Archives For science at home

Solid, liquid, gas. The three states of matter are something I first explored in primary school and water was the best example. You can easily see water frozen in your freezer, and it spews as a gas from your kettle. But if you mess with these normal states, you can do some fun and strange things with something as everyday as water.

We’re all taught in school that water boils at 100°C and freezes at 0°C. This range was defined by a Swedish astronomer(!) called Anders Celsius when he devised the temperature scale that bears his name and which is also known as ‘centigrade’. It’s a very useful way of thinking about temperature[1].

Rather awesomely, there is a way to cool water below 0°C without it becoming ice. The process is called ‘supercooling‘. To try it you’ll need a bucket, some bottled water, lots of salt (we used 700g of the stuff doing this), and a big bag of ice.

Supercooling Kit

Supercooling involves lowering the temperature of a substance but preventing it taking a solid form. In the case of water you can supercool it way down (to about -45°C if you have the right kit) – just so long as you can stop ice crystals forming. Ice forms around ‘nucleation sites’ – places where ice crystals can get a foothold and start growing. These sites are usually impurities in the water or on the surface of some other material that its in contact with. So if you can get very pure water in a very smooth container, you can take it below freezing but keep it liquid. For that reason you should use plastic bottles of mineral water, but filtered water will work too. We used both here.

Salty Slush

Instructions

  1. Put some water into a bucket and pour all the salt in. Mix it up really well, and try and make sure the salt is all dissolved.
  2. Stand the bottles of water in the bucket and pour in the ice cubes until they almost cover them. You want to cover the bottles just enough that you can still pick them up and also turn them.
  3. Mix it up! Salt reduces the freezing point of water and so the ice will melt quite a bit as this goes along. You want to create a bucket of briney slush. This will let the temperature get very low.
  4. Now you need to leave your bottles alone. Every ten minutes or so, go and rotate them gently. This is just to make sure the water cools down evenly and is cooled throughout. After about 45 minutes the process should be complete. You should be able to feel that the bucket is very cold to the touch – ice will be starting to form on the outside.
  5. Now you can play with your supercooled water…

Supercooling Water

Playing with Supercooled Water

Here comes the fun part! Note that if this stuff isn’t working for you then you probably need more salt and ice – or to give it more time to cool down.

Supercooled water is just waiting to turn into ice. If you gently pick up one of the bottles and then violently shake it, it will almost instantly turn into ice. Shaking it creates air bubbles in the bottle on which ice crystals can form. Once a few have started, it’s a runaway process since the best place for ice crystals to form is on other ice crystals. This is pretty amazing and I couldn’t get a good video or photo of this happening – it was too quick.

What we did a lot of was make ice towers! It’s a seemingly magical process that my four-year-old thought was brilliant. To do this you simply take an ice cube and pour supercooled water onto it. (It’s best to place this on something to contain the excess water!). As the water touches the ice it quickly crystallises and starts to create a tower, building upward as you pour. It’s pretty cool (ahem!). Again: the best place for ice crystals to form is on other ice crystals.

The towers were quite beautiful too, made of pure, transparent crystals. They were quite solid for a minute or two and then quicky disintegrated (it was 22°C in our house).

Why Salt?

Salt is used to grit roads in the Winter because it lowers the freezing point if water and makes the ice melt. In the process it actually lowers the temperature further. If you have a thermometer handy then you can watch the water in the bucket drop in temperature as this experiment progresses. This is why this method works to supercool the bottles – the temperature quickly dropped to around -8°C in our bucket and the slushy mixture makes good contact with the outside of the bottles for quick cooling of the water inside. I suppose this might work in the freezer but opening the lid our of chest freezer would disrupt the cooling and frankly the slushy brine is half the fun!

Science at Home

My four-year-old loved doing this. We talked about molecules slowing down and ice forming. We talked about the Winter and frozen puddles, road gritters, and cold drinks. Mixing in the salt and ice is fun and making ice towers blew her away. The 45-minute wait was the hardest part for her – so we made glowing jelly at the same time!

I’m not sure if supercooling is necessarily something that young kids will take away from this but there’s lots to understand about ice and freezing. The ice towers were very exciting and next time I do this I’ll have some gloves ready for the kids so they can do the pouring and shaking – the bottles are too cold otherwise.

