# Heat

Three Major Effect of Heat on Substances

1. Change in Volume: generally expand on heating and contract on cooling.

2. Change in Temperature: gain of heat raises the temperature and loss of heat causes a drop in temperature

3. Change in State: gain of heat leads to melting & vaporisation and loss of heat in condensation & solidification.

Expansion of Solids

Generally solids expand on heating and contract on cooling. Different solids expand and contract by different amounts. Solids expand a lot less than liquids and gases.

Expansion of Liquids

Generally liquids expand on heating and contract on cooling. Different liquids expand and contract by different amounts: alcohol expands five times more than mercury. Liquids show greater expansion and contraction than solids but less than gases. Liquid in glass thermometers (mercury, alcohol) rely on expansion and contraction response of liquids to change in temperature. Water expands on heating but also gets bigger when it is frozen!!!

Expansion of Gases

Gases expand on heating and contract on cooling. Different gases expand and contract by the same amounts. They expand and contract by approximately one three hundredth of their volume for each °C rise and fall in temperature. A gas of 300 cm3 at 20°C will have a volume of about 301cm3 at 21°C and a volume of 299cm3 at 19°C.

Investigate and Describe the Change in Volume of Solids, Liquids and Gases when Heated and Cooled.

a) Expansion and Contraction of a Solid

(i) The metal ball will just pass through the metal ring at room temperature.

(ii) Heat the ball in a Bunsen flame – it will not pass through the ring; expanded ball.

(iii) Cool the ball in a beaker of ice – it easily passes through the ring, contracted ball.

b) Expansion and Contraction of a Liquid

(i) Fill a test tube with water and insert a one hole stopper with a length of clear glass tubing.

(ii) Hold the test tube of water in your hands to heat it – note that the water rises up along the glass tubing; the water has expanded.

(iii) Stand the boiling tube in a beaker and allow it to cool back to room temperature – note that the level of water drops in the glass tubing; the water has contracted.

c) Expansion and Contraction of a Gas

(i) Fit a test tube with a one hole stopper and al length of clear glass tubing – the test tube is full of air.

(ii) Hold the test tube of air in your hands to heat it with the tip of the glass tubing just dipping below the surface of water in a beaker – note that bubbles of gas escape from the tubing.

(iii) When the bubbling stops hold the test tube by the stopper to allow the air in the test tube to cool down– note that water moves into the tubing indicating that the air in the test tube has contracted.

Change of State The three states of matter are solid, liquid and gas.

Change of state is always caused by a gain or a loss of heat by the substance.

Heat Gain »

Solid «» Liquid «» Gas

«Heat Loss

Solid » Liquid: Melting

Liquid » Gas: Vapourisation

Solid » Gas: Sublimation

Gas » Liquid: Condensation

Liquid » Solid: Freezing or Solidification

Demonstrating the Expansion of Water on Freezing Fill two bottles with water. Screw the cap on tightly on one and leave off the cap on the other. Stand both in the freezer to freeze in plastic bag. The closed bottle will be broken or greatly expanded; the open bottle will have a pinnacle of ice sticking out from the top.

The Difference Between Heat and Temperature

Temperature is a number that indicates the degree of hotness of a substance. Heat is the energy that transfers from a region of higher temperature to a region of lower temperature. Temperature is the average kinetic energy of the particles of the substance. Heat is the total kinetic energy of the particles of a substance.

If heat energy is added to a substance the average kinetic energy of its particles rises and so its temperature increases. It is possible for a substance to have a very high temperature but little heat and also for a substance to have a low temperature but a lot of heat. (tiles on outside of the Space Shuttle)

Measuring the Temperature of Various Solids and Liquids

Use a liquid in glass thermometer, mercury or alcohol, for measuring the temperature of liquids - mercury gives quicker results as it responds faster. Make sure the bulb is completely submersed in the liquid.

A thermoelectric thermometer can be used to measure the temperature of solids by placing the sensitive probe onto the object and reading the temperature from the recording instrument. A thermistor

Determining the Melting Point of Ice. Place ice cubes of ice in a beaker with a little water. Place the bulb of a mercury thermometer into the water. The heat of the room will gradually begin to melt the ice. Record the temperature every minute. What point does the temperature stay constant as the ice turns to water?

Determining the Boiling Point of Water. Half fill a clean beaker with water. Place the bulb of a mercury thermometer into the water. Heat the water with a Bunsen burner. Record the temperature every minute until the water boils. During boiling the water temperature stays steady at what temperature?

Investigate the Effect of Pressure on the Boiling Point of Water.

a) Increased Pressure

Quarter fill a Buchner flask with water.

Connect rubber tubing to the side arm and close with a clip.

Heat with a Bunsen burner bringing the water to the boil.

Put a thermometer in a one-hole stopper and place the stopper in the flask.

The pressure builds up in the flask because the steam cannot escape.

Note that the mercury rise up above 100°C and rises as the pressure rises.

Remove the heat at 107°C.

Allow to cool and then open the clip to equalize the pressure so the stopper can be removed.

b) Decreased Pressure

Quarter fill a Buchner flask with water.

