Last week Cordie thought up a fun liver and hydrogen peroxide enzyme experiment. The idea is an interesting extension of elephant toothpaste. And it extends the chemistry learning into biology (useful for homeschool records).
When we make elephant toothpaste we use yeast as a catalyst in the breakdown of hydrogen peroxide into water and oxygen gas. By adding soap and food dye, we get oodles of colourful foam that make for a fun and memorable science lesson.
Cordie recently discovered that liver also contains a catalyst which breaks down hydrogen peroxide. She decided to try to inflate a balloon with the gas produced and to test it for oxygen. (Is it just my kids that love experiments where they get to play with fire?)
You can watch Cordie demonstrating her experiment in the video [4:39] below (with crumpet cameo from Jasper).
What you need
Liver (we used about 200g)
Hydrogen peroxide (we used about 75ml / 1/3 cup of 9% / 30 vol)
Balloon
Small plastic water bottle
Funnel
Peg or clip
Knife
If you want to test for oxygen you’ll also need:
Splint (thin piece of wood)
Lighter/matches
What you do
1. Chop the liver and put it into the bottle
2. Pour the hydrogen peroxide into the balloon via the funnel
3. Carefully put the neck of the balloon over the bottle so that the hydrogen peroxide pours onto the liver
4. Hold the balloon in place as it inflates with gas, then clip it closed
5. If you want to test the gas, light the splint then extinguish the flame. Immediately insert the still-glowing splint into the bottle
What happens
As soon as the hydrogen peroxide touches the liver, foam appears and the bottle gets warm. After a few seconds the balloon begins to inflate.
When you lower the glowing splint into the bottle, the flame rekindles. (My kids’ favourite bit!) There should be enough oxygen to do this over and over again.
What’s happening?
Just as with elephant toothpaste, the hydrogen peroxide is broken down into water and oxygen in the presence of a catalyst. (A catalyst speeds up chemical reactions without being changed itself.) The reaction is exothermic – it produces heat.
2H2O2 —-> 2H2O + O2
Liver contains a biological catalyst, the enzyme catalase.
Just as the liver in our experiment breaks down a poisonous chemical into harmless substances, an animal’s liver breaks down toxins and renders them harmless.
Take it further
Heat and cold affect how enzymes work. In Cordie’s science class she timed her experiments using boiled and frozen liver alongside liver at room temperature.
Further resources
BBC Bitesize – Webpage and video about liver, hydrogen peroxide and enzymes
Do let me know if you try this. I love hearing from you. 🙂
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Science is one of the easiest and most enjoyable subjects to learn without a curriculum. Science experiments are also surprisingly easy to strew.
What kid – big or small – can resist the temptation to find out what will happen when we add this liquid to that powder, or when we connect a battery to this strange contraption?
What’s in my unschooling science video?
In my video this week I talk about – and show you – a fun afternoon we spent experimenting. As you’ll see, my children each took the initial idea to make red cabbage indicator in a completely different direction.
And you’ll hear about the shocking discovery I made when I recently browsed a science curriculum for KS3 children (aged 11-14).
My son would (approvingly) call the previous sentence ‘click bait’. Sorry about that. I first wrote ‘surprising discovery’ but I went back and changed it because my jaw really did drop at what I saw!
I plan to compile two more mini videos from the footage of our afternoon’s science:
(1) Our demonstration of how to make red cabbage indicator, and
(2) Cordie’s liver and hydrogen peroxide experiment that I talk about in this video.
You might also like to look at my science page for other fun experiments we’ve done.
* * *
Thank you so much for all your lovely comments about my first video, and for your inspiring ideas for future videos. I did record another last week in which I talked about how we decide what to learn, but I’m not sure about it. (Perfectionism? Or fear of not being seen as a ‘proper’ unschooler? Maybe I’ll quietly put it up on YouTube anyway.)
Every summer when I declutter my science supplies cupboard I come across a few hidden treasures. (The reality: “Oops, we never did get around to making shadow leaf prints / home made light bulbs / popsicle stick trebuchets,” accompanied by a pang of guilt. Is that just me?)
The Epsom salts were bought to make bath fizzies with C(11) last Christmas. The fizzies never happened, but on the bright side, we had an unopened pack of Epsom salts when CSIRO’s cool crystals email landed in my inbox.
J(10) asked if Epsom salt was like the stuff we put on our fries, which was a good opportunity to remind ourselves what we learned about salts when we concocted our own fizzy drinks a few months back: A salt is created when an acid and a base neutralise each other.
Epsom salt is another name for magnesium sulfate. We looked up magnesium and sulfur in our book, The Elements, and noticed how very different the salt is from its constituent elements.
All you need to make quick crystals
What you need
Epsom salt (1/2 cup)
Hot water (1/2 cup)
Food colouring (optional)
Glass, spoon
What you do
Put the salt and water in the glass together with a few drops of food colouring. Stir for about five minutes, then put the glass in the fridge for at least three hours.
Dissolving the Epsom salts
What happens
After just a few hours in the fridge, you get beautiful crystals like these.
Epsom salt crystals
We carefully drained the water to get a better look at our crystals.
