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Dr. Christopher S. Baird

What is the difference between a chemical process and a physical process in chemistry?

Category: Chemistry      Published: September 24, 2013

ice cubes melting
There is no fundamental difference between a chemical process and a physical process. This misleading distinction is made mainly by school teachers. For example, the melting of ice to liquid water is both a chemical and a physical process. Public Domain Image, source: Christopher S. Baird.

There is no real difference between a chemical process and a physical process in chemistry. Some chemistry teachers like to define a chemical process as any process that involves a chemical reaction and all other processes as physical processes. According to such teachers, things like burning fuels are chemical processes and things like dissolving salt in water or freezing water into ice are physical processes. But this distinction is really arbitrary and non-fundamental. While such teachers may make such a distinction with the good intention of teaching their students, they are really setting up the students to be confused in the long term.

All processes that involve the interaction of atoms are chemical. Dissolving salt in water is a chemical reaction. You start with two distinct reactants (salt and water), get the atoms to bind to each other in new ways (each salt ion becomes bound to a crowd of water molecules), and a new chemical is formed (salt water). Typical of all chemical reactions, heat is exchanged with the environment as part of the process. Dissolving salt in water may not be as glamorous as exploding a balloon filled with hydrogen, but it is still a chemical reaction.

Even processes as simple as changes in phase (solid to liquid, liquid to gas, etc.) are really chemical in nature. In the process of freezing to ice, the molecules in liquid water start in one configuration, form bonds as they take on the new configuration, and release energy in the process. Some teachers don't like to treat phase changes as chemical reactions because the basic chemical equations are not very useful in teaching students. For instance, the basic chemical equation for water freezing to ice is: H20 → H20. This equation is misleading. It seems to imply nothing is happening at all. For this reason, some may think that phase changes don't really count. But a more detailed chemical equation is more illuminating: H20(liquid) – heat → H20(solid) . Contained in that arrow is the formation of stable hydrogen bonds between water molecules after the removal of energy (energy is always released when chemical bonds are formed). The formation of bonds is the key feature of chemical reactions.

In fact, just about every day-to-day experience that we are familiar with is fundamentally chemical in nature. Kicking a football, changing gears on a bike, singing, and writing words on paper are all described on the fundamental level as the interaction of atoms. On the fundamental level, the only processes that are not chemical in nature are gravitational processes and nuclear/subatomic-particle processes.

Furthermore, the term "physical process" is so vague as to be useless. Every observable process in the universe is physical. The only things in the universe that are not physical are abstract concepts such as love and faith. All chemical processes are physical, as are all biological, geological, astronomical, gravitational, subatomic, and nuclear processes. The book "Misconceptions in Chemistry" by Hans-Dieter Barke states:

It is traditional in chemistry lessons to separate chemical reactions from physical processes. The formation of metal sulfides from its elements by releasing energy is described in every case as a chemical reaction. In contrast, the dissolving of substances in water is often regarded as a "physical process" because matter "does not actually change", the dissolved substance can be regained in its original form through "physical" separation procedures. If one takes sodium hydroxide and dissolves it in a little water, a colorless solution appears and releases heat; the solution conducts electricity and produces a high pH value. Critical students regard this solution as being a new material and the production of heat shows an exothermic reaction. From this example one can see that it does not make any sense to separate the transformation of matter into "chemical" and "physical" processes [8]. If we routinely continue to do this in the sense of "we've always done it this way", automatic school-made misconceptions would arise based on teaching traditions in school.

Topics: chemical process, chemical reaction, phase, phase change, phase transition, physical process