It is dangerous to sleep in an unventilated room with fire
burning inside because the fire produces carbon monoxide and carbon dioxide
gases. Carbon monoxide is poisonous and can cause death
The filament of an electric bulb is made of tungston because
it has a high melting point and can be heated to a high temperature to emit
Water kept in an earthen pitcher becomes cold because the
pitcher has minute pores on its surface which absorb water. Water in the
pores evaporates when it comes in contact with air, and produces a cooling
A sliced apple, when exposed to air, turns brown after some
time as it contains iron which gets oxidised and gives the fruit a brownish
The freezing compartment inside a refrigerator is at the top
because the air in contact with it becomes cold and heavy and sinks
downwards, cooling the rest of the compartment.
A copper vessel left in the air for a long time turns green.
This is due to the formation of copper carbonate when copper reacts with
carbon dioxide and moisture present in the air.
A wick in a stove keeps burning continuously as kerosene
rises in the wick due to capillary action.
The Earth rotates on its axis from west to east. This
rotation makes the Sun and the stars appear to be moving across the sky from
east to west.
The sky appears blue because the light of the Sun is spread
or scattered by the dust particles in the air. In space the sky would appear
black as there are no dust or air particles to scatter the light.
Food is cooked quickly in a pressure cooker because the
boiling point of water increases at high pressure. Food cooks faster at high
In mountainous regions, the atmospheric pressure is less
than it is at sea level. Water, therefore, boils at a lower temperature
(less than 100° C) and food takes more time to cook.
When ice floating in a glass of water melts, the level of
water remains unchanged because as a solid, ice displaces an equal volume of
A man weighs more at the poles than at the equator because
the polar radius of the Earth is less than the equatorial radius. Hence the
gravitational pull is more at the poles than at the equator.
Standing in double-decker buses, particularly on the upper
floor, is not allowed because on tilting, the centre of gravity of the bus
gets changed and it is likely to overturn.
The boiling point of seawater will be more than the boiling
point of pure water because the former contains salt and other impurities.
An ordinary clock loses time in summer because the length of
its pendulum increases, and therefore, its time period also increases. The
pendulum takes more time to complete each oscillation and thus loses time.
Whenever there is water loss from the body, secretion of
saliva is reduced resulting in dryness of the mouth and stimulating the
sensation of thirst. Intake of fluid then helps in restoring the loss of the
A swimmer just out of the river feels cold particularly if
it is windy, because of the evaporation of water from his body surface. The
evaporation is more on a windy day.
Alcohol is sometimes rubbed on the body of a person
suffering from fever. As soon as it is applied on the body, it evaporates
taking away some heat from the body. Since evaporation has a cooling effect,
the body temperature can be reduced by rubbing alcohol.
Soft iron is used as an electromagnet because it remains a
magnet only while the current passes through the coil around it and loses
its magnetism when the current is switched off.
The person jumping out of a moving train is carried forward
in the direction of the train because the person himself is in motion
sharing the velocity of the train and will continue in its state of uniform
motion unless it exercises some force to prevent it (Newton’s first law of
A lightning conductor is fixed to tall buildings to protect
them from the destructive effects of the lightning.
An electric bulb makes a bang when it is broken because
there is a vacuum inside the electric bulb; when the bulb is broken air
rushes in at great speed from all sides to fill the vacuum. The rushing of
air produces a noise generally referred to as the “bang”.
A small space is left between each set of two rails of
railway line to allow for their expansion in summer.
A bad egg floats in water because the up-thrust produced on
account of displaced water by the immersed portion of the bad egg is greater
than the weight of the egg.
Moisture gathers on the outer surface of a glass tumbler
containing cold water because the water vapours present in the air get
cooled and appear as droplets of water on coming in contact with the cold
surface of the glass tumbler.
The launching of Earth satellites should be from a place
near the equator to take the fullest advantage of the Earth’s movements. The
regions of the Earth closer to the equator are moving faster through space
as compared to regions elsewhere. Thus at the equator, rockets would start
off with a speed of nearly 1600 km/hour. Further, rockets are usually fired
in the direction of the Earth’s rotation.
