Posted by on Aug 1, 2011 in General Chemistry | 2 comments

# The Scientific Method

Introduction to physical chemistry

The Scientific Method:

If we start off with no information and make a guess at something logical, then the development of that idea is mathematics, not science.  Science is based upon observation first and after doing experiments, trends may be noticed and we can perhaps summarize the data into a Natural Law.

For example, Robert Boyle found that at constant temperature the volume of a gas is inversely proportional to the external pressure.

Boyle's Law

Guy Lussac and Avogadro also discovered natural laws for gases:

Charle's Law

These can be written in terms of equations involving pressure, P, volume, V, temperature, T, and number of moles, n.  Of course the Ideal Gas Law is a combination of all three and the constant is found by comparing to experiment:  That is we find that one mole of a gas at 1 atm. Pressure and 273 K occupies 22.4 liters and this gives us the gas constant (click to enlarge):

These forms are deduced (deduction) from the experiments but are not necessarily understood.  They are called “empirical formulae”:  and work to describe the data but do not tell us why.  For example, why is Boyle’s law,

and not

?

In order to understand a Natural Law we have to make some assumptions.  These are called hypotheses and based on these, attempts are made to understand the empirical laws.  In this case, the Kinetic Molecular theory makes some reasonable assumptions:  the gas is dilute; particles are small relative to the total volume V; they collide elastically with each other and the walls, temperature is well above the boiling point.  We assume we can use classical or Newtonian mechanics. From these hypotheses we obtain the ideal gas law and learn something else:  The kinetic energy is proportional to the temperature.  The hotter the gas, the more kinetic energy the gas has.  At absolute zero of temperature, there is no kinetic energy.  These ideas give us deeper insight into why the ideal gas law works. They follow from the molecular kinetic model which is called a Theory. A theory is an explanation of a phenomenon.

Recall we use logic (mathematics) to understand Nature.  We assume that Nature follows logical steps and we, being logical beings, can understand those steps.  We then derive a theory from the hypotheses that we have deduced and find the theory that explains the phenomenon.  We then test this theory.  For the ideal gas law, as the pressure increases; the temperature decreases or the density increases, the ideal gas law fails.  Hence Laws are tested and usually limitations are found.  We can extend the law (for example the ideal gas law can be extended to Van der Wall’s Equation), by improving it by changing the hypotheses.

Sometimes, however, laws fail when new data is found.  Think classical mechanics and its breakdown to quantum mechanics.  Black Body radiation could not be explained by classical mechanics.  It lead to ridiculous conclusions: it predicts the night sky glows dark blue!  This led to a major change after Max Planck introduced quantum ideas in 1898.

Science is a never ending effort to understand natural phenomena by observation and experiments followed by the deduction of empirical laws, then hypotheses and finally theories.  The theories are used to predict and test new phenomena.

Technology is important.  New effects are discovered as our technology evolves.  For example J. J. Thompson could not discover the electron until it was discovered how to embed metal in glass (to make a Cathode Ray Tube).