now chapter 11 we skip the chapter 10
it's about gases I will advise you just
to read it it is simple now chapter 11
is about liquids and intermolecular
forces it's again an easy chapter not
hard I'm sure you will follow and easily
now we have three states of matter right
as I start the chemistry course actually
when talk about the states of matter I
say there are two three states and one
of them is the solid state the other one
is liquid state and the other is gaseous
state now if you compare these three
different states at molecular or atomic
level we actually see this when we have
a solid as we see here the molecules or
the ions or the atoms makes the solid
are very regularly arranged in the three
dimensions and the solid in fact is hard
we can't really change the volume of a
solid by applying pressure if we take a
solid and put it in a container it will
not fill the container or take the shape
of the container but will just stay as
it is and the liquid if you take a
liquid again apply pressure you can't
change the volume much off the liquid
that means that the liquid just like
same as the solid are not compressible I
can't change the volume munch but the
liquid will take the shape of the
container that you put in right so
simply you can move and change its shape
easily as a matter of fact now the gas
on the other side if you take a gas and
apply pressure we can reduce the volume
easily
that means the gases are compressible
and if you take a gas and put it in a
container doesn't matter what volume it
is it will fill out the whole container
now these observed properties can be
explained in terms of intermolecular or
inter atomic in tyrannic interactions in
these three different states
we know that the solid in fact the
molecules or the atoms all the ions
whatever the units makes the solid are
very strongly attracted to each other
that means intermolecular forces are
very very high so that is why the solids
does not change it is shape when you
pour it when you put it in a container
it will just stay as it is now what doll
what do the molecules or the units in a
solid they are not stationary right in
solid they move but they make vibration
emotions simply like this but they're
more less the average point do not
change it just just move like this make
vibration of motions in the solid but
they cannot separate from one another
why because they are attracted each
other that means there's a very strong
attraction force between the molecules
of the solids or the units of the
solvent is a matter of fact now the
other things is the distances between
the molecules of the solid in the solid
is not much that means there's not much
space between the solid molecules or the
solid units so that's why the solids are
not compressible that means you can't
really apply pressure and change the
volume reduce the volume of the solid
there's not much space actually
between the units molecules or the atoms
of the solid is a matter of fact now if
you go to the liquid the attraction
between the molecules of the liquids are
also very strong because the liquids
cannot simply leave the liquid state and
go into the gaseous state or other state
they have to stay together they have to
stay together what keeps them together
actually the intermolecular force of
attraction right intermolecular force of
attraction but since the liquid
molecules are moving freely as they
touch each other like this you can
change the shape of the liquid easily
actually the shape of the liquid will be
the same as the shape of the container
that you put in right but they will not
fill in the whole volume of the
container as a matter of fact so what we
say that the intermolecular forces or
attraction forces between the liquid
molecules or the unit that make the
liquid are weaker than this solid but
they are pretty strong still strong that
the liquid stays together that means it
in the liquid state now when you go to
the gas the molecules are simply moving
freely in three-dimensional space like
this why because the attraction between
the gas molecules is very very weak
actually if you consider the ideal gas
in the ideal gases we assume that the
intermolecular attraction is zero in a
gaseous state there's no attractions so
every gas molecules moves in
three-dimensional space freely as a
matter of fact like this now also there
is a lot of space between the gas
molecules right so if you simply
compress the gas Moloch gases sample
apply pressure you can change the volume
but if you increase the volume since the
gas molecules are moving they will fill
out the whole container right they will
fill out the whole container so that
means
the attractions between the molecules in
the solid is very strong if the liquid
is still strong but not as strong as
solid and then for the gas the
attractions is very very weak as a
matter of fact almost negligible right
this is how we describe the three states
and molecular level properties and these
actually explains why we observe those
physical properties of the solids
liquids and the gases that means change
in volume and compressibility and so on
now let's see what we have okay this
this explains very shortly those
properties of gases different states of
matter here it seems both volume and the
shape of the container we say that
expands to fill the container and is
compressible and flows readily diffusion
within the gas occurs rapidly this is
the gas phase and all of these are
explained with the model where you have