In deserts, day temperatures are very high and night
temperatures are extremely low because the specific heat of sand is very
low. It, therefore, absorbs the heat readily and its temperature rises by a
large degree during the day. At night sand radiates the heat equally
readily, making the temperature low.
If you are sweating, you will feel cooler on a hot day than
on a cooler moist day because on a hot dry day the perspiration gets
evaporated quickly producing more cooling effect.
The air escaping from a punctured tyre feels cold because
the air escaping from a punctured tyre enters a region of low pressure from
high pressure and thus suffers a fall in temperature.
White roof keeps the house cooler in summer than black roof
because white roof reflects more and absorbs less heat rays whereas black
roof absorbs more and reflects less heat rays.
It is hotter on a cloudy night than on a clear night because
clouds prevent the heat radiated by the Earth from escaping into the sky.
Ice wrapped in a blanket does not melt away quickly because
woollen blanket is a bad conductor of heat.
A rose appears red when day light falls on it because it
absorbs all the constituent colours of white light except red, which it
reflects to us.
Grass appears dark in blue light because it has the property
of absorbing all other colours except its own colour. The blue rays falling
on the grass are absorbed by it and hence it appears dark in colour.
A group of soldiers on a bridge are advised not to walk in
steps because their movement causes the bridge to vibrate. It they walk in
step the frequency of vibration may match the natural frequency of the
bridge structure, thereby causing resonance. This resonance of frequency can
cause the bridge to collapse.
When the wind is blowing at high speed, the roofs of the
hutments are sometimes blown away. The pressure at the top falls much below
the pressure below the roof. This difference in pressure across the roof
causes it to be blown away.
Human breath is visible in winter but not in summer because
in winter air is cold and so water vapours present in the human breath
condense, making it visible.
Steam causes more severe burns than boiling water because
steam at the same temperature has more latent heat which causes more severe
burns than boiling water at the same temperature.
It is not advisable to wash a clinical thermometer with
boiling water because the boiling point of water is much above the maximum
provided in the clinical thermometer. As a result, pressure from undue
expansion of mercury may break it.
The face turns red when one feels hot and turns blue when
one feels cold. This is because in the first case the blood rushes towards
the skin to get cooled, and in the second case it rushes away from the skin
to keep itself warm.
A petrol fire cannot be extinguished by pouring water over
it because of the high temperature of burning petrol the water breaks up
into hydrogen and oxygen which only helps in burning. Further, petrol being
lighter than water, floats on water and continues to burn.
We experience difficulty in breathing on mountains because
the pressure of the air outside is less as compared to the pressure of air
inside the lungs.
If a highly corked glass bottle full of water is left out of
doors on a frosty night it will burst because the water contained in the
bottle will freeze on a frosty night and convert into ice. There is no room
available for the increased volume and this may result in bursting of the
Water extinguishes fire because as it evaporates, the
temperature of the burning body is lowered, thus retarding or stopping the
burning action. Also the resulting vapour surrounds the burning substance
cutting off the oxygen supply thus inhibiting the burning process. In fact,
hot water will extinguish fire more quickly than cold water as hot water
will vaporise faster.
A perspiring man feels relief when air floats by his side
because the flow of air increases the rate of evaporation of perspiration
from the body.
Water from a hand pump is warm in winter and cold in summer
because in winter outside temperature is low and in summer outside
temperature is higher as compared to the temperature of water obtained from
underground which remains practically unchanged due to Earth being bad
conductor of heat.
A thick glass tumbler often cracks when very hot liquid is
poured into it because the inner surface of the thick glass tumbler coming
in contact with the hot liquid expands more in comparison with the outer
surface which has relatively lower temperature. This unequal expansion of
inner and outer surfaces causes the tumbler to crack.
When a gun is fired at a visible distance, the sound is
heard a little after the smoke is seen because the velocity of light is much
higher than that of sound.
