States of Matter

What a thermometer is really measuring

Most people finish this topic knowing that heating something does two different jobs, and never find out what the first of them actually is. Here it is. Temperature is not something a substance has. It is a measure of how fast its particles are moving. A thermometer does not report how much energy is in something. It reports how briskly the particles are jiggling about, and it reports nothing else whatever. So the question for the whole topic becomes: is the energy going in making the particles move faster, or is it changing how they are arranged? If the first, the thermometer climbs. If the second, the thermometer sits absolutely still while the heater is running, which is the single oddest thing in this topic and stops being odd the moment you know what is being measured. ⭐ And it reaches further than the graph. If a warm gas is a fast gas, then a warm gas hits the walls of its container harder and more often, which is why a sealed can left in the sun becomes dangerous with nothing added to it. Density, and how to measure it, is the other module on this topic.

Five quantities, said precisely

The first is the total; the next two are what it is made of. The last two are the ones examiners most often see confused, and they differ in one thing only.

Heat a sealed container of gas

A rigid sealed can of gas is warmed on a bench. Put what happens inside it into order.

• Energy is transferred into the gas from the warm surroundings
• The particles gain kinetic energy, so on average they move faster
• A thermometer in the gas reads higher, because that is what faster particles mean
• Moving faster, each particle reaches a wall more often than before
• And each collision with a wall is harder than it used to be
• More collisions, each of them harder, means the pressure inside the can rises

The thermometer that will not move

A beaker of crushed ice at its melting point is heated steadily. For several minutes the thermometer reads exactly the same, although the heater has been on the whole time. Where is the energy going?

• Into changing how the particles are arranged, pulling them out of their fixed positions. A thermometer reports only how fast they are moving, so it shows nothing while that is happening
• It is being lost to the surroundings, so none of it reaches the ice
• It is being stored up and will make the temperature jump suddenly once the ice has gone
• The thermometer has stopped working properly at that temperature

Two places the energy can go

Every heating question in this topic is asking which of these two columns you are in. Decide that first and the equation to use chooses itself.

Five observations, five reasons

Work out the energy needed

The specific heat capacity of water is 4200 joules per kilogram per degree Celsius. How much energy is needed to raise the temperature of 2 kilograms of water from 20 degrees Celsius to 70 degrees Celsius?

Speed, or arrangement

For each one, decide where the energy is going, or what the observation is about. Three lives.

Why a pump gets warm

Two questions that look like separate facts and are both the claim applied to a gas.\n\nWhy does a gas push on its container at all? Because its particles are moving, and every one that reaches a wall bounces off it, giving the wall a tiny push. Pressure is nothing more than an enormous number of those tiny pushes. Warm the gas and the particles move faster, so they reach the walls more often AND hit harder, and the pressure rises. Both halves of that sentence are needed: more often, and harder. ⭐ And why does a bicycle pump warm up? Push the piston in and you are not just squeezing the gas into less room. You are pushing on particles that are coming towards you, and a push against something moving towards you speeds it up. You have done work on the gas, its particles are now faster, and faster particles are what a higher temperature IS. ⚠️ Notice what is NOT happening in either case. No energy has been added by heating in the pump, and nothing at all has been added to the sealed can. The temperature changed because the particle speeds changed, which is the only thing temperature has ever meant.

Two claims to strike out

Five statements from a set of revision notes. Tap the TWO that are wrong.

• The flat part of a heating graph is where a change of state is happening
• Specific latent heat is the energy to change the state of one kilogram, with no temperature change
• During the flat part of the graph, no energy is being supplied to the substance
• Warming a sealed gas raises its pressure because the particles get bigger
• Specific heat capacity is the energy to raise one kilogram by one degree Celsius

Moving house

A family is moving out, and somebody with a stopwatch is timing how fast the boxes come down the path. For twenty minutes the rate is steady and the watch tells a clear story. Then it drops to nothing for a quarter of an hour, and stays there. It would be quite wrong to conclude that everybody has stopped for tea. They are wrestling the wardrobe over the door frame. Every bit of effort is going into getting one awkward thing out of the position it has been in for years, and none of it into moving anything along the path. The stopwatch measures the path. It has nothing to say about the door frame, and the moment the wardrobe is through, the readings resume exactly as before. ⭐ And one more thing from the same afternoon. The hallway is narrow, and the faster the movers walk, the more often somebody clips the wall with a corner, and the harder each of those knocks is. Nobody added any movers and the hallway did not get narrower. They are simply going faster, and everything about how often and how hard the walls get hit follows from that alone.

Put the proper words back

The total energy stored in the particles of a substance is called its _____ energy, and it has two parts: the _____ energy of the particles as they move about, and the energy stored in how they are arranged. Heating something without changing its state raises the temperature, and how much energy that takes depends on the specific heat _____ of the substance. Heating it during a change of state raises no temperature at all, and that energy is the specific _____ heat; for boiling it is the one called the latent heat of _____. In a sealed container, faster particles strike the walls more often and harder, so the _____ rises.

A sealed can in the sun

A rigid sealed can of gas is left in strong sunshine and warms up. Nothing is added to it and nothing escapes. Select every statement that is correct.

• The average speed of the gas particles increases
• The particles collide with the walls more often than before
• Each collision with a wall exerts a greater force than before
• The individual particles expand, which is what raises the pressure
• There are more gas particles inside the can than there were

Four puzzles from one bench

Four things happen in a single lesson, and each is answered by asking where the energy is going. Each stage sets out where things stand and hands you the decision.

• A class heats a beaker of crushed ice steadily and plots temperature against time. For several minutes the line is flat, then it starts climbing again. Decide what the flat section shows.
• A student inflates a tyre with a hand pump and notices the barrel of the pump is warm afterwards, although nothing has been heated. Decide what has happened.
• Two blocks of equal mass, one aluminium and one lead, are given exactly the same amount of energy by identical heaters for the same time. The lead ends up much hotter. Decide what that shows.
• A student writes that when water boils away it has been destroyed, because there is nothing left in the beaker. Decide what to say.

Build the two-part answer

Assemble the sentence that answers a heating question and a gas question in one go.

Explain the graph and the gas

Explain why the temperature of a substance stays constant while it is melting, even though it is still being heated, and explain why the pressure inside a sealed container of gas rises when it is warmed.

• Say what a thermometer is actually measuring about the particles
• Explain where the energy goes during a change of state, and why the reading does not move
• Name the quantity that decides how much energy a change of state needs, and the one that decides how much a temperature rise needs
• Explain what happens to the particles in a sealed gas when it is warmed
• Finish by explaining the rise in pressure, giving BOTH reasons the collisions change