Resource 2: Corn starch and water
Background information / subject knowledge for teacher
Corn starch and water – a curious mixture!
Caution: Always dispose of the mixture in a rubbish bin. Do not put it down a sink as it will cause a blockage.
Your students will probably be familiar with the properties of solids, liquids and gases. They will be able to describe their properties and, classify a substance correctly on the basis of its properties. This is fine so long as a particular substance falls neatly into one or other of the categories. But what happens if it doesn’t? You have seen, for example, that sand, though composed of tiny grains of solid behaves, in some ways, like a liquid. Only one individual grain on its own would satisfy all of the criteria for a solid.
So some substances are definitely difficult to classify. However, you can use this as an opportunity to probe your students’ understanding of the nature of solids, liquids and gases. In this activity you will make a substance that is difficult to classify. The substance is made from water and cornstarch. In order to experiment with it you will need the following materials:
- One box of cornstarch, 450 g (16 oz), or equivalent (a powder with a high starch content)
- A large mixing bowl
- A jug of water
- A spoon
- A large plastic food bag
- Newspaper or similar to cover the floor
- Food colouring
- A cup or beaker.
- Pour approximately 1/4 of the box (about 100 g, 4 oz) of cornstarch into the mixing bowl and slowly add about 1/2 cup of water. Stir. Sometimes it is easier (and more fun) to mix the cornstarch and water with your bare hands.
- Continue adding cornstarch and water in small amounts until you get a mixture that has the consistency of honey. It may take a few tries to get the consistency just right, but you will eventually end up mixing one box of cornstarch with roughly 1 to 2 cups of water. As a general rule, you're looking for a mixture of approximately 10 parts of cornstarch to 1 part water. Notice that the mixture gets thicker or more viscous as you add more cornstarch.
- Sink your hand into the bowl of cornstarch and water, and notice its unusual consistency. Compare what it feels like to move your hand around slowly and then very quickly. You can’t move your hand around very fast! In fact, the faster you thrash around, the more like a solid the mixture becomes. Sink your entire hand in and try to grab the fluid and pull it up. That’s the sensation of sinking in quicksand.
- Drop a small object into the cornstarch mixture and then try to get it out. It’s quite difficult to do.
- Slap the surface of the mixture hard. If you have used just the right proportions it will not splatter all over the place as you might have expected.
Explaining the properties of cornstarch ‘quicksand’
Cornstarch mixed with water is an example of a heterogeneous mixture. That’s a bit of a mouthful! Basically it means that both components of the mixture can be seen in the mixture, or they could be if the particles of cornstarch were not so small. Over time the particles settle out and sink to the bottom so do not pour any remaining mixture down a sink – the water will evaporate and leave a solid lump of matter that will block it.
In fact the cornstarch and water mixture acts like a solid sometimes and a liquid at other times. The mixture is in fact an example of a suspension – a mixture of two substances, one which is finely divided (the solid) dispersed in the other (the liquid).
When you slap the surface with your hand you force the long starch molecules closer together. It feels like a solid. This impact traps water molecules between the starch chains and forms a semi-rigid structure. When the pressure is released, the cornstarch flows again.
If you push your finger slowly into the mixture, it goes in easily and it feels like a liquid.
All fluids have a property known as viscosity – or resistance to flow. The more resistance to flow a liquid has the greater its viscosity is; e.g. honey,. Water has a low viscosity. Sir Isaac Newton proved that viscosity is affected by temperature. So, if you heat honey, its viscosity is less than that of cold honey. Cornstarch, water mixtures and quicksand are regarded as non-Newtonian fluids because their viscosities change when a force is applied, not when heat is applied.