Connect rubber tubing to the side arm and close with a clip.

Heat with a Bunsen burner bringing the water to the boil.

Allow to boil for five minutes so the ‘steam’ can drive out all the air.

Put a thermometer in a one-hole stopper and place the stopper in the flask.

Turn off the Bunsen and allow the flask to cool.

The pressure inside the flask falls as the steam condenses.

Note that the water is still boiling even though the temperature is below 100°C.

Note the temperature at which boiling stops.

Open the clip on the rubber tubing to equalize the pressure so the stopper can be removed

Increased pressure raises the boiling point. Decreased pressure lowers the boiling point.

Food cooks faster in a pressure cooker because the temperature is higher in the pot as the water is boiling at 120°C. Water boils at 70°C at the top of Mount Everest as the pressure at the top is only half that at sea level.

Latent Heat is the heat gained or lost by a substance while it changes state without it changing temperature.

As ice melts it gains heat from the room but the temperature of the ice remains at 0°C. The temperature only rises above 0°C when all the ice has melted; then the gain in heat produces a temperature rise.

As water boils it gains heat from the room but the temperature of the water remains at 100°C. All the heat is going into vapourisation; as time passes the volume of water in the beaker is decreasing.

Plot a Cooling Curve and Explain the Shape of the Curve in Terms of Latent Heat.

Over time the gain of heat by ice from the freezer produce a two-step curve. The temperature gradually rises from -18°C to 0°C as the ice from the freezer gradually warms up. The temperature remains steady at 0°C as the gain in heat melts the ice. After melting, further gain of heat results in a temperature rise. The temperature rises to 100°C and then vapourises. During vapourisation the temperature remains steady at 100°C despite heat gain.

Heat Transfer

Heat can travel from a region of higher temperature to one of lower temperature by 3 methods

Conduction is the transfer of heat through a substance from a hot to a cool region without the substance moving.

Convection is the transfer of heat through a substance from a hot to a cool region by the flow of the substance.

Radiation is the transfer of heat from a hotter to a cooler region by electromagnetic waves.

Conduction and convection need a medium, a material substance; radiation does not. Electromagnetic waves can travel in a vacuum therefore heat can be transferred through a vacuum by radiation.

Carry Out Experiments to Show the Transfer of Heat Energy by Conduction, Convection and Radiation

a) Simple Experiment to Show the Transfer of Heat Energy by Conduction

Identical rods of different materials are inserted by a similar short length into a empty metal container.

Glass beads are stuck to the underside of the rods at one centimetre intervals with a small amount of petroleum jelly. Boiling water is poured into the container. Record the time at which each bead falls off. Heat transferred along the rods melting the jelly and the beads fall. The greater the number of beads that fell and the sooner they fell the better is the material at conducting heat. Set up a league table of heat conduction ability.

Metals tend to be good heat conductors – copper, aluminium, zinc and iron in that order.

Non-metals tend to be poor conductors (good insulators) – glass, wood, rubber, plastic.

Graphite carbon is an exception – it is a good heat conductor despite being a non-metal.

b) Simple Experiment to Show the Transfer of Heat Energy by Convection

Set a beaker of water on a tripod. Using a dropper place a small amount of purple potassium permanganate solution on the bottom of the beaker in the center. Heat the water using a fine yellow flame of a Bunsen burner. The purple water moves up the centre to the surface spreads across the surface and down along the sides. The movement of the colour indicates the movement of the water due to convection.

c) Simple Experiment to Show the Transfer of Heat Energy by Radiation

With a marker pen blacken the bulb of a mercury-in-glass thermometer. Observe and record the temperature of the room. Place a sheet of clear plastic between an infrared lamp and a blackened bulb. Note the increase in temperature – the mercury rises up the stem of the thermometer.

Investigate Conduction in Water Sink a small cube of ice in a boiling tube of water by placing a coil of wire over it. Heat the top of the water with the flame of a Bunsen. A ‘heat shield’ of tin foil can be used to stop heat radiation. The water at the top boils but the ice cube is not melting any faster. There is a difference of 100°C in the temperature over just a short distance. Therefore water is a bad conductor of heat.

Heat Insulation (opposite of conduction) is the prevention of heat transfer. Heat transfer can be prevented by using a vacuum, using non-conducting materials, by trapping liquids and gases so they cannot move and by giving surfaces a high polish.

A thermos flask uses a vacuum between the two walls preventing conduction and convection. The walls are also highly polished to reduce radiation. Hair and clothing traps air preventing its convection – air is a bad conductor.

Compare Insulating Ability of Different Materials

A covered beaker with 200 cm3 of boiling water at 100°C is set on a pipe clay triangle and its temperature measured and recorded every minute for 15 minutes. A second beaker with 200 cm3 of boiling water at 100°C is set in a box is completely surrounded by a cotton wool - its temperature measured and recorded every minute for 15 minutes. Other materials are tested in a similar fashion. Graphs are drawn using the same axes so that the insulation abilities of the different materials can be compared to each other and to that when insulation was not used.