I’ll leave you with another ice tower video:

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[1] I cannot fathom why anybody prefers Fahrenheit, which starts with frozen brine at 0°F, the freezing point of water at 32°F and it places the boiling point of water 180° higher at 212°F. Because those are all such handy numbers. Human ‘blood-heat’ was supposed to be at 100°F but things got rearranged – so now it seems odd. Sheesh.

How to Make Glowing Jelly

September 7, 2013 — 28 Comments

Here’s a really simple and fun experiment to do at home: make glowing jelly (or jello, American friends)! The method is really easy – you’re just making jelly – but you do need some kind of UV light source to see the effects[1].

Ingredients

It’s a very simple idea: you make jelly but use a 1:1 mixture of plain ol’ water and tonic water (e.g. Canada Dry). Tonic water contains quinine which is naturally fluorescent and so the resultant jelly will glow under a UV light. Here’s what I did:

  1. Boil 100ml of water and mix with the jelly so it all dissolves.
  2. Top up with cold water up to 300ml.
  3. Add 300ml of tonic water.
  4. Mix it up really well and put it in the fridge.

Quinine glows a blue-ish colour under UV light so I used green jelly to maximise the effect. I’m assuming that the jelly acts as a kind of filter to the fluorescence, so using red jelly would probably result in a very poor glowing jelly. And nobody wants that. There was no blue jelly in the shop; not much blue food in general, actually.

Fluorescence occurs in some materials when they absorb high-energy light photons and then re-emit that energy as lower-energy photons. In this case, the quinine is absorbing UV photons and re-emitting them as visible light (UV light is higher-energy than optical light).

Glowing Jelly!

The results were pretty much awesome. You can see our glowing green jelly above. It tasted great too. Lime turned out to be a good flavour to accompany the tonic water, which would normally have a bitter taste. The kids gobbled it up – under the glow of the UV lamp.

Glowing Jelly

This project was really quick and easy and the kids loved it! I think there may be glowing jelly – and maybe other glowing foods – at Halloween this year.

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[1] You can buy UV torches and lights at Maplin or Amazon. There are likely other places too, of course 🙂

Note: This experiment involves sharp objects and should only be performed by children if under supervision. As long as care is taken, this is a fun experiment with effective results. It can be done without the razor blades, but the results are not as good.

Spectrometers are used, like prisms, to spread light out into the component colours. This enables us to understand the compositions of everything from stars to streetlights. Here I show you how to make your own spectrometer and give you a few examples of what you can see with it.


What You Need:

  • A cardboard tube (toilet roll or kitchen roll tubes are just perfect, in the pictures here, I have used more black card to make a tube myself.)
  • 2 square pieces of black card (approx 8cm x 8cm)
  • Black tape or masking tape (something that blocks out light)
  • Razor blades (nothing fancy just cheap blades that are not attached to anything)
  • A stanley knife
  • An old CD

Make a Diffraction Grating:

Cut a small square hole (approx 1cm across) in the middle of one of your 8cm x 8cm black cards. Break the CD into pieces, just snap it. You’ll need a section of the broken CD that can nicely cover the 1cm hole in your card.

Using a bit of sticky tape, peel away any cover remaining on the piece of CD, so that it is transparent. Use tape to stick it over the hole, creating a sort of window. This will be our diffraction grating.

Make a Very Fine Slit:

Using a stanley knife, cut a slit in the middle of the other piece of 8cm x 8cm black card. This slit should be about 2cm long and just a few millimetres wide. Tape the the two razor blades either side of the slit. They should make an even narrower slit, just 1mm or less if possible!

The aim is create a very fine, narrow slit though which light can travel. Make sure the blades are securely attached with tape for safety.

Make the Spectrometer:

This is the easy bit! You now attach the two square cards to either end of the tube using the dark tape. You have to attach it in such a way that no light is let into the tube accidentally (hence the dark tape). When you look through the diffraction grating, you only want to see light coming from the slit.

Testing Out Your Spectrometer:

The best way to see how this works is to use daylight. Just point the spectrometer toward a window during the day or up at a cloud if you’re outside. You should not ever look directly at the Sun. You should see a nice, smooth spectrum (rainbow) somewhere in your field of view in the tube. Here is a photo of a cloud taken through my own spectrometer. The bright white light is the slit and spectrum is just off to one side.

What’s Happening?

