The Material World Class 6: Fun reactions

Our last class this term, which I nicknamed FUN FRIDAY. We made fizzing bath bombs, butter and ice cream.

Younger group

The younger group started out with an experiment you find all over the internet, often called “The Cat’s Meow”. It is one of the experiments in the series I linked to at the start of the term. My youngest son had tried it and wanted to show it to his friends. We did two different versions – one with detergent and one with oil.

We set out two plates of full-fat milk, and dropped tiny amounts of food colouring in positions evenly spaced around the plates. The food colouring (which is water soluble) did not spread out very much.

The Cat's Meow


We then added detergent to the centre of one plate, and oil to the centre of the other.

The Cat's Meow after oil or detergent added

The oil did not do much, although if I stood with my head at the right angle, I could see a thin-film interference pattern (not something that I could easily capture on my smartphone). I added the oil as I had seen it mentioned on the internet and wanted to see if it would work.

'The Cat's Meow' after oil added

When we added detergent to the other plate, the coloured spots started to swirl around. That’s what I was looking out for!

First plate after detergent added

I believe the effect we see here is not completely understood, but is undoubtedly to do with the nature of detergent. Each detergent particle has one end that bonds with water (the hydrophilic end) and one end that bonds with fat (hydrophobic). When we wash dishes or clothes, the detergent particles surround the oil or fat globules so they can be carried away in the water. Perhaps the same process in milk sets up currents which are made visible by the food colouring.

To explore this with the children, we poured oil and water into a glass. They all predicted correctly that the oil would float on the top of the water. I talked a little about how milk has microscopic fat globules inside it. I am not sure that they wanted to pursue the explanation very much more, and so we moved on to the next activity.

Here I set up some white powders and some colourless liquids that we had encountered before: sodium bicarbonate, cream of tartar and baking powder, plus demineralised water and ‘white vinegar’. We are all familiar with the reaction of bicarb with vinegar. This time I allowed the children to set up their own ‘experiments’: each child had two containers and could choose what to put in them. They wrote down what was in each container and we briefly discussed the result (trying to clear up at the same time). I have not asked them to be quantitative yet, but I am hoping that this will make it clear that not all white powders react with vinegar to give effervescence. For example, my son tried tartaric acid and vinegar and nothing happened. I briefly recapped that the vinegar + bicarb reaction is an example of ACID plus ALKALI. Maybe in another term I can set them an analytical chemistry activity where they have to work out what a few ‘white powders’ are, by trying out different reactions. We can also use red cabbage water as a home-made indicator solution.

Finally, we came to making our bath bombs.

I had prepared a mixture of bicarb and citric acid the previous night, and despite keeping it in a screw-top container, it had obviously absorbed water from the atmosphere and was pretty rock hard in the bottom of the container. We had to jab at it with a knife but managed to dislodge the whole mixture into a mixing bowl where we added oil. Each child wore gloves and had a go at moulding the mixture. I had neglected to supply safety glasses and one child got citric acid in their eye but a parent used plenty of water to wash it out. I will definitely supply safety glasses next time.

Bath bombs, if you are not familiar with them, are yet another example of the ACID + ALKALI (or ACID + BASE) reaction. And if we use sodium bicarbonate as the base, it reacts to give a salt, water and carbon dioxide. So, drop your bath bombs into some warm water and you have a fizzy bath. We did not add any food colourings, or essential oils for fragrance, but they are easily included in the recipe.

I probably should have checked beforehand whether the children all had baths at home. At least one family only has showers, so will try the bath bombs in the wash basin. But it should be fun anyway.

I used instructions I wrote myself ages ago when I was working for Planet Science in the UK. I see my version has disappeared from their site (I said it was ages ago, didn’t I?) but they have new instructions here.


Older group

I went over time (again) and started the older group late, but they did not mind as we got to make ice-cream with no freezer, and eat it too:

Noodle eating ice cream Drama King eating ice cream Dray eating ice cream (2) Princess eating ice cream

We froze our ice cream by nestling our small containers of cream and sugar in a bowl of ice cubes. We added salt to the ice to bring its temperature down – ask me if you want an explanation of this one.

(The girls aren’t officially in the older group but scored some ice cream simply by being in the right place at the right time.)

