Easy science experiments with table salt

Here's a really simple set of experiments using plain table salt and water (and a little food colouring if you want to).  You can swap the salt for sugar if you want an even safer version for toddlers if you think they might try to drink the experiment, and glass for plastic cups.

I set up paper place mats with small glasses (old Nutella glasses in this case) half filled with water - one glass cold water, the other warm water.  I added a pot (an old Gu pud ramekin) full of table salt and a spoon, and wrote labels on the paper to help with  identification and word recognition.

The task for the boys was to add a level spoonful of salt to each glass and stir it (lolly sticks) until the salt dissolved.  They would know it had dissolved when the water went fully transparent again.  They observed which one went transparent again first - it should be the warm water - and we used the science terminology to describe it 'the salt dissolved in the warm water faster, we could see that the warm water went transparent again faster'.  They didn't ask why, so I didn't volunteer the information (it's a mixture of the water molecules moving faster in hot water and therefore coming into contact with the salt faster, and the heat of the water reducing the strength of the hydrogen bonds enabling the salts to form new solute-solvent bonds - I think we're not quite ready for this level yet).

The boys could then repeat this process, making a mark if they wanted to record how many spoons of salt were added, until the water wouldn't go transparent again.  At this point the water couldn't dissolve any more salt and it was saturated with salt. 

The boys had a lot of fun transferring salt to the water and stirring it.  When the solution was saturated I encouraged them to tip the rest of their salt into one of their glasses and give it a good stir up to make sure it really was saturated with salt.

   You could stop at this point and leave it as a stand-alone dissolving experiment if you wanted to keep it short and sweet, or continue for the extension in part 2.  Part 1 is deceptively simple but develops fine motor skills in transferring and stirring, plus language and observation skills.  Part 2 builds on language and observation skills further.

 For part 2 we wanted to look at whether salt crystals would be larger if they were produced by fast or slow evaporation of the water in the solution, so I added a little food colouring to aid visibility.  Ollie predicted that evaporating the water off quickly would result in small crystals, based on what he knows about the formation of rocks from cooling lavas which was nice evidence of transferring existing ideas to new situations (rapidly cooling lavas produce glassy rocks with crystals so small to be indistinguishable, such as obsidian, while slower cooling lavas and magmas produce larger crystals which can be seen more easily, such as in granites which form when magma cools fairly slowly in underground chambers known as batholiths). 

We put some of the salt solution onto glass slides, but also some into the bottom of more glass ramekins - we happen to have slides but, if you don't, any small dish will do fine.  One set of slides was left to evaporate slowly on a windowsill on the shady side of the house, the other on a windowsill in full sunshine.  I used the salts which formed in the ramekins as a touch exploration media, so the boys could feel the difference in the texture of the salt crystals and be less likely to immediately remove the crystals on the slides by poking them.  The boys could clearly see there was a difference between the slides, and talked about chunkier crystals on their own on the slow dried slide compared with the moosh on the fast dried slide.

As before, you could leave it at that, but since we happen to have a microscope we continued (it was bought years ago at Lidl for £40, but the same model available on Amazon for £88 at time of writing - I wouldn't personally bother with microscopes from toy shops as they tend to disappoint but even a hand lens or a magnifier app on your phone will give you some extra investigative power if you haven't got access to a microscope).  The microscope I have comes with a camera attachment, but we couldn't get the software to work any more since I had to upgrade my laptop.  This camera doesn't have great resolution anyway, but was handy last year when the boys were smaller as it meant they weren't fighting to look down the same eyepiece and constantly changing the focus.  Now however they both really enjoyed using the eyepiece and even got to grips with changing the slides and focussing the microscope for themselves (I only let them do this on the lowest magnification as they couldn't wind down too far and damage the lens on this one). 
 
 

 I wasn't sure how well they would respond to using the microscope with just the eyepiece, so the night before I took some images using my phone in order that the boys would have something to see even if they didn't get on with the eyepiece.  This was a highly untechnical operation - I cupped my hand as a tube around the eyepiece and balanced my phone on top of my hand.  I got the best results by turning down the light intensity under the stage, turning the phone setting to 'close up', turning off the flash and just making small adjustments to how high above the eyepiece and what angle it was at until I got good shots.  The benefit is that the resolution was good and worth the frustration of a dozen crummy shots for every good one.

