Heat & Changes of State · HeatLab

Using a Thermometer

The common lab liquid thermometer works on the expansion and contraction of a liquid: the liquid in the bulb expands when heated and rises up the narrow tube.

How to use it: ① check the range and the smallest division; ② the bulb must be fully immersed in the liquid being measured, without touching the bottom or sides of the container; ③ wait until the reading is steady before reading; ④ keep the bulb in the liquid while reading, with your eye level with the top of the column.

Try letting the bulb touch the bottom of the beaker (the bottom sits on the hot plate and is hotter than the water), or lifting it out; then read from above and from below and see whether the reading comes out too high or too low.

Celsius Temperature

Temperature tells how hot or cold an object is. The common unit is the degree Celsius (°C).

The Celsius scale defines: at standard atmospheric pressure, an ice-water mixture is 0 °C and boiling water is 100 °C; the interval between them is divided into 100 equal parts, each 1 °C.

Put the thermometer into each of the three beakers and watch where the column stops. Temperatures below 0 °C are read as “minus”, e.g. −5 °C is “minus 5 degrees Celsius”.

Melting and Solidifying

A substance changing from solid to liquid is melting; from liquid to solid is solidifying (freezing).

Heat the test tube of hypo (sodium thiosulfate) in hot water (a water bath: even heating, slow temperature rise), recording the temperature at regular intervals; the right panel draws the temperature–time graph.

Hypo stays at 48 °C while it melts — this is its melting point, and solid and liquid coexist in the tube; only when it has all melted does the temperature rise again. Paraffin has no definite melting point: it softens and thins while its temperature keeps rising.

Solids with a definite melting temperature are crystals; those without are amorphous solids. Melting absorbs heat, solidifying releases heat; a crystal's freezing point equals its melting point. Press “Stop heating” to watch it solidify.

Boiling water

Boiling is violent vaporization happening inside the liquid and at its surface at the same time.

Watch the bubbles while heating: before boiling, bubbles formed at the bottom get smaller as they rise (the upper water is cooler, so the vapour inside them condenses); during boiling, lots of bubbles rise and get bigger, bursting at the surface.

The temperature at which a liquid boils is its boiling point. While boiling it keeps absorbing heat but its temperature stays the same; remove the heat (press “Stop heating”) and boiling soon stops.

The boiling point depends on pressure: the higher the pressure, the higher the boiling point. On high plateaus the pressure is low and water boils below 100 °C, so rice doesn't cook well; inside a pressure cooker the boiling point can reach 120 °C.

Evaporation and Condensation

A substance changing from liquid to gas is vaporization, which happens by evaporation or boiling. Evaporation takes place only at the liquid's surface and at any temperature.

Wrap the bulb of one thermometer in cotton soaked with alcohol: as the alcohol evaporates it absorbs heat from the bulb and the reading drops; once the alcohol is gone, the reading slowly returns to room temperature.

What speeds up evaporation: a higher temperature, a larger surface area, and faster air movement over the surface. Fanning yourself feels cool because it speeds up the evaporation of sweat.

A substance changing from gas to liquid is condensation, which releases heat. Hold a cold glass plate above boiling water: the vapour condenses into droplets on it. The “white steam” from a kettle spout is not water vapour either — it is tiny droplets of condensed vapour.

Sublimation and Deposition

A substance changing directly from solid to gas is sublimation; directly from gas to solid is deposition. Sublimation absorbs heat; deposition releases heat.

Put a sealed iodine tube with a few iodine grains into hot water: the grains shrink and the tube fills with purple iodine vapour. Take it out to cool, and the vapour deposits as tiny iodine crystals on the glass.

Why a hot-water bath? Iodine melts at about 114 °C and hot water stays below 100 °C, so the iodine cannot melt — the purple gas can only come from sublimation. With “Direct heating” the temperature passes the melting point and the iodine first melts, then vaporizes, which no longer shows sublimation.

In daily life: mothballs shrinking and frozen laundry drying in winter are sublimation; frost, rime and ice patterns on windows are deposition.

Comparing How Substances Absorb Heat

Use two identical electric heaters to heat water and kerosene of equal mass: in the same time both absorb the same amount of heat (controlling variables).

The temperature–time graph on the right: kerosene warms up fast, water slowly. To reach the same temperature rise, water must be heated longer and absorb more heat — water has a greater ability to absorb heat.

Physics describes this with specific heat capacity: the heat a substance absorbs divided by its mass and its temperature rise. Water's specific heat is 4.2×10³ J/(kg·°C), kerosene's about 2.1×10³ J/(kg·°C), sand's about 0.92×10³ J/(kg·°C).

Q = c m Δt

That is why coastal areas have small day–night temperature differences while inland deserts have large ones, and why car engines are cooled with water.

Conduction

Heat flowing through an object from its hotter part to its colder part is conduction.

Copper, aluminium, iron and glass rods of the same size have one end in the same heat source, with matches stuck on with wax at regular intervals. As heat travels along a rod, the wax melts and the matches fall one by one.

The matches on the copper rod fall first and most, then aluminium, iron more slowly, and on the glass rod they never fall: metals are good conductors of heat, copper and aluminium the best; glass, wood, plastic and air are poor conductors.

Switch to the thermal view to see the temperature creep along the rods. Wooden handles on spatulas and down jackets (full of air) both use poor conductors.

Convection

In liquids and gases, warmer parts rise and cooler parts sink, circulating until the temperature evens out. This way of transferring heat is convection.

Heat the bottom of the tank on one side and drop a few crystals of potassium permanganate at the bottom: the purple stream first rises on the heated side, flows across at the surface, then sinks on the other side, forming a loop.

The more heating power, the faster the convection; stop heating and the flow slowly dies down.

Radiators placed under windows, air conditioners mounted high in a room, and kettles heated from the bottom all make use of convection.

Radiation

Heat sent out directly from a hot object as electromagnetic waves, without needing a medium, is thermal radiation. That is how the Sun's heat crosses empty space to reach the Earth.

A desk lamp shines on a black can and a white (shiny) can holding equal amounts of water: the black can heats up clearly faster — the darker and rougher the surface, the better it absorbs radiation. With the lamp off, the black can also cools faster: good absorbers are also good emitters.

A thermos flask blocks all three ways heat travels: the space between its double glass walls is a vacuum, stopping conduction and convection; the glass is silvered to reflect radiation back; the stopper reduces convection and evaporation. Compare how fast hot water cools in a plain flask, a vacuum flask and a silvered vacuum flask.

From Ice to Boiling

Keep heating a beaker of crushed ice at −20 °C on the hot plate; the right panel draws the whole temperature–time curve:

① the ice warms to 0 °C; ② the ice melts and the temperature stays at 0 °C (ice is a crystal with melting point 0 °C); ③ the water warms to 100 °C; ④ the water boils and the temperature stays at 100 °C while the water boils away.

On both plateaus the substance is absorbing heat, but the heat goes into changing its state rather than raising its temperature. Compare the lengths of the two plateaus: vaporizing 1 kg of water (about 2.26×10⁶ J) takes nearly 7 times the heat of melting 1 kg of ice (about 3.34×10⁵ J).

Change the heating power and the initial temperature of the ice and see how the curve changes.