HEAT – EFFECTS OF HEAT

Effects of Heat

Some daily life examples on effect of Heat on Matter.

1. Snow Melts, Water boils and gradually disappears.

2. Rise in temperature of the body.3. Expanding of objects.4. Change of states.5. Speeding up of chemical reactions.6. Killing of organisms eg. Bacteria present in milk

and water

How does the temperature of a body changes on heating?

This change in temperature depends on the quantity of heat absorbed or given out by the body and the material of the body.

When heat is absorbed, the temperature of the body rises.

When heat is given out, its temperature falls.

How does the shape of a body changes on heating?

This change in shape is due to thermal expansion.

When a body is heated it expands.

When it is cooled it contracts.

TEMPERATURE VERSUS TIME CURVE

How does the state of matter changes on heating?

A) FUSION OR MELTING - It is the process during which a solid changes to liquid state at a constant temperature by the absorption of heat.

Wax melts at 63 Ice melts into water at 0.

SOLID LIQUID GAS

B) Freezing - It is a process during which a liquid changes into its solid state at a constant temperature by releasing heat.

The melting point and freezing point for the same substance are same.C) Vapourisation or Boiling - It is the process during

which a liquid changes into its vapour state at a constant temperature by the absorption of heat.

Boiling point or vapourisation point of water to steam is 100D) Condensation - It is the process during which

vapours change into liquid state at a constant temperature by releasing heat.

E. Sublimation - It is a process in which solid directly converts into gaseous state. E.g. camphor, naphthalene, ammonium chloride and iodine.

F. Solidification : It is a process in which a gas condenses directly into solid state. E.g. dry ice is conversion of carbon dioxide gas to solid state

G. Expansion : A substance expands on heating and contracts on cooling.

All substances expand on heating and contract on cooling. Except water which on heating from 0 , contracts and on heating further it expands.

When a substance is heated, its molecules start moving faster and the intermolecular space increased. Hence, hotter the substance, the larger will be its volume. Comparatively, gases expand the most while solids expand the least on heating.

Measurement of Temperature – UNITSdegree Celsius denoted as ⁰Cdegree Fahrenheit denoted as ⁰FKelvin (K)Instrument of measurement – Thermometer.Working principle : A given length of liquid

(mercury) column rises with the rise in temperature.

The basic qualities of a good thermometer.1. Bulb should be thin walled, so that the liquid inside the bulb

should quickly attain the temperature of the body in contact.

2. Stem must be made of thick glass, so that it does not easily break.

3. Capillary bore should be narrow, so that the liquid moves through a large length in the stem even if there is a small increase in the volume of liquid in the thermometer.

4. Liquid used must expand uniformly with the rise in temperature and even for a small rise in temperature. This will help in calibration of thermometer.

5. Liquid should have a low freezing point and high boiling point, so that the instrument can be used over a wide range of temperature.

6. Liquid should be opaque, so that it is visible through glass.

7. The liquid should be practically non-volatile so that its expansion is not affected by its vapour pressure.

8. Liquid should have low specific heat capacity. Lower the specific heat capacity lesser the heat it will absorb from the body to expand.

9. Liquid should be available in its pure state.

10. Liquid should not stick to the glass.

Advantages of using mercury as thermometric liquid.

1. Does not stick to the glass, therefore readings are more accurate.

2. Is opaque and shiny liquid, hence easily visible through glass.

3. Expansion is uniform at all temperatures.4. Low freezing point (-39⁰C) and high boiling point

(357⁰C). Therefore can be used over a wide range of temperatures.

5. Is easily available in pure state.6. Is non-volatile substance and hence vapours are not

formed in the empty part of the thermometric tube.

Advantages of using alcohol as thermometric liquid.

1. Freezing point is -139⁰C and hence can record very low temperatures.

2. Has a fairly even expansion or contraction or a large range of temperatures (about six times more than mercury)

3. Difficulty of visibility through the glass is removed by adding a dye to it.

However alcohol starts boiling at 78⁰C and cannot be used to measure very high temperatures.

Water cannot be used as a liquid in thermometer due to the following reasons.1. Sticks to the glass.2. Is transparent, so it is difficult to read its level through the

glass.3. Is volatile and hence vapours apply pressure on the surface

thereby affecting the reading.4. Is poor conductor of heat.5. Expansion is not uniform over a wide range of temperatures.6. Freezes at 0⁰C and boils at 100⁰C and hence the range is not

wide.7. Specific heat capacity is high which means that it will absorb

more amount of heat from the object which is kept in contact with the thermometer.

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Steps for Making of a Mercury Thermometer.1. Construction2. Marking the lower fixed point and the upper

fixed point.3. Calibration i.e. temperature scale.

