its centre, and one electron orbiting the nucleus.

Then we made one big pancake for our oxygen atom.

Oxygen has 8 protons and 8 neutrons in its nucleus.

Oxygen also has 8 electrons – one pair in its first orbit, and 2 more pairs in its second orbit. The second orbit also contains 2 single electrons.

To make our water molecule, we put the 2 small pancakes beside the large pancake, lining up the 2 sets of unpaired electrons.

In reality, electrons are really far away from the protons and neutrons. If a proton were as big as a grape, you would need to walk an hour before you set down your electron!

After eating up our atomic pancakes, we moved onto making the real thing. Come back soon to find out how we made oxygen and hydrogen out of water!

Resources

The Mystery of the Periodic Table – A wonderful living book about the history of chemistry – a great read aloud for all ages.

CSIRO – Australia’s national science agency’s website. I came across atomic pancakes via their (free) Science by Email program.

Chemistry, a Volatile History – Fascinating BBC documentary series with a whole episode on the phlogiston blind-alley. We saw it a few years ago and would love to see it again. YouTube has clips. Please let me know if you find the whole thing available somewhere!

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I’m appreciatively linking up here:

Science Sunday at Adventures in Mommydom

After School Link Up at Planet Smartypants

Weekly Wrap-Up at Weird, Unsocialized Homeschoolers

The Hip Homeschool Hop

The HomeEd Linkup Week 7 at Adventures in Home Schooling

Science Saturday at Suzy Homeschooler

{This post contains Amazon affiliate links.}

If you blow out a candle and then put a lighted match close to the wick (but not touching it), the wick will re-light. Most of us intuitively know this, but have you ever wondered why?

This week we did two simple experiments investigating the science of how candles burn. Both come from a free online Kitchen Chemistry course we’re enjoying.

Experiment 1 – Lighting gaseous wax

What you need

• Candle
• Match or lighter

What you do

Light the candle. Notice how you have to hold the match very close to the wick for a second or two before it ignites. Allow the candle to burn for a few minutes. As it burns, observe what happens to the wax.

Now blow out the candle and quickly hold a lighted match near the wick. This time the wick should easily ignite, even without the flame actually touching it.

Repeat the process a few times, experimenting with how far away from the wick you hold the match.

What’s happening?

When you light the candle, the solid wax melts and liquid wax is drawn up the wick. As the candle gets hotter, the liquid wax evaporates into a gas. This gaseous wax burns in the oxygen of the air.

The gaseous wax remains in the air after you blow out the candle. If you hold a lighted match near the hot wick, the wax ignites and the flame spreads to the wick. If you allow the candle to burn for long enough that it produces a visible white vapour when you blow it out, you can light the vapour from above.

Experiment 2 – Soot on a spoon

What you need

• Lighted candle
• Metal spoon

What you do

Briefly hold the spoon in the candle flame. Remove it and observe what you see on the spoon.

You should see black soot. You may also see a tiny bit of wax. (Don’t worry, it all washes off.)

What’s happening?

As the candle burns, solid wax becomes liquid and then evaporates to become a gas. The gaseous wax burns in oxygen to produce water, carbon dioxide, heat and light.

The burning candle also produces carbon, in the form of the black soot we see on the spoon.  It is glowing soot that causes the candle give out light.

If there were enough oxygen to burn all the wax, only carbon dioxide and water would be produced and the flame would be blue, like in a gas burner.

The small amount of wax on the spoon is the unburnt gaseous wax which has condensed on the cold spoon and turned back into solid wax.

Golden gas and solid methane

These experiments are an interesting way to explore states of matter. My kids were surprised to discover that every element can exist as a solid, liquid and a gas.

C(10) wondered if gold can be a gas. The answer is yes – gold boils and evaporates at 2,800°C.  And solid gold becomes a liquid at just over 1000°C.

Meanwhile J(9) wanted to know if gaseous bodily emissions can take solid form (and if so could we google photos). He phrased it differently, as you can imagine. This led us to the fascinating topic of solid methane, a source of fuel which exists in very cold conditions at the bottom of the ocean and at the poles.

Trick re-lighting candles

We also wondered how trick birthday candles work –  the ones that re-light by themselves after you blow them out.

The “magic” ingredient is usually magnesium in the candle’s wick. When the candle is burning, the magnesium is shielded by the liquid wax being drawn up the wick. But after the candle has been extinguished, the wick is no longer hot enough to draw up the liquid wax. The magnesium is exposed to the wick, which is hot enough to ignite the magnesium The burning magnesium in turn ignites the gaseous wax.  This article explains the process very clearly.

Apparently you can see tiny flecks of magnesium going off around the glowing ember of re-lighting candles.  I’ve ordered some so we can observe this for ourselves.

Further Resources

Scientist Michael Faraday gave a series of children’s lectures about the chemistry of candles at the Royal Institution, London in 1860. You can read an abridged version here.

How does a candle work?  (How Stuff Works)

The Mystery of the Periodic Table is a wonderful living book about the history of chemistry. This week we read about the discovery of oxygen, which was the perfect background to our experiments.

Kitchen Chemistry  Free online course from the University of East Anglia. (Runs until 26 May 2014.)

Main photo credit: Windliest

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I’m appreciatively linking up here:

Science for Kids at Adventures in Mommydom

After School Linky at The Educators’ Spin On It

The Hip-Homeschool Hop

Wonderful Wednesdays at Solagratiamom

Weekly Wrap-up at Weird Unsocialized Homeschoolers

Collage Friday at Homegrown Learners

The Sunday Showcase at Mom to 2 Posh Lil Divas