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.


Volcanic eruptions and rock formation

We started off our third Marrickville science session by watching a short animation from the BBC about the structure of the Earth and how this relates to volcanoes. The children were not all paying attention at this point, but the YouTube clips of volcanoes went down well and prompted some discussion. We talked about different types of eruption and how hot the lava gets.


Essentially, volcanoes (and earthquakes) occur near the edges of continental plates. New rocks are formed when magma cools. Even with the same minerals in the magma, the rocks that form may look very different depending on how they have formed. If the magma cools slowly you can get very large crystals forming in the rock. In general, the faster the cooling, the smaller the crystals. (This is something that you can test out yourself with more crystal growing experiments. I will try to post some more instructions in the next week.)

If magma cools really quickly you get obsidian, which is one of the rocks featured in Minecraft and which is also actually a glass. In a glass the particles are not held together in regular repeated units, but in varying orientations relative to each other.

There is also pumice, which is quite a unique rock that actually floats in water. This contains many holes (it is porous) due to the gases from the volcanic eruption.

My middle son remembered a game he’d played with a friend where you can vary the conditions of a volcano to see what kind of eruption you can get. One of the other mums found the link, and I’d encourage everyone to have a go at this:

Discovery Kids Volcano Explorer

I had planned to split the group into younger and older children but they all wanted to come out into the garden together. We put out some different rocks on the table and tried to identify which were igneous (formed from magma), sedimentary or metamorphic. I hadn’t really talked about the other two types of rock but we had a few pictures of the rock cycle and an A4 page with some descriptions and pictures of typical rocks to help us. I can e-mail copies of these to anyone interested. The New Zealand website Science Kids has some information and pictures. You might also like to look at Rock HoundsKids Love Rocks or The last one is more wordy.

We had samples of pumice, granite, basalt, sandstone, limestone (with fossil crinoids), mica schist and several that I hadn’t been able to identify myself! You can find pictures of all these rocks on, and crinoids are explained here with loads of good pictures. (If you do an internet search for Australian crinoids you can find out about ones that are still alive today.)

Personally, I find the metamorphic rocks the hardest to identify. Sometimes it’s hard to tell the difference between them and igneous rocks that have very small particle size. Hand lenses (magnifying glasses) can help but in some rocks, the crystals can only be seen with a microscope.

I had brought along some colouring sheets for the younger children but by this point they were all more interested in playing in the garden.

A couple of them, however, were enticed back when I produced an obsidian spearhead. They then wrote down a list of all the rocks in Minecraft and identified that all but about two existed in real life.

Next time we plan to make fake ‘fossils’ from modelling clay and plaster of Paris. We will also look at chocolate versions of the rocks we were looking at in the last session. I found a good rock identification game on which I suggested my students tried out in preparation for the next class. (Watch out for the typo where they wrote calcium instead of carbon!)

We might also try making sugar crystals and honeycomb. It won’t be a good day for our teeth but I hope it will be a fun way to end the classes.