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℃.
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
100℃.
D) 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℃ 𝒕𝒐 𝟒℃ ,
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 – UNITS
degree Celsius denoted as ⁰C
degree Fahrenheit denoted as ⁰F
Kelvin (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.
d
Steps for Making of a Mercury Thermometer.
1. Construction
2. 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 thermometer
Lower fixed point and Upper fixed point
Step 3 Reading a thermometer
Filling of mercury in the capillary tube.
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.
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 thermometer
Lower 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 35⁰C
5. 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 Scales
Celsius 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 0⁰C
Upper 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 K
Upper 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 32⁰F
Upper fixed point is marked 212⁰F.
RELATION BETWEEN CELSIUS, FAHRENHEIT AND
KELVIN SCALES
℃
℉ − 𝟑𝟐
𝑲 − 𝟐𝟕𝟑
=
=
𝟏𝟎𝟎
𝟏𝟖𝟎
𝟏𝟎𝟎
Sources of Heat - FIRE
Fire as known to early man was for
Protection from cold,
Frighten wild beasts
Melt 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 to
Drive 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.
THE END
HOME WORK
Pg 59 QC 1TO 7 ALL
DO all the QA,QB in text book