1. Construction

A thermometer is made up of a cylindrical stem with a bulb at one end.

Inner walls of the bulb are connected to a capillary tube.Initially the other end is open.

Steps involved in thermometer construction

Step 1 Construction - Filling of mercury in capillary tube

Step 2 Calibration of the thermometerLower fixed point and Upper fixed point

Step 3 Reading a thermometer

1. A clean and hard capillary tube about 30 cm long is taken.

2. A cylindrical bulb is connected to the capillary tube at one end and the other end is kept open.

3. A small glass funnel is connected to the tube with the help of a short rubber tube.

4. Place this arrangement vertically with its bulb in an oil bath.

5. Pour some mercury into the funnel and the oil bath is heated.

6. Air of the bulb expands and escapes out in the form of bubbles through the mercury in the funnel.

Filling of mercury in the capillary tube.

7. The tube is taken out from the oil bath and allowed to cool.

8. The air contracts and a little amount of mercury goes down into the bulb through the capillary tube.

9. This process of heating the tube in the oil bath is repeated many times, till the bulb is completely filled with mercury.

10. The bulb is then heated to a temperature higher than the maximum temperature to be measured by the thermometer.

11. The top end of the capillary tube is sealed by heating the end over a gentle bunsen flame.

Calibration of thermometerLower and upper fixed point :Steps :1. Keep the bulb of the capillary tube in ice. At 0⁰C taking care

that the temperature of ice is maintained at 0⁰C.

2. Because of this the mercury starts contracting till the temperature of mercury in the tube is also 0⁰C.

3. When the temperature of mercury in the tube is 0⁰C, it stops contracting and its level becomes fixed.

4. Mark this position of the mercury level as 0⁰C. This is known as the lower fixed point.

For upper fixed point the capillary tube and bulb is kept inside an arrangement called hypsometer.

Steps :

1. The capillary tube’s bulb should be continuously in contact with the steam.

2. As the bulb is in contact with steam mercury inside it rises. It is expanding till the temperature of mercury is same as the temperature of steam - 100⁰C.

3. On attaining this temperature, mercury stops expanding and its level becomes fixed.

4. This point is marked as 100⁰C and is known as the upper fixed point.

Reading a thermometer.Steps :1. Note the temperature difference indicated between the two

bigger marks.2. Note down the number of divisions between these two

marks. Then read one small division.3. Now wash the thermometer, preferably with an antiseptic

solution.4. Hold it firmly and give it a few jerks to bring down the level

of mercury down. Ensure that it falls below 35⁰C5. Now place the bulb under your tongue for 1 minute.6. Table the thermometer out and note the reading.7. This is your body temperature.

Temperature ScalesCelsius Scale : Length of the thermometer between the lower fixed point and upper fixed point is divided into 100 equal parts.Lower fixed point is marked 0⁰CUpper fixed point is marked 100⁰C.

Kelvin Scale : Length of the thermometer between the lower fixed point and upper fixed point is divided into 100 equal parts.Lower fixed point is marked 273 KUpper fixed point is marked 373 K.

Fahrenheit Scale : Length of the thermometer between the lower fixed point and upper fixed point is divided into 180 equal parts.Lower fixed point is marked 32⁰FUpper fixed point is marked 212⁰F.

RELATION BETWEEN CELSIUS, FAHRENHEIT AND KELVIN SCALES

Sources of Heat - FIREFire as known to early man was forProtection from cold,Frighten wild beastsMelt copper and iron to make weapons and tools.We use fire to cook food,And the heat produced by burning of fuels like diesel,

petrol etc., is used toDrive engines,Run vehicles,And also produce electricity.Thus FIRE is one of the major sources of HEAT

But FIRE causes CERTAIN damage to PROPERTY and also LOSS OF LIFE.

Petrol, diesel, LPG, synthetic fibres, wood, dry grass, paper etc., are substances which can easily catch fire – INFLAMMABLE SUBSTANCES.

Water, sand are best known as fire resistant substances – NON-INFAMMABLE SUBSTANCES.

If an unlucky person suffers from burns injuries, it would be best to pour water on the burn injuries as first aid. Water helps to remove heat from the burnt part most efficiently.

Source of Heat - SUN

Sun is the major natural source of heat.Living organisms on the earth get heat from the

sun.Heat when falls on water reservoirs or masses

evaporate – received as RAIN.Plants synthesise their food from heat of the

SUN.

Source of Heat - ELECTRICITY

We get heat from electricity in various forms.Room heater,Geyser to heat water,Mixer, Grinder, Toaster and electrical gadgets,

electricity is used which heats the filament of the gadget.

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