 

The top photo is of the salt crystals that were on the slide that was evaporated rapidly.  The bottom one is from the slowly evaporated solution.  The magnification is the same for each so you can really see the difference in the size and formation.  The shape is a sort of pyramid - if you want to see this form large enough to handle and see without magnification then Maldon Sea Salt (available in the UK from supermarkets) has crystals in the same shape but a couple of millimetres or more across. 

The magic of microscopes is the kids reaction - they kept looking at the slide on the stage, then back in the eyepiece as they couldn't believe what they were seeing was the same little dot of salt.  I can well understand the craze for the unseen miniature world around us that sprang up following the invention of the microscope.  It really is an addictive past time when you get going with one.

So there it is, three simple things to do with plain old table salt.

Play dough volcano

 

Ollie was 'reading' one of his science books in the car the other day and wanted to do 'an experiment with volcanoes'.  It sounded like a great idea to me, so when we got home from swimming I decided that making a bicarbonate volcano would be a fun activity.

The first step was to make some play dough.  I like the recipe I'm giving below because it measures in cups and tablespoons rather than grams, which makes it easier for the kids to help to measure out the quantities:
2 cups plain flour
2 cups water
1 cup salt
1 tablespoon vegetable oil
1 tablespoon cream of tartar (optional but helps the play dough to keep longer without going slimey - unlikely you'll want to save the dough after this activity though so you may as well leave it out)
1-2 teaspoons colouring - we used blue because ours is a bit rubbish at being blue but makes a good rocky grey colour.

Help your kids to measure, add and stir the ingredients, then put into a saucepan and cook on a medium heat stirring constantly until the mixture thickens and looks like dough. it will obviously be hot at this point, so when we make play dough I add in another activity while we wait for it to cool.

 For our 'cooling' activity on this day Matt and Ollie made a volcano picture.  Matt drew the cone, magma chamber and vents, then Ollie coloured it.  Matt drew guide lines for Ollie's words and they worked out what to write letter by letter "What is this red stuff you've drawn inside the volcano? That's right it's magma.  What do you think the word 'magma' starts with..." with some help over shaping the letters by writing each out on a separate piece of paper (while Ollie tuts "I know how to write a 'm'!"). 

I was surprised when Ollie said that the stuff coming out of the volcano was lava and chunks of rock blown off the top - I didn't realise he had remembered that the molten rock is magma when it's inside the volcano and lava when it's outside - I've had A-level students that didn't remember the difference.





In the end they labelled the vent, cone, lava and the magma chamber, by which time Toby had made a birthday card for his friend and our play dough was cool enough for the next stage.








I save plastic pots and tubs for our experiments and activities, and today a pot which had previously contained a small fairy cake in an Asda children's lunch pack was the perfect one - small, plastic and with a push fit lid into which I could easily cut a small round hole in the centre.  I added a couple of teaspoons of red food colouring to the pot, then four teaspoons of bicarbonate of soda and put the lid on.  This formed our magma chamber and the boys modelled the volcano around it.

Next I filled a large medicine syringe with vinegar and the boys took turns squirting it into the magma chamber and laughing at the explosive results.  The first couple of goes where a bit too tame, just gently bubbling down the volcanoes sides, so Matt added some sticky tape to make the aperture in the lid of the chamber smaller.  The resulting fountains of bicarb and vinegar caused great delight.  This video isn't the biggest eruption we had, but for some reason the other footage isn't working.

So what are the science bits from this?  The bicarb and vinegar experiment is an old favourite - adding the acid vinegar to the alkaline* bicarbonate of soda causes the release of carbon dioxide bubbles.  If you add pressure by having a lid on the container with a hole in it, the results can fountain up quite impressively - for a really impressive version look online for videos of the 'mentos and cola' experiment.

What has this got to do with volcanoes?  Quite a bit surprisingly, since there are different types of volcanoes but the one that every kid draws, the cone shape, is an explosive volcano**.  Famous examples of this type include Mount St Helens and Mount Pinatubo.  These volcanoes are explosive because of the presence of gasses dissolved under pressure in hot, highly viscous (thick) magma.  When sufficient pressure builds up to blow a hole in the top (or sometimes side if the vent to the summit is plugged) of the volcano, the pressure is released and the gases suddenly come out of a dissolved state into a gaseous state which froths up the newly emerging lava to form volcanic ash, which, along with other pyroclastic material, rockets up into the atmosphere until it runs out of energy and then collapses on itself, at which point it rushes down the sides of the volcano as a pyroclastic flow, a very fast deadly wave of burning hot poisonous gases and ash.  Very few people actually die from lava flowing from a volcano - it's the pyroclastic flow which is the real danger.  The bubbles you create by adding vinegar to bicarb are about the safest way you can demonstrate how important gases are in an explosive eruption.