the gas molecules are freely moving
attractions between the gas molecules is
very very weak as a matter of fact now
in the liquids this gain assumes the
shape of the portion of the container it
occupies does not expand the fill it's
all container here it fills the whole
container and is virtually
incompressible not much space between
the liquid molecules Falls readily
diffusion within the liquid occurs
slowly much slower than the gas in this
solid retains own shape and volume
doesn't change and does not expand to
fill its container it's virtually
incompressible does not flow diffusion
within the solid occurs extremely slow
very very slow so this simply gives you
idea about three different states as a
matter of fact winters day yeah
how can we explain these behavior
actually here it says the answer to this
question largely relies on the balance
between the kinetic energies of the
particles and inter particle energies of
attraction now this is again can be
explained as following we have the
attraction between two molecules like
this and we also have the kinetic energy
of the molecules in the solid state they
simply move like this vibrational type
motions but they have kinetic energies
in the liquid they can move freely by
touching each other like this they're
not separate in a gas they are more free
to move so simply we can say that the
attraction forces in this solid is
greater than the kinetic energy of the
molecules
therefore the molecules cannot simply
break off break that attractions and
become free right so that means
attraction forces in the solid greater
than the kinetic energy now if you go to
the liquid in this case the attraction
forces are about the kinetic energy or a
little bit less than kinetic energy why
because the molecules in the liquid can
go by and move in the liquid freely as a
matter of fact right they simply just
break even with the attraction forces in
fact attraction energy between the
molecules but if you go in the gaseous
state kinetic energy is much greater
than the energy of attraction between
the molecules as a matter of fact right
therefore the molecules simply move
freely from one another and that is the
gaseous state so this is what try to
explain that here is the kinetic
energies in the gas phase is much
greater than the
fraction energies of attraction in the
liquid case comparable kinetic energies
and the energies of attraction solid
energies of attractions is greater than
the kinetic energies so this is it
now here is a question how can we
increase the kinetic energy of the
molecules that means by increase in
temperature yes if you just give heat to
the system temperature will increase
when the temperature increases that
means that the kinetic energy of the
molecules will increase in three
different states
so what does that means that means if I
take a solid which have the kinetic
energies actually less than the
attraction energy is right and heat the
solid what will happen the kinetic
energy will increase and I will get to
the point where the kinetic energy will
be greater than the attraction energies
what does that means that means the
solid will go from solid to the liquid
state that means there's going to be a
transition between the stage the phase
now if you take that liquid in fact and
keep heating it give heat and the
temperature increases as temperature
increases the kinetic energy of the
molecule increases increases and now
they will be a point where the kinetic
energy of the molecule will be much
higher than the attraction energy
between the molecules what does that
means that the molecules will simply
leave the liquid state will go into the
gaseous state right will go into the
gaseous state so in that case it will be
simply evaporation another change of the
state so we can simply say that the
kinetic energy at the same temperature
if I have gas liquid and solid I can
simply say this kinetic energy of the
molecules highest in the liquid in the
gas and then lower much lower in the
liquid but much lower in the solid state
right in a solid state now all we said
so far attraction energies that means
there is an attraction force
between the molecules so now let's look
at what kind of attractions between the
molecules are possible and we call these
forces intermolecular forces
intermolecular forces now
let's we have simply in in in in in a
solid or liquid and a substance actually
we can talk about two kinds of
attraction forces one is called the
intra molecular attraction forces the
other one is called inter molecular
attraction forces now intra molecular
attraction forces is actually the
chemical bonds right and intermolecular
attractions are the forces that we have
talked about to explain the properties
of solid liquid and gases right the
gases and we should also know that the
intermolecular attractions that means
between the molecules is much weaker
than the intra molecular attraction
which is a chemical bond so the chemical
bond is also an attraction but it is
much much stronger than the
intermolecular attraction this is what
it simply shows that here we have a
chemical bond and that's an attraction
also but here we have the attraction
between two molecules this is an
intermolecular and this is an intra
molecular attraction intra inter so we
should simply remember those again and
many physical properties that reflects
intermolecular forces like boiling
points Mountain points viscosity surface
tension and capillary actions all of
these properties which are simply
