States of Matter

States of Matter

Heat a block of ice and it warms up, melts, warms again, then boils. But the energy you supply does not always raise the temperature: twice on that journey it does something else entirely, and the thermometer sits completely still while you keep pouring energy in. To follow where it goes, meet internal energy, heat capacity and latent heat.

The words for it

Five terms. The third one is the key to everything that follows, and it is the one most often stated loosely:

The thermometer that will not move

Ice at 0 °C is heated steadily. For several minutes it melts and the thermometer stays at exactly 0 °C, even though the heater never stops. What is the best explanation?

• The energy is going into potential energy, separating the particles from each other, and temperature measures only their average kinetic energy, which is not changing
• The energy is being lost to the surroundings as fast as it is supplied
• The ice cannot absorb any more energy until it has finished melting
• The thermometer cannot read accurately while ice and water are mixed together

Two equations, two jobs

Both tell you how much energy a change needs. Choosing between them is a single question: does the temperature change?

Warming it up

A 2 kg block of a substance with a specific heat capacity of 100 J/kg°C is warmed from 20 °C to 25 °C. How much energy is transferred to it, in joules (J)?

Melting it

A 3 kg block of a substance sits at its melting point. Its specific latent heat of fusion is 200 J/kg. How much energy is needed to melt it completely, in joules (J)?

Which equation?

A kettle contains water at 100 °C. It keeps boiling until the last of the water has turned to steam at 100 °C. Which calculation gives the energy that took?

• E = m L, because the temperature never changed: all the energy went into turning liquid into gas
• ΔE = m c Δθ, because energy is being transferred to the water and it is getting hotter
• Both, added together, since energy is being supplied throughout
• Neither: no energy is needed, because the temperature does not rise

The heating curve

Plot temperature against time while heating a substance steadily and the shape tells you the whole story. It rises whenever the temperature is climbing, and goes flat whenever the substance is changing state: once at the melting point, once at the boiling point. The flat sections are the give-away, and their heights are the melting and boiling points. One detail worth noticing: the boiling plateau is usually much longer than the melting one. Melting only has to loosen the particles enough to flow past each other. Boiling has to separate them completely, and that costs far more energy.

Plot the melting point

This heating curve rises, then flattens while the substance melts. The points (0,0), (2,3) and (4,3) are shown; the flat part sits at temperature 3. Plot the missing point on the flat melting section, at time 3.

Read the curve

Why water is the awkward one

Water has an unusually high specific heat capacity, about 4200 J/kg°C, which is several times most common substances. Three consequences you have met without noticing. A kettle takes minutes rather than seconds, because every kilogram of water needs 4200 J for each degree. Car engines are cooled with water, precisely because it can absorb a great deal of energy without its own temperature shooting up. And coastal places have milder winters and cooler summers than inland places at the same latitude. The sea warms and cools slowly, and it drags the air above it along with it.

Which are true?

Select ALL THREE statements that are TRUE.

• Energy is still being supplied during a flat section: it is going into potential energy rather than kinetic, so the thermometer does not move
• For the same substance heated at the same rate, the boiling plateau is longer than the melting one, because separating the particles completely costs more than loosening them
• A substance with a high specific heat capacity needs more energy to warm by each degree, and also releases more as it cools
• During melting the heater must have been switched off, since the temperature is not rising
• Specific latent heat is measured in J/kg°C
• Temperature is a measure of the total energy of the particles in a substance

Order the experiment

Put the steps for investigating how a substance heats and melts in order.

• Heat the solid steadily and start a timer
• Record the temperature at regular time intervals
• Plot a graph of temperature against time
• The flat sections of the graph give the melting and boiling points

The heating rules

Heating a substance can raise its temperature or change its _____. Temperature measures the average _____ energy of the particles, so during a change of state, when the energy goes into potential energy instead, the temperature stays _____. The energy to raise 1 kg of a substance by 1 °C is its specific heat _____.

Explain the plateau

Exam practice. In about 50 words, explain why the temperature of boiling water stays at 100 °C even though the hob is still supplying energy. Include:

• what temperature actually measures about the particles
• where the supplied energy is going instead during the change of state
• why this means no temperature change does not mean no energy transferred