Investigating and comparing materials

Michael is thinking about buying a new water bottle, so he asks his friends for recommendations. - I have a metal one and it’s great! - No, I think a reusable plastic one like mine is much better! - Mine is made of glass and I love it! Michael is only more confused now. All these bottles look very similar. They all hold 500 millilitres of liquid, have walls of approximately the same thickness, and they all look really cool. The biggest difference between the three bottles is what they are made of.

Metal, plastic, and glass are three different materials used to manufacture water bottles. Each of these materials has different characteristics, or properties. To help Michael figure out which bottle he should buy, let’s investigate his friends’ bottles, and compare the properties of the materials they are made of! The first thing Michael notices is that he can easily see how much water there is in the glass and in the plastic bottles, but not in the metal one. The plastic and glass used for these bottles are see-through — transparent, while metal is opaque — it doesn’t let light through.

Michael then tries weighing all three bottles in his hands. Despite having the same thickness of the walls, the plastic bottle is lighter than the ones made of this specific metal and glass. The metal and glass used to make these bottles have more mass in the same volume - they are denser. Try filling the three bottles with ice cold water, Michael! How do they feel now? - The metal bottle gets very cold, very quickly!

The glass and plastic ones — not so much. Try hot water now, but be careful! - Ouch! The metal one is hot! This metal bottle quickly gets hot. The glass one gets hot too, but not as fast.

And the plastic one stays a little cooler to touch. - Different materials transfer heat differently! What else do you notice about these materials, Michael? - Hmm, all three feel quite hard to the touch, but when I squeeze the bottles, only the plastic one gives a little. That’s right! Plastic is more flexible than glass and metal of the same thickness. And which material, do you think, is most resistant to scratches? - Hmm, I can see that Philip’s plastic bottle has quite a few small scratches on the surface.

But Lina’s glass bottle and Maria’s metal bottle don’t seem to be scratched at all. Metal and glass are harder materials than plastic, so you’d need to use a hard object to scratch them. Plastic, on the other hand, is softer, and can get scratched just from being carried around in your bag. What else can we test about these materials? - Maybe I can check what would happen if I dropped them on the pavement? I’m clumsy, so that could happen often... - Ooops, sorry!

When dropped on the pavement, the plastic bottle cracks slightly at the point of impact, the metal bottle gets dented, and the glass one shatters into tiny pieces. Each of the three materials has different brittleness. Every material is different, and some work better for certain purposes than others. Testing and comparing those properties is the best way to find out which one to choose. And sometimes it’s just a matter of preference!

Have you made your decision, Michael? - Uhm… I might have to get more bottles than just one…