Taking a closer look at the crystals
How do crystals form? The scientific explanation
Epsom salt is an ionic compound. It’s made up of magnesium and sulfur ions joined together by ionic bonds. When we dissolve the salt in hot water, these bonds break and the two elements become separated .
Later, when we cool the salt solution in the fridge, the magnesium sulfate ions no longer have enough energy to move about freely. The ions begin to re-bond, first as single molecules and then – as the molecules themselves begin to join together – as crystals.
Science Kids at Home has some cool diagrams showing what’s happening at a molecular level.
Epsom salt crystals
More crystal science
Different types of molecule always the same shape of crystal, every time they form.
In the past we’ve also made crystals from table salt (sodium chloride), borax and sugar. The process for each of those is slightly more fiddly, but comparing the different shaped crystals is interesting. Borax crystals make pretty decorations, and sugar crystals are yummy!
Sugar crystalsSalt crystalsBorax crystals
And now the Epsom salt packet has been opened, we’re one step closer to making those bath bombs. 🙂
***
I’m appreciatively linking up with Weird Unsocialised Homeschoolers’ Weekly Wrap-Up and All Things Beautiful’s Science Sunday.
Close your eyes and imagine taking a long sip of your favourite soda. How does it taste? Now imagine drinking a different type of soda – Sprite, or Pepsi, maybe. What taste do the different fizzy drinks have in common? Are they salty? Acidic? Something else?
In this fun Science Buddies lab we discovered how sodas get their fizz, then we experimented to find our personal favourite soda recipes.
1. Mix 1/16 tsp baking soda with 1/4 tsp citric acid in an empty cup.
2. Add 1/4 cup of cold water and quickly stir, then taste. (You’ll probably want to have somewhere to spit out, too, especially for the first few mixtures.) Observe the reaction between the chemicals in the water, and start your timer for 1 minute.
Adding water to our fizz powder
3. Discuss (and, if you wish, record) your observations. How bubbly is the mixture, on a scale of 1-5? How ‘gritty’ is it?
4. Observe and taste again after 1 minute. Has the taste changed? Is the drink more or less bubbly? Discard any remaining liquid.
5. Repeat in a clean cup, increasing the amount of baking soda to 1/8 tsp. (The amount of citric acid stays the same throughout.) Repeat again using 1/4, then 1/2 and finally 1 tsp of baking soda.
6. Make a note of the formula that tasted best. Did everyone like the same?
7. Experiment by adding different amounts of sweetener to your preferred base recipe, beginning with 1/4, 1/2 then 1 tsp.
Nothing happens when you add the two white powders (citric acid and baking soda) together. But when you add water, bubbles are produced. More bubbles are produced when you increase the proportion of baking soda, and the reaction lasts longer.
How does it taste?
Depending on the amounts of baking soda used, our drinks ranged from fairly disgusting to reasonably palatable. J(10) hated every single unsweetened beverage, confirming our suspicion that he has only persuaded himself to endure fizzy drinks because of their ton of added sugar.
Fizzy drink flavourings (if your’e feeling brave)
We also tried adding a few flavourings. J(10) had run away to clean his teeth by this point, but C(11) was keen to try chocolate flavour soda. I suspected that if that combination worked we’d already know about it. I was right.
Vanilla soda wasn’t much better, but lemon juice worked nicely (of course, adding lemon juice also increases the ratio of citric acid to baking soda). Finally, we taste-tested our fizzy drinks against shop-bought lemonade, and decided our formula stood up pretty well against Schweppes.
Making fizzy drinks is a great demonstration how an acid and a carbonate react in the presence of water to form carbon dioxide, a salt and water.
Citric acid + Bicarbonate of soda ——> Sodium citrate (a salt)+ Carbon dioxide + Water
If you want to talk ions:acids ionise in water. This means they lose electrons, producing positively charged hydrogen ions. Meanwhile, a carbonate is a mild alkali. Alkalis in water generate negative ions, which combine with the positive ions from the acid in a neutralisation reaction.
You can also make a batch of ‘fizz powder’ by mixing citric acid, baking soda and icing (powdered) sugar. This time the reaction happens on your tongue!
We followed Science on the Shelves’ recipe. Mix 6 tsp citric acid, 3 tbsp bicarbonate of soda and 2 tbsp icing sugar, then crush with a spoon to make a fine powder.
My husband and I found the fizz powder charmingly reminiscent of the sherbet dib-dabs we’d buy with our pocket money as children, but – as you can see from the photo – our kids weren’t convinced.
Taste testing our fizz powder
More fizzy drink science
Try testing the acidity of your home-made sodas with indicator paper or a home-made indicator.
Remind your kids not to drink too many sodas by showing them the effect on their teeth – see Ticia’s What soda does to teeth.
We don’t have fizzy drinks at home but when we were in Spain C(11) and I sometimes treated ourselves to a coke at the beach cafe. J(10) has never liked anything fizzy, but while we were away he decided to overcome his aversion. The 1-minute video below shows how that turned out.
