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 5: Bread rolls, fairy cakes and crystals

I love cooking, and I love science. ‘McGee on Food and Cooking‘ is one of my favourite non-fiction books, and Heston Blumenthal one of my favourite celebrity chefs. Materials science easily overlaps with food science, so this term’s topic was an ideal way to share some of my passion with the children.

Both groups covered similar material in this class. We looked at how egg white changes when cooked, and when we whisk air into it. We looked at yeast (which bubbled more successfully in the second class).

We carried out the classic bicarbonate plus vinegar experiment, then introduced baking powder and discussed why this might be a better, more controlled way of creating bubbles in cooked foods.

To illustrate how yeast works, and how we change the properties of materials, the younger group made some bread rolls. I suggested a small experiment to see how much difference it made whether we kneaded the dough or not. It was great to see how much they all enjoyed the hands-on aspect. The room was probably the quietest when the children were focussing on kneading the dough.

I admit that I went overboard! I prepared too much for this class, in terms of the breadth of subject matter. I wanted to show the children

-how egg proteins change with cooking,

-how wheat proteins change with cooking,

-how yeast works

-several different ways to introduce gas, or air, into cooked foods.

Covering all these could easily take three sessions, and I probably should have narrowed my focus a little. I was also not quite on the ball in terms of having the equipment to hand while doing demonstrations (and I probably should have started off with the bread dough instead of trying to fit it in at the end). I still had good feedback after the class and the children had some great questions, which is always reassuring as it shows they are really thinking about the topic.

Demonstrating that sand is insoluble in water
Demonstrating that sand is insoluble in water
Dissolving salt in water
Dissolving salt in water

In the older group, we introduced the terms soluble and insoluble, miscible and immisciblereversible and irreversiblephysical reaction and chemical reaction. It was interesting to hear their ideas about how we could retrieve the salt from the water, or the sand from the water! I set them some homework – to ask a barista how come the coffee grounds don’t go in the coffee that people buy. Let’s see what answers they receive.

They made some mini fairy cakes to illustrate how baking powder works, and enjoyed topping them with caramel or chocolate syrup and eating them straight away.

The older children also looked at crystallization. I was relieved, after my failure last year, that my ‘fast crystallization’ demonstration actually worked. I had made up the supersaturated solution the night before the class, and introduced some seed crystals on a pipe cleaner at the start of the class. Watching carefully, we could see the crystals forming in solution and falling to the bottom or adding to the growth on the pipe cleaner. It wasn’t quite as exciting as some rapid crystallization videos on YouTube (like this one with sodium acetate) but the boys were still impressed, and I was satisfied.

sodium carbonate crystals growing from a supersaturated solution
sodium carbonate crystals growing from a supersaturated solution

They each took home a solution of sodium carbonate with a pipe-cleaner star to grow ‘snowflake’ crystals.

photo 1 (4)

In the next two weeks, the children will be bringing their own ideas for experiments and demonstrations, loosely based on the topic of The Material World. We will try them out in class and discuss how and why we get the results we do. I think it will be loads of fun. We have now pretty much covered everything the NSW syllabus expects children to cover at these stages (Stage 1 and Stage 2). There are a few other things we can talk about, but they won’t take long. I am looking forward to the children’s input, and to work that is not constrained by the syllabus. I am planning a few surprises on the last day, too.