L2 : Physical Change - Physical & Chemical Changes, Science, Class 7 Video Lecture

hello friends this video on physical and
chemical changes part 2 is brought to
you by exam for your calm no more fear
from exam now broadly there are two
types of changes which we are going to
discuss one is a physical change and
other one is a chemical change so let's
see what is physical change and what is
chemical change so we will start our
discussion with physical change now as
the name suggests the change is with
respect to the physical property so when
the change is such that the differences
are noticed with respect to physical
properties what do you mean by physical
properties those properties which can be
observed which can be measured which can
be seen from outside they have called
physical properties so let me give you
some examples of physical properties for
example color shape size appearance
texture smell melting point boiling
point so these are all physical
properties because you can actually see
them or you can actually measure them
from outside so they are not internal
property so they are all external so
these kind of properties when there are
changes in these physical properties the
change is said to be a physical change
now mostly the physical changes are
reversible change reversible something
which can be reversed so that means the
change has taken place and again the
change can come back so let us take an
example a very simple example of your
haircut so here is a physical property -
something which can be seen from outside
the length of the hair can be measured
from outside so it is a physical
property so when you go by a haircut
what happens the size and the shape of
the hair changes so it is a physical
change now do you think that it can be
reverted back yes of course because your
hair will again grow so again your hair
will grow and again you can go for a
haircut right so basically these kind of
changes they are physical changes let me
give you an example let us suppose you
take
water so water ice and water vapor so
all of these if you look at their
chemical composition all of them is
nothing but H 2o this is also h2o this
is also h2o this is or vegetable but
there is a change that is happening
values that change happening due to
difference in their physical properties
so if you look at ice ice is solid the
appearance of the eyes is very much
different from the appearance of the
water which is liquid again the
appearance of the vapor which is a gas
is a lot different from solid and liquid
so due to these changes and physical
properties this change has taken place
but the chemical composition still
remains the same so this is an example
of a physical change so we are now going
to discuss about some more examples of
physical change so the first example we
will take is breaking of a glass if a
glass breaks what happens what changes
the shape of the glass changes of course
initially it was one complete glass now
it is like broken pieces of glass of
different shapes and sizes so the size
changes shape changes what else changes
the texture the appearance also changes
but do you think that because of this
change a new substances being formed
chemically whatever glass was the same
chemical composition remains even after
breaking into pieces so there is no
change in the chemical composition just
the size and shape of the glass orders
so therefore this is a physical change
though you might ask that once the glass
break I can cannot join it to form the
same glass yes you can if you become
little more imaginative you can actually
think how the small pieces of glasses
can be used to make a new glass it can
be used may be you cannot do it
instantly but it is not impossible it is
possible maybe if you take small pieces
of glass if and that is how all the
glass objects are being
Manufacturing the factories the pieces
of glass molded together so of course it
is reversible as well so this is an
example of physical change again if you
take example of a paper being cut into
pieces so just think of this newspaper
and just think that you have cracked
this newspaper and tore it into pieces
so what do you think changes in this
cases initially it was just one single
rectangular piece of paper now it is
like multiple pieces each of different
shapes so the shape has altered the size
has altered but again the chemical
composition of paper is still the same
no new substance no new chemical
substance has formed so this is again an
example of a physical change think of
breaking and end now please note that
here I am talking about breaking an egg
so we are just breaking the egg so when
we break the egg
we are not ordering the chemical
composition of the eggs the egg list is
remaining the same there is no change in
its composition it's just that the size
changes the shape changes the appearance
changes so in fact you can even think of
restoring the same egg with a little
more imagination thinking that you will
just join all the pieces together so
this is again a physical change think of
chopping a log of good so when you
chopping the whole log of wood what
happens the size changes of course
because you are actually breaking it so
you see it is actually coming out in
small small pieces so size shape and
clearance they are changing but again
the composition remains the same so
weather predictor Miami for change is a
physical change or a chemical change one
important trick is that if any new
substance with new chemical composition
is being formed that means definitely it
is a chemical change but as long has no
new substances formed it is always a
fizz
the change then if you observe more
carefully you will see that all physical
changes are mostly reversible so you can
actually revert them to get back to the
original substance and some are the
other way
think of a mixture of water and sugar so
when you mix sugar with water what
happens so you get a sugar solution
because sugar is highly soluble in water
so it easily dissolves in water so you
get a solution but do you think that
chemically the new substance is being
formed that Ganesha also it was sugar it
was water so chemically it was h2o and
chemically what was sugar maybe let
let's say it is c6h12o6 or whatever now
even after you mix both of them they do
not react with each other to form a new
substance it is just a mixture of the
two substance but they are chemically
still the same so no new chemical
substance is being formed and moreover
if you want you can separate it as well
sugar and water even though they
dissolve with each other but there are
so many processes by which you can also
separate them not only sugar solution if
you think of a solution like salt and
water if you mix salt with water then
also you will get a salt solution that
is NaCl when mixed with water so there
also they do not really react with each
other if you start boiling the salt
water what happens the water will
evaporate and you will be left over with
the salt so you can actually separate
water and salt so it is again a
reversible process so this is again a
physical change so when you mix these
kind of substances which makes together
to form a solution but actually
chemically they are still remaining the
same they are not forming a new chemical
substance and moreover they are also
reversible changes so this is also a
physical change thank you please visit
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