We also made our own butter:

Home made butter


Again, you can find instructions on Planet Science, although they are not specific enough for me. I reckon it takes between 10 and 15 minutes to make the butter, and it can get very tiring while you are shaking. Perhaps the new writers didn’t want to put off their readers!

In both of these activities, we were using the properties of milk and the properties of salt, and looking at physical changes rather than chemical changes.


I have to move house now. I will not be running classes for about six weeks, and may have sporadic internet access. I will try to post articles and interesting links when I can. Next term’s classes start on 15 August, and will cover The Physical World: heat, light, sound and forces.


The Material World Class 4: Keen observation, and bouncy balls

In this class the younger group looked at properties of materials, and the older group were looking at materials processing.

I asked the children in the younger group to bring along a variety of small objects made of natural materials, and they brought some wonderful items.

items 5

items 3 items 2 items 1

We examined them under the magnifying glasses, again, and used all our senses except taste to find out what properties they had. I actually used an activity that Julie Bogart from Brave Writer calls ‘keen observation’. It might also be familiar to anyone who has tried mindfulness practices. We looked at the object with our eyes, looked at it under a magnifying glass, held it up to the light, touched it, stroked it, banged it, squashed it, listened to it, rubbed it on our cheek, rolled it up if we could, weighed it in our hands and smelled it.

The children liked saying what the items smelled like! Then the children came up with a couple of words to describe the item they were holding.

 Hard, soft, shiny, springy, silky, fluffy, ridged…

I was trying to get them to think about what properties are important when you want to make something or build something.

Would you build a bridge with pompoms?

Why not?

What would happen if a car went across it?

I read them the story of Mr Silly, who lives in Nonsenseland where things are not the same as they are in our world.

I think I could have done more quantitative work, e.g. getting the children to order the objects from lightest to heaviest and then weighing them to see if they were right. Or we could have classified the objects according to different criteria. But that’s something they can build on at home or in future classes.

I started off the older group with three problems that materials scientists have solved by using gels, and one that hasn’t been solved yet, but gels may be part of the answer.

(1)    Providing food for athletes in ultra-endurance events. The food needs to be lightweight, easy to digest (low in protein and fat), low fibre so you don’t need to poo during the race, doesn’t make your stomach feel like loads of liquid is slopping around, but also give lots of energy.

Scott Jurek. Image from
Sports gels, image from


(2)    Convenient way of delivering liquid medicines without people having to measure them out in small quantities using a syringe.

Household measuring spoons are not precise enough for measuring medicine. Image from
Gel capsules. Image from







(3)    Also a convenient way of delivering medicines that you don’t want to be digested until after the stomach.

Stomach and small intestine. Image from

(4)    This hasn’t been solved yet, but what if we could grow more teeth the way that sharks do? Currently, humans only grow teeth inside the embryo and then they push their way through our gums when ready. But what if we could stimulate our gum cells to produce teeth the way they do in embryos? (See this link from Harvard to find out how gels can promote tooth formation.)

Image from

Of course the ‘solutions’ themselves are not perfect. I particularly dislike the use of wasteful packaging for all these performance gels. Some athletes are trying to move back to more natural food for endurance events.

Nevertheless, all these are examples of how novel materials can lead to exciting developments in many different disciplines.

Discussion of gels led on to discussion of polymers and their structure at a microscopic or molecular level.

Image of cross-linked polymers. From

Then it was time for some fun, messy stuff – making our own bouncy ‘silly putty’.

There are many recipes for these on the internet. The problem is, many recipes that are circulating are just not reliable. Children can get very disappointed if you try an activity and it fails to give the desired effect. (Yes, it’s a learning experience, but not one you want to happen too frequently.) I used these instructions from the WOW lab at McGill University, Montreal, Canada, although we only had PVA glue so we didn’t make the latex equivalent.

 photo 1 (1)     photo 3     IMAG4138    IMAG4136

We just about had time to watch a video from the Canadian TV series How It’s Made, showing how robotic arms are manufactured. I didn’t have time to show the video of woven wickerwork or electrospinning, or tell them about Stephanie Kwolek inventing Kevlar. But here is a great video from Penn and Teller of a microbiologist turned fashion designer who makes fabric from milk.

Next week the younger group will be changing properties of foodstuffs, and the older group will start growing crystals.