Chemical reactions, Year 9: fuels and rusting

temperature

提示It is read from the thermometer before and after.

为什么The better the fuel, the more energy it passes to the water and the further the thermometer reading climbs. The fuel is what you choose to change; the reading is what you measure as a result.

A control variable

提示In a fuel test, the volume of water in the can is one.

为什么If two things changed at once, you could not tell which had caused the result. Keeping everything else the same means any difference in the readings must come from the one thing you changed on purpose.

Fuel A

提示Think how much of the heat has the chance to drift away before it reaches the can.

为什么The further the can is from the flame, the more of the heat escapes into the air on the way. Fuel A is not worse; it has simply been tested less fairly. The gap must be the same for both.

Heat escaping to the surrounding air

提示Draughts carry something useful away from the flame before the water can take it in.

为什么Not all the energy from the flame reaches the water: much of it warms the air and the can instead. This is the biggest source of error in the experiment, and it makes every measured temperature rise smaller than it should be.

A single result might be a one-off

提示Think how far you would trust one roll of a dice to tell you whether it is fair.

为什么Small things vary from run to run: a draught, the height of the flame, how well the water was stirred. Repeating shows whether the readings agree, lets you spot an odd one, and gives a mean you can trust more than any one reading.

Water

提示Think what a drying agent removes from the air.

为什么Rusting needs two things to reach the iron together. This tube still has plenty of oxygen in its air, and the nail stays bright, so oxygen on its own is not enough.

Oxygen

提示Boiling drives dissolved gas out of the water, and the oil stops more getting back in from above.

为什么Tap water has air dissolved in it. Boiling removes that air and the oil seals the surface, so the nail has water but none of the gas it also needs. Water on its own is not enough to rust iron.

barrier

提示Think of a fence or a wall: something that stands in the way.

为什么If water and oxygen cannot reach the metal, the reaction cannot start. The protection lasts only while the coating is whole; a chip in the paint lets both in, and rust spreads from that spot.

Galvanising

提示Metal dustbins, buckets and farm gates with a dull grey, speckled finish have been treated this way.

为什么The zinc layer keeps water and oxygen away from the iron underneath, and it goes on protecting the iron even where the coating gets scratched.

Zinc is more reactive, so it reacts instead of the iron

提示Compare where the two metals sit in the reactivity series.

为什么Oxygen and water attack the more reactive metal first. While any zinc remains nearby it is the zinc that corrodes, and the iron is left alone. Blocks of zinc are bolted to ships' hulls for the same reason.