A train stops when the chain is pulled because of the
following mechanism. When the chain is pulled, one small valve gets opened
and air enters to the under side of the piston head of the brake cylinder
through pipe connections. Initially there is vacuum on both top and bottom
side of the piston head. When air enters the under side of the piston head,
the piston raises up due to difference of pressure and pulls the brake thus
stopping the train.
Archimedes Principle: When a body is immersed either wholly
or partially in a fluid at rest, the apparent loss of weight suffered by it
is equal to the weight of the fluid displaced by it.
Avogadro’s Law : Equal volumes of all gases under the same
conditions of temperature and pressure contain equal number of molecules.
Black Body Radiation : A black body absorbs heat or radiates
heat more quickly than any other body.
Boiling Point : It increases with the increase of pressure.
The presence of impurities also raises the boiling point of a liquid.
Boyle’s Law : At constant temperature, the volume of a given
mass of gas varies inversely to its pressure.
Centre of Gravity : A body will remain at rest only if the
vertical line through its centres of gravity passes through the base of
support of the body.
Charle’s Law : The volume of a given mass of gas is directly
proportional to its absolute temperature when the pressure remains constant.
Coulomb’s Law : The force between the two electric charges
reduces to a quarter of its former value when the distance between them is
Dalton’s Law : At a specific temperature and for a container
of fixed volume, the total pressure of a mixture of non-reacting gases is
the sum of their respective partial pressures.
Doppler’s Principle : When the distance between the source
of a wave and the observer increases due to their relative motion, the
frequency of the wave appears to decrease. The converse condition is also
Faraday’s Laws of Electrolysis
1. The amount of chemical change during electrolysis is proportional to the
2. The masses of substances liberated or deposited by the same quantity of
electric charge are proportional to their chemical equivalents.
Heisenberg’s Uncertainty Principle : The velocity and
position of an electron in the orbit of an atom cannot be simultaneously
Inverse Square Law : The force of attraction between two
unlike magnetic poles and the force of repulsion between two like poles is
inversely proportional to the square of the distance between them. A similar
law is true of electrical charges also.
Law of Conservation of Matter : In chemical changes, matter
is neither created nor destroyed. The sum total of the masses of all the
products of a chemical change is exactly equal to the sum total of the
substances from which these products have been ,formed.
Laws of Thermodynamics
1. The amount of heat given to a system is equal to the sum of the increase
in the internal energy of the system and the external work done.
2. It is impossible to construct a continuous self-acting machine that can
pump heat energy from a body at lower temperature to a body at higher
Lenz’s Law : When an electric current is induced by a change
in magnetic field, the induced current is always in such a direction that
its magnetic field opposes the change of field which causes the induction.
Mass-Energy Equation : E = mc2, where E = quantity of energy
released from the annihilation of matter of mass ‘m’, c = velocity of light.
It implies that mass and energy are interchangeable.
Newton’s Law of Cooling : The rate at which a body cools or
looses its heat to its surroundings is proportional to the excess of mean
temperature of the body over that of the surroundings, provided this
temperature excess is not too large.
Newton’s Universal Law of Gravitation : Everybody in the
universe attracts every other body with a force, directly proportional to
the product of their masses and inversely proportional to the square of the
distance between them.
Newton’s Laws of Motion
1. Everybody continues in its state of rest or of uniform motion in a
straight line unless it is compelled to change that state by a force [called
Law of inertial].
2. The rate of change of momentum of a moving body is proportional to the
applied force and takes place in the direction of the force.
3. For every action there is an equal and opposite reaction.
Ohm’s Law : The amount of current flowing in an electric
circuit is governed by the voltage of the battery or dynamo which powers it.
In other words, the current through a conductor is directly proportional to
the potential difference across the conductor and inversely proportional to
1. When pressure is applied to a fluid, the pressure change is transmitted
to every part of the fluid without loss. Hydraulic machines work on this
2. Atmospheric pressure decreases with increase in height.
Rectilinear Propagation of Light : Light travels in a
straight line. Total internal reflection takes place when a ray of light
tries to pass from a denser medium to a rarer medium at an angle of
incidence more than the critical angle.