considered as physical properties can be
explained or can be compared by
comparing the intermolecular attractions
intermolecular - intermolecular forces
okay types of intermolecular forces we
simply classify them as follow
dispersion forces this is the one first
one and it's sometimes called London
dispersion forces dipole-dipole forces
hydrogen bonding it's a special type of
dipole-dipole forces ion dipole forces
this is actually appears only in liquids
in solutions like this and here says the
first two types are also referred to
collectively as van der Waals forces so
when we talk about Venerable forces we
mean dispersion forces and dipole-dipole
forces not the hydrogen bonds again now
let's try to compare these and explain
what these forces are now dispersion
forces it says here we see that usually
actually dispersion forces are present
in all substances as an intermolecular
force as a matter of fact and but it
becomes the main intermolecular force if
you have a non-polar molecule nonpolar
molecule as a matter of fact now well
how can we explain that let me just try
to show you how the dispersion forces
are explained now
consider for example neon gas right now
neon gas is more atomic noble gas it is
gas at room temperature but if you lower
the temperature they will be liquefied
you can have a liquid neon as a matter
of fact now that means that there is any
traction forces between neon molecules
now what is the source of its attraction
the source of attraction between two
neon neon atoms actually is the
the dispersion forces dispersion forces
and it is again as followed this neon
has ten electrons right ten electrons
let's say how this is the simply the
nucleus and we have the neon atom like
this and this is one neon atom and then
I have the other neon atom here like
this
the gases form right same size now 10
electrons are distributed put in
orbitals but they move in the neon atom
now suppose that in one instance in a
very short time the electrons of neon
are more on this side than on that side
what happens if you have this
unsymmetrical distributions of the
electrons in the neon one side more
negative the other side will be more
positive that means that the neon atom
will behave as a dipole one side will be
a little bit negative and this side will
be a little bit positive again right
very short time now but when you have
this polarization this is called the
polarization of the neon that means more
electrons there and less there then the
neon atom which is next to this one will
also be affected by this and as soon as
that happens here I will have simply
more electrons on this side and less
electron on that side this side of this
neon will be Delta minus this side will
be Delta plus now there's going to be
another neon atom here actually and the
same things will happen there and so on
so what will happen in an instant time
very short time here I have the neon
atom one sign is a little bit negative
due to the unsymmetrical distributions
of the electrons on homogeneous so you
can call it one side a little positive
now this one is affected by that this
atom also this side negative this side
positive this side becomes
again negative positive negative
positive what happens it becomes like
dipole dipole dipole attractions right
dipole-dipole attraction this creates
the attraction force between the neon
atoms actually even though the neon
molecule atom is not polar right it is a
non-polar atom spherical actually but in
a very short time this happens now an
instant later then the whole electrons
can move on that side now that side will
be positive this side will be negative
positive negative positive but still
there's going to be attraction due to
this polarizations or formations of a
dipole actually on the atoms and this
kind of force or intermolecular forces
are called London forces dispersion
forces now the question is in which
molecules and which atoms this simply
takes place it takes place it is present
in all substances between all molecules
all right now let me try to ask you this
what is the main factor which will
determine this dipole actually
attraction that means is this dispersion
forces what will be the main factor the
main factor actually will be this how
easy to move the electron in one atom to
this fashion that means how easy to
distort the electron distribution in in
a molecule or an atom will determine how
strong will be the dispersion forces the
attraction actually right now if it is
very easy to disturb we call that
polarizations polarize the atom of the
molecule then the attraction dispersion
forces will be high if the polarization
is not easy but difficult then the
dispersion forces will
now this polarization will be determined
by the size of the molecule or the size
of the atoms the larger the size of the
atom and the larger the molecule that
means more electron there and it will be
easy to polarize those kind of molecule
and that will result actually stronger
dispersion forces between the molecules
so let's say remember that this is what
is the dispersion forces actually the
dispersion forces now the factors gained
a fair amount dispersion forces in the
molecule factors which affect the amount
and number of electrons in an atom or in
a molecule doesn't matter the more
electrons we have that means the more
stronger will be the dispersion forces
and size of atom or molecule simply the
molecular weight again the size shape of
molecules with the similar masses or