When light enters the tube though the slit it spreads out – all waves do this when passing through small slits. The CD then makes the separate colours visisble to your eye. You see a nice, even spectrum from daylight sources because daylight is made up of all the colours of visible light from the Sun. Once you can see this pattern, you can start trying to find the spectra of other things.

In our physics lab we have lamps of different chemical make-ups. These let us see pure light from different sources. Here are a few I took today, all photos taken by my own camera through my own, homemade spectrometer.

Here is the spectrum for Zinc, which you can see contains some red and blue but very little green.

Cadmium is very distinctive, with short sections of each of the three primary colours and very little between them. It is less spread out than Zinc. There is a big gap between the green and red sections.

Krypton is seen to be fainter than the others here, but the spectrum is still visible. The blue section has become much more violet or indigo here and the green is greener than it was in Cadmium.

The Astronomy Connection:

This is how astronomers know what stars are made of. They use advanced spectrometers to measure the spectrum of stars and pull out the ‘fingerprint’ patterns of colour that you see above. Each element has a unique set of spectral lines (colours) and these can identify the presence of different chemicals in stars, nebulae and just about everything else.

This is the whole spectrum of the Sun. It is so detailed that it had to spread onto multiple lines to see it properly! You’ll see that in fact it is not perfectly evenly spread out as I suggested earlier. This was taken with a very advanced spectrometer that has a greatly increased sensitivity compared to one made here, but its based on the same principles.

Things to Look At With Your Spectrometer:

  • Sodium streelights
  • Compare daylight to a lightbulb.
  • Different light bulbs look different (that’s why energy saving bulbs light up the room in a different way).
  • Neon signs.
  • TV  and computer screens.
  • LEDs from computers or remote controls (these give very pure spectra, often only one colour).

Have fun with your spectrometer and why not try and take a photo through it? It worked fairly well for me. I’d love to see any photos you take with it, or of it. Let me know how you get on. Thanks to the Science Made Simple team for this great idea!

 
 

 

I recently did a piece on measuring the speed of light using your microwave. Well here is some more physics you can play with in your kitchen. This time let’s create a vacuum and then use it to crush something. I like crushing things. Don’t we all?

What you will need:

  • A regular drinks can
  • A pot of cold water big enough to submerge the can
  • A pair of tongs
  • A kitchen hob (gas or electric is fine).

What to do:

Now you have to be careful with this one. The tongs have to be good or you’ll burn yourself. If you’re a child reading this, then make sure someone supervises you while doing this experiment. Reading though all the instructions before you start out is vital. I recommend having a couple of attempts, so maybe have two or three cans ready. So let’s begin:

Whilst you are filling up the pot of water why not drink the coke or whatever is in your drinks can. We don’t need any of the contents for this experiment, just an empty can. Once it is empty, rinse it out and place about two tablespoons of water in the can.

Now take your tongs and get a firm hold on the can. Hold it over the kitchen hob. We need to boil the small amount of water we have put in the can. This won’t take long and you’ll know when it’s worked because you’ll see steam coming out of the hole at the top of the can. Let it steam for a minute or two to be sure the water has all boiled.

Now here’s the cool bit. Keeping the can in between the tongs, take the can directly from the hob and dunk it, upside down, into the pot of water. The can will instantly and violently be crushed! It will happen very quickly so be ready. When I did it, it made a loud smacking sound as it went under water. I did it twice because I missed it the first time!

What is happening?

There is some great physics going on in this simple experiment. When you heat up the can and boil the water inside, the can fills with steam and pushes out all the air. Then when you dunk the can into cold water, the steam quickly condenses into water and there is no air pressure inside the can to support it. The can cannot resist the forces pushing on all sides from the water and air above it. Therefore it is crushed instantly!

Balloons:

Air pressure is also at work in balloons. When you blow air into a balloon you are artificially increasing the air pressure inside it and the rubber skin expands outward, forced by the force of the air molecules bounding around inside it.

You can ‘crush’ balloons by dipping them into liquid nitrogen. This condenses the air inside into a liquid and the balloon goes flat as a pancake. Here can see a video of a balloon that has been dunked into liquid nitrogen thawing out. The air boils back into a liquid and the balloon re-inflates. We filmed this last year in our first year undergrad physics lab.

Enjoy playing with air pressure and feel free to send me any images of your crushed cans!