As always, don't be afraid to use the proper terminology if you know it when explaining and describing what's happening as kids are amazingly receptive and will surprise you with what they retain over time.  If you don't feel confident explaining what's happening, do the activity anyway and then have fun together looking up explanations in books or the internet.  Finding videos of real volcanoes is a good follow up activity, and if you have some don't forget to try floating your pumice stone foot rubber in a bowl of water - rock shouldn't float right?  Pumice is special and it's all down to those gasses that were in the magma.

Safety bit: Things to be careful of - usual care to be taken with hot stove/hot play dough while you're making the dough; during the chemistry bit be careful of getting bicarb/ vinegar in eyes - wear eye protection if you are concerned, wash eyes immediately with luke warm water if you do get any in there as both will sting.  It can be fun to see how high you can make the volcano jet out, but you don't want to be showering bystanders with the discharge so do exercise sense and start small.


*ok chemistry purists, technically it's a base because it's dry, but acid and alkali are everyday terminology.

** the other main type is an effusive volcano - one that oozes over a long period of time, such as those on Hawaii.  Once upon a time there were also vast lava flows called 'flood basalts' but we can all be glad we haven't any of these now - look up 'Deccan traps' if you want to know why.  Other less impressive but still interesting types of volcanoes are less well known, including mud volcanoes.

Milky surface tension experiment

This experiment is another kitchen cupboard favourite that we learned how to do after a lovely friend shared a video of it on my FaceBook wall.  All you need is milk, food colourings, a container and some liquid soap.

Put some full fat milk into your container, add a few drops of food colourings to make little blobs of it floating in the milk- you may want to experiment with what colouring works best as we found our blue sank and red floated.  Add a few drops of liquid soap, e.g. washing up liquid, on to the blobs of food colouring and your little ones will be amazed as straight away the colouring starts swirling out into the milk.

 The original video that was shared with me didn't have an explanation, but if your little ones are looking for an explanation this is what I think is happening: for very little ones you can tell them that the soap is helping the milk and the colours to mix together.  For older children you can talk about surface tension.  Surface tension is the ability of a liquid to resist an external force - a really good example that you can observe this effect with is to find video footage on the internet of a pond skater walking on water.  The effect is caused by the cohesive properties of molecules in the liquid - the ability of the bits to stick together.  It is responsible for lots of the properties of liquids, for example the way rain falls as drops instead of just remaining in the atmosphere as separate water molecules.

 This experiment works best with full fat milk I think because the fat molecules in the milk are evenly distributed  and when you add the food colouring to the milk the fats prevent the colouring from diffusing (mixing) quickly into the liquid as would happen if you dripped colourings into plain water.  When you add soap I think two main things happen, firstly the soap is a surfactant so it breaks up the bonds between the fat and water in the milk, plus it disrupts the surface tension of the water in the milk - it makes the sticky molecules come unstuck, both of which allows the colours to spread out.  Even more exciting, this is not now a gradual dilution that you get in plain water, the colours shoot around the dish in waves as the colours are driven by movements in the milk as the stronger surface tension areas pull away from the soapy areas where the surface tension is weaker.

After we played with the original set up a few times by replacing the ingredients with a fresh set up in a cleaned dish, we then brought in the idea of scientific method by keeping some parts of our experiment the same and just changing one part - on this day we tried using a different type of liquid soap - hand soap - to see if one soap made the colour mix in faster.  We used simple equipment to measure out our quantities to keep them standard - medicine spoons for the soap, medicine syringes for the colouring and glasses for the milk.  The boys then added the ingredients of their experiment at the same time and watched to see what happened - in our basic set up we didn't see much difference.  I thought there might be because our hand soap was a standard brand while the washing up liquid is an eco brand so I expected it to be less strong of a surfactant.  As it happened it looks like it was just as effective at breaking up the fat molecules and surface tension as the regular soap.