Density
Density is an important concept and we used it to sort and identify polymers in our last session. This time we explored the idea further. First, Alison placed a can of Coke and a can of Diet Coke in water. The Coke sat lower in the water than the Diet Coke, showing that Coke is more dense, ie has more mass per unit volume. Each cubic centimetre of Coke weighs more than Diet Coke.
We discussed how to find densities of other objects. Measuring the mass is easy if you have a good balance, but for density we also need the volume - the amount of space the object takes up. With a regular object like a cube, it’s easy to calculate volume by measuring the sides and multiplying height x width x length, but what about irregular objects like the lumps of Mystery Metal that Angie had found? We tried to calculate the density by placing samples in a displacement vessel and measuring how much water was displaced. This could work well but we found that, if the sample was very small, the water in the displacement vessel just “bulged” and didn’t overflow. This means the surface tension on the water was able to absorb the extra volume. So, we ideally would need to know what range of equipment was accurate for.
Properties of metals
We had twelve rectangular pieces of sheet metal, each labelled with a letter. We passed them round and tried to work out what each mystery metal was, and how we might investigate that without damaging the sample. Using magnets, observing the colour, and comparing the masses and feeling how easily they conducted heat, we were able to deduce most of them - but there were some tricky ones, eg non-magnetic steel. Our samples included tin, copper, brass, steel, iron, zinc, lead, bronze.
Thermal Insulators
Thermal insulators are materials which don’t allow heat to pass through them easily, whereas thermal conductors do. We wanted to investigate which was the best insulator :- bubble wrap, woollen cloth, or aluminium cooking foil. Each group was provided with three identical empty cans and the insulation materials, hot water, thermometers, and sticky tape. Groups designed their own method of testing and chose how to write up their results. For example, one group produced bar charts showing how much the temperature changed over 30 seconds, while another took several measurements over 10 minutes and charted the rate of cooling. We discussed how our tests might be biased or inaccurate, eg :
- was it easier to coat the can in one insulating material compared to the others?
- Did we have to remove insulation to take temperature readings, and if so, did heat escape while we did it?
Our findings were consistent overall, that bubble wrap was the best insulator, followed by wool, and then cooking foil last of all. This was a surprise for some people because aluminium foil is used in cooking, but it’s used for a different purpose - usually to help distribute heat evenly over the surface of food, to stop it burning.
For a recap on thermal insulators, and a quick quiz, see BBC Bitesize on Heat .
