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Physical Science: Session 4

A Closer Look: Chemical Vs. Physical Change

ice melts
Ice melting: an example of physical change.

Physical change:

Although some extensive properties (like shape, phase, etc.) of the material change, the material itself is the same before and after the change. The change can be “undone.”


rusty nail
A nail rusting: an example of chemical change.

Chemical change:

The substances present at the beginning of the change are not present at the end; new substances are formed. The change cannot be “undone."


Mass conservation:

Mass is neither created nor destroyed.

What are the macroscopic and microscopic differences between physical and chemical changes?

  Macroscopic Definition Microscopic Definition
Physical Change The matter is the same.
The original matter can be recovered.
The particles of the substance are rearranged.
Chemical Change

The matter is different.
The old matter is no longer present. The original matter cannot be recovered.

The particles of the substance are broken apart, and the atoms are rearranged into new particles, forming a new substance.

 

Examples:

Many changes of state are easy to characterize based on the definitions above, including the following:

Physical Changes Chemical Changes
Aluminum foil is cut in half. Milk goes sour.
Clay is molded into a new shape. Jewelry tarnishes.
Butter melts on warm toast. Bread becomes toast.
Water evaporates from the surface of the ocean. Rust forms on a nail left outside.
A juice box in the freezer freezes. Gasoline is ignited.
Rubbing alcohol evaporates on your hand. Hydrogen peroxide bubbles in a cut.
  Food scraps are turned into compost in a compost pile.
  A match is lit.
  You take an antacid to settle your stomach.
  Your body digests food.
  You fry an egg.

 

Is the distinction always “clear-cut”?

There are many cases where the distinctions between physical changes and chemical changes are unclear. For example:

The dissolution of salt in water: This seems like a physical change because we know we can recover the salt from the water. However, if we look at the microscopic level, we see that the two types of atoms in salt, sodium and chlorine, separate from one another. In this example, we don’t have a new substance, therefore this salt in solution doesn’t fit the microscopic definition of a chemical change; but we also don’t have the substance in its original form — a stack of alternating sodium and chlorine atoms. Does this mean the change is half chemical and half physical? Though it has aspects of a chemical change, scientists would still classify the dissolution of salt as a physical change.

The creation of a metal alloy: If we melt two types of metal together, we create an alloy metal that has different properties than either of its components (e.g., heat conductivity, electrical conductivity, density, etc.). This might lead us to think that we’ve witnessed a chemical change. In fact, a new particle is not created by melting two metals together. This indicates they did not undergo a chemical reaction. Brass, for example, is about 60% copper and 40% zinc, and is composed of individual copper and zinc atoms (i.e., there is no “smallest unit” that is still brass). There is no such thing as a brass molecule.

The heating and cooling of certain rubbers and plastics: One might think that exposing certain rubbers and plastics to heat or cold would cause a chemical reaction because the properties change (e.g., the materials become more rigid and brittle). While chemical reactions do take place, they simply bind together different parts of the large molecules that compose rubber and plastic. These new bonds add to the rigidity of the material, but the particles of the substances remain the same.

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