| What makes ice melt?
|Question Date: 2015-02-09|
In short, one word: heat! Also known as
thermal energy. You've probably experienced this
first hand eating a piece of ice or watching it
melt on a hot summer day. Now the question is,
why is that so?
The basic idea is that all materials have some
sort of interaction or bonding between each of the
species (e.g., atoms, molecules). When you heat
something up, you give the atoms or molecules more
energy to move around. At some point, there is
enough thermal energy that the interaction between
the atoms or molecules would be disrupted, and
your material changes phase (e.g., melts). Let's
see this in action...
Ice is the solid phase of H2O.
Although we call various phases of H2O
ice (solid), water (liquid), and vapor (gas), they
are all composed of the same atoms, namely two
hydrogen atoms and one oxygen atom. Atoms are the
building blocks of all matter. An atom itself
consists of a nucleus with protons and neutrons
surrounded by electrons. Protons are
positively charged, neutrons have no charge and
electrons are negatively charged. Protons and
electrons are electrostatically attracted to each
other. If you've ever played with magnets before,
you will have an idea what this attractive force
is like (although for magnets the force is of a
different kind, but the idea is the same).
Here is approximately what a carbon atom is
composed of. The nucleus containing the protons
and neutrons is much smaller than what is shown
and how the electrons fill around the nucleus is
quite complicated, so the picture is not
Nevertheless it is useful for explaining
Hydrogen and oxygen are similar, but have
different numbers of protons, neutrons, and
electrons. For a neutral atom, the number of
electrons and protons are the same. These are
captured with the
periodic table , which looks like it is
arranged in a weird way, but is actually one of
the most important tools we use in the research I
Here is what a water molecule looks like
(i.e., when two hydrogen and one oxygen atom come
together to form a molecule).
If you were to look at the periodic table, you
would find that hydrogen contains 1 electron, 0
neutrons, and 1 proton while oxygen has 8 of each.
Because oxygen has so many protons, its nucleus
has a much larger positive charge compared to the
nucleus in hydrogen. This means that the oxygen
atom draws in the electrons from the two hydrogens
a little closer to itself. This makes the oxygen
atom slightly negative and the two hydrogen atoms
slightly positive. What arises out of this uneven
distribution of electrons is something known as
hydrogen bonding, in which hydrogens of one water
molecule are attracted to the oxygen of other
water molecules. You might also notice that the
hydrogens are squished together (we call it a bent
At this point, you might be wondering what this
all has to do with ice melting. It turns out it is
this hydrogen bonding and bent configuration that
gives rise to a lot of interesting properties to
water. Ice is a solid, but is less dense than its
liquid form. This arises from the bent structure
(and the hydrogen bonding to a certain extent),
which forces the water molecules to adopt
particular orientations (i.e., directions) when
frozen. It is also why snowflakes have very
particular symmetries (like the ones in this
cool photo gallery of snowflakes; pun
When ice is melted, you give the water
molecules enough energy to wiggle out of these
particular orientations. If you heat the water
further, you completely disrupt the hydrogen
bonding, and you get water vapor.
It turns you can also change the melting point
(and boiling point) of ice, and pretty much any
other material. There are many more variables you
can change to do this, the most common being
pressure. That water boils at 100 C or freezes at
0 C assumes that it doesn't have impurities and we
are at atmospheric pressure. But this doesn't
necessarily have to be true!
Hope this helps!
This answer relies on a related question asked a
few weeks ago "How does water and air act like
a heat reservoir? ".
read it here
Heat is what makes any material melt. Like
everything on earth, ice is made of atoms. Many
many atoms of Hydrogen and Oxygen combine together
to form ice. These atoms are stuck together by a
force of attraction between the atoms. But,
when one supplies energy to ice, the atoms get
excited and try to move around a bit. If the
energy supplied to ice is large enough, the atoms
that are moving can overcome the attractive force
between each other, and break free. When atoms
break free, ice turns into liquid water. In other
words, ice melts.
If the supplied energy is even larger, then
atoms completely move away from each other turning
liquid water into water vapor.
Ice is made of water molecules which take
different shapes depending on the temperature. At
high temperatures, the water molecules have a lot
more energy and can move freely through the air
which is what we call steam. At room temperature
the molecules have less energy so they pack
tightly together in liquid water. Finally, at low
temperatures, the water molecules don’t move as
much anymore and form a solid crystal which we
call ice. So ice melts when the water molecules
get enough energy to move around more which allows
them to become liquid water.
Ice and other materials are capable of holding
energy in the form of heat. If you put an ice
block in a hot environment, it will absorb some of
that heat energy. If the ice absorbs enough heat,
it reaches a point where it will melt into water.
This is its melting point, which is usually given
as the melting temperature (32 degrees
Fahrenheit). Water is more energetic than ice,
meaning it is holding more heat than ice.
To explain WHY this melting occurs, you need to
know that materials like ice are made up of many
tiny, identical building blocks called molecules.
When molecules absorb heat energy, they
vibrate. The more they are heated, the bigger
the vibrations get until they break apart from
each other and form a liquid. This is why a liquid
has less structure than a solid, and gets,
Ice melts because of either warmth or pressure.
Above 0 degrees Celsius (32 degrees Farenheit),
ice melts to become water. Enough pressure can
also melt ice (this is how ice skates work)
Water can exist as a solid (ice), as a liquid
or as a gas, depending on what is the temperature
and the pressure.
When we take ice cubes (solid) out of the
freezer to the room, we are taking them from a
very low temperature to a higher one. Eventually,
the temperature of the ice cube has to reach the
temperature of the "infinite" temperature "bath",
the room. So, the ice absorbs energy, in the
form of heat. The water molecules are getting
warmer, they have more kinetic energy, they
translate and vibrate more and they don't want to
stay nicely organized as a solid. So, they
disorganize and the ice cube becomes a liquid.
Another way to melt ice is to increase the
pressure. Water is quite atypical in this respect.
Usually, if we want to melt something you would
think we want to decrease the pressure, to allow
the molecules to move, and not be tightly packed.
But with water it's the opposite. In the ice
phase, the molecules are well organized with
specific distances between them, prescribed by the
hydrogen bonding, and there are specific gaps
between them. If we increase the pressure, instead
of packing them better, we disorganize their
structure and it melts and we achieve this at a
fixed temperature. At even higher pressures this
thinking breaks down, like for ice found on
All these transitions are shown in a graph called
the "phase diagram", which also includes the
transitions between liquid and gas water as well
as all the different structures of ice. There are
more than a dozen of them!
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