compact less dispersion actually when
you have the same size of molecule but
different shape of the molecules like in
this case here we have n pentane c5h12
and then we have no pentane c5h12 both
molecule has the same number of carbon
and hydrogen atoms same weight same
number of electrons but the shape are
different so one is sort of longer again
and the other one is sort of you know
compact like this which one will be more
easy to polarize this one will be more
easy to polarize right so that simply
means that the dispersion forces in
normal pentane will be stronger than the
dispersion forces in neopentane now how
this reflects this reflects in the
boiling point of these two compounds you
see again in this end pentane the
boiling point is
3:09 right this is 3 or 9.5 Carolyn and
neopentane 2.80 2.7 Calvin that means
that the boiling point of Noah pentane
is much lower than the boiling point of
the normal pentane what does that means
that means that the degree of dispersion
forces or the strength of dispersion
forces between the molecules or the
atoms affects the boiling point of that
compound and actually affects the other
properties as we will see viscosity
surface tension and so on now this is
again the polarizability and the boiling
point what we simply compare here neon
argon Krypton xenon now all of these are
atomic actually and they are not polar
and also fluorine molecule chlorine
molecule bromine and iodine again they
are nonpolar molecule so the only
attraction forces between two nonpolar
molecules is the dispersion forces
nothing else but the dispersion forces
so as we see here that the mountain
point of the substances for example
going from neon to argon Krypton and
xenon increases an increase in melting
point simply means that increase in the
inter atomic attraction dispersion
forces increases dispersion forces and
again you see the polarizability will
increase this way it would be easy to
polarize large xenon atom or large
iodine molecule compared to small neon
atom or fluorine molecule very difficult
to polarize the fluorine it's very small
you know number of electrons 18
electrons very compact but when you have
iodine a very large nonpolar
lot of electrons so much easy to
polarize the iodine therefore the
dispersion forces in iodine is much much
stronger than the dispersion force and
fluorine molecule that
for the boiling point of the iodine is
much higher than the boiling point of
the fluorine you see again how the
dispersion forces degree of dispersion
forces changes the boiling point or
affects the boiling point yes yes
increase in the temperature will
certainly affect the intermolecular
forces because kinetic energy increase
simply if it close to the intermolecular
forces then the effect of the
intermolecular forces becomes lower in
determining the physical properties it
does but that would be the same for all
actually now the second type of
intermolecular forces is dipole-dipole
interactions and this is only possible
if you have polar molecules right and
very simple again right the negative
side of the polar molecule will be
attracted by the positive side of the
polar molecule and the molecules
particularly for example if you have in
the solid state
they will orient themselves in such a
way to get maximum attraction between
dipoles actually between polar molecules
and in the liquids they will be a little
bit more chaotic way but still the main
attraction will be simply dipole-dipole
attraction and this is only possible for
dipolar molecules actually we have to
have polar molecules to have
dipole-dipole interactions to operate in
that substance
now this simply shows how the
dipole-dipole interactions and the
dipole moment 'strength affects the
properties particularly in this case the
boiling point are given all here you see
now these molecules this is propane this
is dimethyl ether Isetta aldehyde C the
nitrile
and all of these are polar molecules
they are all polar molecules now they
all have this almost same molecular
weight this is 44 46 44 41 that means
that the size of the molecule in other
words the number of electrons is about
the same you can think about that way so
that simply means that since the size is
about the same
so the polarizability will be about the
same that means that the dispersion
forces for all of them will be more or
less the same right so the difference
again of their boiling points results
from the different typo
dipole attractions between the molecules
and as the dipole moment simply
increases the dipole dipole attraction
force actually interaction increases
here the dipole moment point one dipole
moment one three two point seven three
nine you see the boiling point 231 248
234 and 355 you see how the boiling
point increases so stronger
intermolecular attraction means higher
boiling points right a higher boiling
point for the liquids in fact now this
is in other things now we said we have
three types of intermolecular forces
dipole-dipole forces dispersion forces
and hydrogen bond and in solutions we
have ion dipole forces how can we
compare these forces in fact which one
is dominant which one determines the
physical property all has the highest
effect on the physical properties let's
look at here if two molecules says are
comparable size and shape and
dipole-dipole interactions will likely
to be dominating force that means if we
have the polar molecules which have
about the
same molecular weight the same size so
the dipole-dipole interactions will
