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What is boiling?
In order to answer this question, we must be familiar with the
kinetic molecular theory of matter. This theory explains the
conversion of the three states of matter namely, solids, liquids and
gases. The main postulates of the theory are as follows:
1. All matter in solid, liquid or gaseous state consists of
extremely small particles in the form of ions, atoms or molecules,
which are capable of independent existence.
2. Each substance has its characteristic constituent particles,
differing from those of other substances.
3. Particles of each substance are arranged in such a way that there
are empty spaces between two particles.
4. Particles are held together by attractive forces. This attractive
force is strongest in solids at ordinary temperatures, weak in
liquids and weakest in gases.
5. The particles are in continuous motion and therefore possess
kinetic energy. They interact with one another and hence also
possess potential energy. |
Boiling |
Now, let us consider the process of
vaporization or boiling. We know that relatively weaker forces
of attraction hold particles in a liquid state together. This means
that they are loosely packed, as compared to solids, and are not
arranged in a regular manner. The particles enjoy freedom of
movement within the bulk of the liquid. On heating the liquid, the
kinetic energy of the vibrating particles increases steadily until a
point is reached when these particles possess enough energy to
overcome the attractive binding forces. At this stage, the particles
leave the surface of the heated liquid and escape as vapor. This
transition from the liquid to the gaseous state occurs at a constant
temperature and is referred to as its boiling point. Only after
complete vaporization will the temperature of the gas increase on
further heating.
Why boiling points differ for different
substances
Molecules of substances are held together by forces of
attraction, thereby constituting bonds. The strength of the bonds
between molecules varies. Naturally, the energy required to overcome
these bonds will also differ. Consequently, different substances
have different boiling points.
Stronger forces of attraction between
molecules of a liquid result in lower vapor pressure and hence, a
higher boiling point. Some substances are held together by extremely
strong bonds and have almost no vapor pressure at ordinary
temperatures. The thermal energy required to overcome these bonds is
extremely large and thus, these substances have very high boiling
points. For example, iron has a high melting point at 1535ºC and an
even higher boiling point at 2750ºC as a result of the presence of
extremely strong attractive forces between its constituent
particles. |
