Ronaldo Degazon Wednesday 09/05/12 Physics: Lab #8 Thermal Physics

Aim: To investigate the cooling curve of a substance.

Apparatus: candle wax, test tube, test tube holder, Bunsen burner, tripod & beaker, thermometer,

stop-clock, retort stand, water & wire gauze, glass rod

Diagram:

Procedure:

1. The apparatus was set up as shown in Diagram A, with the substance in a water

bath.

2. The substance was heated until it was completely melted and well above its melting

point.

3. The test tube containing the substance was removed and placed in a retort stand,

with the thermometer in the liquid as shown in Diagram B.

4. The stop-clock was started and the temperature was recorded every half minute

until it fell to well below its melting point. The thermometer was not removed

during solidification.

5. The substance was re heated to a temperature above its melting point so the

thermometer could have been removed from the substance.

6. A cooling curve of F versus t was plotted for the substance.

Results:

Time / s Temperature / oC

30 62

60 59.5 90 57

120 56.5 150 55

180 54.5 210 54.5

240 54

270 54 300 53.5

330 53.5 360 53.5

390 53.5 420 53.5

450 53.5

480 53.5 510 53

540 52.5 570 52.5

600 51 630 51

Time / s Temperature / oC

660 50.5

690 50 720 50

750 49.5 780 49

810 48.5 840 48

870 47.5

900 47

Discussion:

The melting point of a substance is the temperature at which it changes from a solid to

liquid. Also the process in which a liquid turns into a solid is called solidification which occurs at the

freezing point; the exact opposite of the melting point. When a substance solidifies, the heat released

during that process is called the latent heat of solidification.

From the experiment, it was deduced that the melting point of candle wax was 53.5 oC

because it remained at that temperature for 180 seconds. Heat was being lost during that period but the

temperature did not fall at all. Therefore it was concluded that the heat was no lost but was used to

repair the bonds of the molecules. The heat used during that 180 seconds is its latent heat of

solidification.

Dark brown flakes were noticed moving around in the candle wax when melted.

However, when solidified, these flakes remained still. This can be accounted for with the Kinetic Theory.

The theory states that liquids consist of clusters of particles that are free to move and that solids consist

of fixed particles which are restricted from moving but only vibrate about a fixed point. When the wax

was melted, the brown flakes moved around from being bombarded by the moving clusters in the liquid

wax. However, when the wax solidified there was no movement amongst the particles and so the flakes

no longer moved.

The Kinetic Theory can also be used to explain the shape of the graph. First it states that

all matter is made of tiny particles. It goes on to say that these particles when supplied with energy,

move faster and hence their average kinetic energy increases. Average kinetic e nergy is proportional to

temperature, hence the more heat supplied the higher the temperature. Applying the theory to this

experiment, you could say that the more heat released by the candle wax, the lower its average kinetic

energy fell and hence the lower its temperature dropped.

From points A to B, as more heat was released by the candle wax, the lower its

temperature fell hence the downwards trend in the line. However, from B to C the line remained

straight – the wax’s temperature remained constant. This is simply because the being lost was not being

absorbed by the wax’s surroundings, it was being used to repair the bonds of the molecules, hence the

average kinetic energy of the particles did not fall. Nevertheless, when the bonds were repaired and the

wax had solidified, the surroundings resumed absorbing the heat from the wax, hence from C to D there

is another downward trend.

Sources of Error:

- Human Reaction Time: there was a possibility that the temperature reading was not

taken exactly 30 seconds after the previous reading.

- Parallax Error: there was a possibility of a parallax error when reading the scale of

the thermometer.

- There was the possibility that the wax would have melted too quickly and vaporized.

- There was a possibility that the thermometer would take the reading of the tube’s

temperature and not the wax’s.

Precautions:

- To reduce parallax error, the thermometer reading was taken with the eye

perpendicular to the level of liquid in the thermometer.

- The test tube containing the candle wax was heated indirectly in a water bath to

control the rate at which it was heated so that it didn’t melt too quickly.

- It was ensured that the thermometer did not touch the bottom of the test tube to

ensure that the temperature of the wax was taken and not the tube.

- A glass rod was used to stir the water to ensure that its temperature remained

uniform throughout.

Conclusion: A cooling curve was successfully obtained for candle wax.