determine the boiling point which is in
the previous cases remember as the
dipole moment increased the boiling
point increase that means that the
dipole-dipole attractions increases with
the increase in dipole moment of the
molecule if one molecule is much larger
than the other dispersion forces will
likely determine its physical properties
again dispersion forces even though it
is present among all molecules but it
becomes more important and more
influential when we have large molecules
and a lot of electrons actually so those
large molecules the dispersion forces
become the dominant intermolecular force
and that determines a lot of physical
properties this is what it simply means
now here again we have a graph which
shows the boiling point of certain
compounds now these are the hydrides
this is the simply hydrides
group four right this is group 5 group 6
and group 7 elements and their compounds
with the hydrogen right and let's start
here we have ch4 methane silane germane
and standing right all the same
structures what is the difference
between them the only difference is the
molecular weight that means that the
number of electrons actually so what we
said as the molecular weight increases
the dispersion forces between the
molecule increases so we see that by
increasing the boiling point of these
compounds for example the boiling point
of silane this molecule sh 4 is greater
than the boiling point of methane and
germane is greater than that greater
than that that means that dispersion
horses highest in this compound and lows
lows in this compound and it just
changes with the molecular weight again
right this is the molecular weight like
this now when you look at the group five
right the simply hydrogen compounds died
being here and then we have the ammonia
molecule here all the others are there
what we see that the boiling point of
ammonia is higher than the boiling point
of simply the germain right here
actually what we have here actually this
is the iodine and then this is going to
be the bromine I guess yes this is going
to people mean and the same thing is
also true for the HF higher than
expected it should be here actually down
here but it's over there and a water
case also the compounds of the group
simply seven hydrogen compounds delirium
start from there and the water there
what is the reason that we have high
boiling points for the water then the
h2s hydrogen sulfide which is the same
you know and hydrogen fluoride then the
hydrogen chloride nh3 actually here and
the previous is the phosphine right here
pH three phosphine and the reason is
there is an extra intermolecular force
very very strong intermolecular forces
in these compounds water hydrogen
fluoride and ammonia that force is
called hydrogen bonding hydrogen bond
all right so we have dispersion forces
we talked about that second is
dipole-dipole now in addition to this
now we have three hydrogen bonding now
this hydrogen bonding doesn't mean a
chemical bond it's just a special name
for that now let's explain what the
hydrogen bonding is first of all to have
a hydrogen bonding we must have in the
molecule a hydrogen atom which is bonded
to the most electronegative atoms
what are those most electronegative
atoms tell me what is the most
electronegative atom fluorine next one
is oxygen right the next one this is
four oxygen is simply four three point
five and then the third one is nitrogen
right these are simply known as the most
electronegative elements most
electronegative element now if I have a
simply a hydrogen bonded to this most
electronegative elements like this and
there may be others here right and one
molecule in on the other molecule if I
have again lone pair electrons on these
most electronegative atoms what does
that means that means I can have simply
a fluorine here or maybe oxygen maybe
nitrogen but I have to have lone pair
electron
and they will be this will be bonded to
others like this you know and the same
things they let's say over here
fluorine maybe oxygen maybe nitrogen
let's say fluorine and maybe oxygen
nitrogen in fact lone pair electrons on
one of the most electronegative elements
like this and there is a very strong
attractions actually stronger than
normal dispersion forces and
dipole-dipole forces between this
hydrogen and this electron pair actually
as the book says it is the attraction
between that hydrogen and this fluorine
but through this electron pairs again
and also this and also this right so
this kind of attractions are called
hydrogen bonds hydrogen bond now if you
think about this hydrogen bond is
actually a special type a different type
of dipole-dipole attractions we can
think about that as because when you
have the hydrogen bonded to a very
electronegative elements one of these
this side of the molecule will be a
little bit Delta positive right and this
will be attracted by this side of the
molecule maybe a little bit negative you
can say due to the electron pairs and
they will be attracted and this will
result what we simply call the hydrogen
bonding and this is very very effective
actually between the molecules and that
is the reason why we have the higher
boiling point for ammonia then expected
and for HF than expected and water than
expected remember the boiling point of
water is hundred degrees normally the
boiling point of water should be
somewhere about over here right this is
again about 200 Kelvin that means you
know less than
degrees as a matter of fact no this is a