This lab appealed to us because we had been speculating, in the light of J(10)’s distaste for all fizzy drinks, what they must all contain, apart from bubbles. And I thought it would be fun to watch J(10)’s face as he taste-tested the various recipes {mwah ha ha}.
Look out for J(10)’s taste-bud theory about why C(11) and I can drink fizzy drinks, and his verdict on alcohol, which I shall be reminding him of on his 18th birthday. 😉
I’m appreciatively linking up here:
Science Sunday at All Things Beautiful
Weekly Wrap-Up at Weird Unsocialized Homeschoolers
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.
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.
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)?
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!
Bleached filter paperAfter 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.
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.
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?
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”!
“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”.
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!”
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.)
Proton and neutron grapes, and white chocolate chip electrons
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 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.
Water molecule
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!
Putting electrons into orbit
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!
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!
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.
Lighting a candle by holding a flame near the wick
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.)
Holding a metal spoon in a candle flame
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.
Frozen methane bubbles: U.S. Geological Survey
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.
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.)
This fun science trick demonstrates that hot and cold water have different densities.
The demonstration reminded my kids that lower density liquids float above higher density liquids. They learned that hot water molecules move faster and further apart from each other than cold water molecules.
The trick also allowed us to revisit the topic of surface tension and to build on the air pressure science we learned last term.
What you need
2 identical jars or drinking glasses (eg babyfood or salsa jars)
hot and cold water
red and blue food colouring
index cards or similar
scissors
a large shallow dish to catch spills
What you do
1. Cut a square of card big enough to completely cover the mouth of a jar.
2. Fill one jar with cold water. Add a drop of blue colouring and stand the jar in the baking pan.
2. Fill the second jar with very hot water. Add a drop of red colouring.
3. Slowly add more hot water to the red jar until you see a bulge of water over the rim. (Take the opportunity to talk about surface tension here.)
4. Lay the square of card on top of the jar of hot water and tap it gently.
6. Put the upside-down red jar on top of the blue jar. Ask your students to predict what will happen when you slowly pull out the card, then do so.
Atmospheric pressure holds the card and water in place (left)
What happens
When you remove the card between the two jars, the hot red water stays in its jar on top and the cold blue water stays on the bottom.
Hot water floating on top of cold water
Reverse the jars
7. Repeat the experiment the other way round. Turn the jar of cold water upside-down and put it on top of the hot water jar. Again, ask students to predict what will happen when you remove the card.
What happens
When you remove the card, the water quickly turns purple, as the cold and hot water mix together.
Hot and cold water mix
The scientific explanation
When water is heated, the water molecules move faster. They bounce off one another and move further apart. The extra space between the molecules means the same volume of water weights slightly less and is less dense than cold water.
When we put the hot water on top, the less dense hot water floats on top of the denser cold water. The red water and blue water stay in their respective jars and do not mix.
When we put the cold jar on top, the more dense cold water immediately sinks to the bottom and the two colours mix.
We talked about a previous demonstration in which we created a column of liquids with different densities, and reminded ourselves by pouring water into a glass of oil and watching the oil float to the surface.
They heated the salt solution in a saucepan, capturing the evaporated water on the oversized lid. The condensed water was then collected as it dripped off the lid.
Separating water and salt
I hadn’t been sure how C(9) and J(8) would get on with this activity. I knew they’d enjoy it, but I thought I might have to help them more than I did.
Seeing how they confidently and intelligently tackled the puzzle reassured me that our science un-curriculum is still working well.
This week we decided to extend our gummy bear science lab see what happens when you leave them for a bit longer.
J(8) wondered if the same would happen with jelly babies (a popular British sweet, a bit like gummy bears but with a harder outside).
Supplies
Gummy bears
Jelly babies
Water
Glass or jar
Refrigerator, if you want to taste test
What we did
We placed our gummy bears and jelly babies in separate glasses of water and left them for a week (but twenty-four hours would do). We put them in the fridge, to minimise bacterial growth, in anticipation of a taste test.
What happened
We used our senses to examine our gummy bears and jelly babies.
We could SEE that the gummy bears had become three or four times bigger and become more translucent
They FELT “squidgy” and “gooey”
They didn’t TASTE good at all!The jelly babies also expanded… but their hard outside layer split
Gummy bear science
Gummy bears and jelly babies contain water with lots of other things dissolved in a concentrated solution.
The water in the glass flows through the gummy bear’s semi-permeable membranes. This flow (diffusion) of water from a region of low concentration of solute to a high concentration is called osmosis.
The sweets became more translucent partly because the colouring had to go further in the larger sweet, and maybe partly because some of the colouring dissolved in the water.
We were intrigued by some of the gummy bear (right) and jelly baby (left) ingredients
In the case of the jelly baby, we hypothesised that although water can pass through the (sugar?) shell, the outside of the jelly baby is unable to absorb water as fast as the inside, causing it to crack.
More fun with osmosis and jelly babies
We also came across this exciting jelly baby oxidisation experiment – screaming jelly babies. Sadly not one to try at home!
And as for osmosis, I can’t believe we’ve been homeschooling for three years and we’ve never done the celery or carnation experiments?! Guess what we’ll be trying next…
