1I s it possible to walk across the English Channel? 21
It would be – if it was frozen. But will this ever happen? We all know pure water freezes at 0 ºC. But salty water doesn’t freeze until it gets even colder. There is a lot of water in the channel. It would take ages for all the particles to get into a regular pattern and form ice. So it would need to be very cold for a very long time to freeze the channel.
Grit is actually rock salt – impure sodium chloride. So how does this compound help? It lowers the freezing point of water. What salt dissolves, its ions attract water molecules. That makes it harder for water molecules to form crystals, so water freezes at a lower temperature.
Anything that dissolves in water will lower its freezing point, so you could use sugar. But salt is NaCl and glucose – the simplest sugar - is C 6 H 12 O 6. Why is this important? Sodium chloride has a smaller formula mass than sugar, so there are more particles per tonne of sodium chloride. The more particles, the bigger the effect on water’s freezing point.
4 It's 4'C today, so how come there's still snow on the ground?
Melting takes time. It’s the same with ice-cream. Why? Water molecules have to absorb energy to overcome the attractive forces holding them in place. Absorbing enough energy takes time.
…but in 10s the driver will skid over the side of a bridge! Why? 5The road is ice-free…
Bridges get icy (and slippery) before the roads on land. Why is this? Modern bridges contain a lot of metal. Strong metallic bonding gives bridge supports high tensile strength. Delocalised electrons make metals good conductors, so they lose heat quickly.
6 His diesel car has stopped working… …but the petrol cars on the road are fine. Why?
The hydrocarbon chains in diesel are longer than those in petrol. What difference does that make? Diesel has stronger intermolecular forces so it is less volatile than petrol. Some of the compounds in diesel freeze around -15 ºC. It has to be much colder to freeze petrol.
7 You throw water up - snow comes down. How come?
The air has to be cold – very cold. In Siberia it is –40 ºC. What happens to the water in the air? It freezes instantly as water molecules lose energy and fit together to form crystals. Throwing it up helps by forming droplets. Their large surface area, makes them lose energy faster.
8When can water pack a bigger punch than a boxer?
Water is a simple molecular substance. So why is frozen water so hard? Covalent bonds are strong, but intermolecular forces are weak. So most simple molecular solids are soft. But the intermolecular forces in ice are quite strong, so ice is harder than most simple molecular solids. intermolecular force very strong covalent bond
If you stayed out in freezing weather for too long, you would die. But wood frogs can survive. How? They fill their cells with sugar. This acts like antifreeze to stop them freezing solid. The frog’s antifreeze stops ice forming the same way salt stops it forming on roads.
liquid water solid water 10 Why is water strange?
If this was any other substance, the solid state would sink. So why do icebergs float? When most substances freeze, their particles get closer together, so they get denser. Water is strange because the particles in ice crystals are more spread out, which makes it less dense than water..
11Santa has a new sledge. Its made of plastic. Is this a good choice of material? 21
Plastic is lightweight so it’s easier for reindeer to pull, but will it survive a subzero Christmas Eve? Maybe not! Plastic particles are like strands of cooked spaghetti. They slip and slide over each other to make a soft, flexible material. But when the temperature drops, particles lose energy and stick together. The plastic goes rigid and brittle like glass. Any small pushing force could make the sledge shatter.