boiling point yes Kelvin and the ammonia
should be down here and so on but you
see the methane is here now there is no
intermolecular or hydrogen bonding
actually between two methane molecules
let's consider those so methane remember
it's a tetrahedral molecule hydrogen
hydrogen hydrogen like this and there's
another methane here hydrogen hydrogen
hydrogen and there will be no hydrogen
bonding why because to have a hydrogen
bonding I should have a hydrogen bonded
to fluorine oxygen or nitrogen
do I have fluorine oxygen or nitrogen
here no that means there is no hydrogen
bonding between two fluorine molecule
what is the force of attraction
intermolecular force between two methane
London or dispersion forces right just
the dispersion forces because it's
nonpolar molecule actually and
dispersion forces is always there
remember and here in this case the only
force of attraction is dispersion forces
of London forces the same things now if
I simply have the nh3 this is remember
tetrahedral geometry this is trigonal
pyramidal geometry right I have the
electron pairs here and now here I have
another ammonia like this okay and here
I have the lone pair electrons so now I
have all conditions to form hydrogen
bond between two ammonia molecules and
what is that the hydrogen bonded to
nitrogen and electron pairs on the
nitrogen so there's going to be
a hydrogen bond between this hydrogen
and that nitrogen over here through that
lone pair electron lone pair electron so
what does that means in addition to the
other intermolecular forces I also have
hydrogen bond now what others
intermolecular force will be operating
in ammonia first of all will it have
London dispersion forces yes
remember dispersion forces exists
between all molecules now second do I
have a dipole-dipole attractions ammonia
is a polar molecule right even though
the polarity is very very low but it's a
polar molecule is not much but this is
this hydrogen bond is the main force of
intermolecular attraction which changes
the boiling point simply in way of
increase in the boiling point of ammonia
then we expected only based on the
dispersion forces again right now same
thing is true for the hydrogen fluoride
in a hydrogen fluoride we have simply
hydrogen bonded to the fluorine and then
we have fluorine simply have lone pair
electrons actually three lone pair
electrons and I will have very strong
simply hydrogen bonding between this
hydrogen and this fluorine now this
hydrogen and another fluorine this lone
pair another hydrogen now here I have
the simply like this and actually the
structure of the HF liquid HF is zigzag
type to get a maximum hydrogen bonding
and so that hydrogen bonding forces in
fact increases the boiling point of HF
compared to the HCL actually much higher
than we expected now for the water
another is again let's take the water
remember we have the electron pairs
the oxygen and now we have hydrogen
bonded to the oxygen um and I also have
electron pairs on the hydrogen lone
pairs so there's going to be again
hydrogen bond between this oxygen and
that hydrogen through the lone pair
electrons see again to have an
intermolecular attraction as hydrogen
bonding between two molecules you have
to have a hydrogen bonded to most
electronegative element on one molecule
and then you should also have lone pair
electrons again on the most
electronegative elements those are
fluorine oxygen and nitrogen right to
have the hydrogen bonded otherwise we
will not have hydrogen bonded so this
unexpected behavior or much higher
boiling points of ammonia HF and water
is due to the hydrogen bonding possible
between the molecules of these liquids
actually which is not present in methane
which is not present in methane okay
this these are again the same type of
thing which shows how the hydrogen
bonding will simply take place the
dipole-dipole interaction experiences
when hydrogen is bonded to nitrogen
oxygen or f are usually strong we call
these interactions hydrogen bonds it
says hydrogen bonds is an attraction
between a hydrogen atom attached to a
highly electronegative atom and nearby
now here I didn't like this this is this
is not really true
here it says attached to a highly
electronegative atom right I have to
have a hydrogen bonded to nitrogen
fluorine or oxygen electronegative
that's what means atom and nearby small
electronegative atom in another molecule
or a chemical group that means it says
there must be a nitrogen oxygen or
fluorine on another molecule but this is
not enough
why we should also say that there must
be lone pair electrons on those nitrogen
in fact fluorine oxygen to have a
hydrogen bond if you don't have the lone
pair electrons on
nitrogen oxygen and fluorine it will not
form hydrogen bond so that is we have to
be careful here remember that the
possibility of hydrogen bonding is
possible if you have lone pair electrons
on most electronegative on one molecule
and again if you have hydrogen bonded to
the most electronegative elements okay
now this is simply says that the
hydrogen bonding is more or less like a
dipole-dipole attractions yes you can
look it that way
hydrogen bond arises in part from the
high electronegative the Oh hydrogen
nitrogen and fluorine because those are
the most electronegative these atoms
interact with nearly bare nucleus which
contains one proton simply inform the
hydrogen bonding but there have to be
lone pair electrons if there's no lone
pair electrons no hydrogen bonds okay I
guess we will stop here question