Every substance has a characteristic set of properties - chemical and physical - by which it may be identified. (Some properties can be observed only by sophisticated equipment, but this unit uses only easily observed properties.) The identification of a substance's properties, and the use of properties to characterize a new substance, are a significant part of what chemists and other scientists do.

Why should students be interested in learning how chemists discover a substance's properties? Someone recently asked: "don't we already know the properties of everything?" The answer to this is, first all, we don’t know all the properties of even the best-known substances; but, probably more important, scientists make and discover new substances all the time.

The Registry of the American Chemical Society contains more than 22 million substances.  Between 1997 and 1999, 3 million new substances were added.  And the annual rate of growth is still increasing. These new substances represent both newly synthesized chemicals and those discovered in nature.

In an average working day, the CAS registry adds approximately 11,000 substance registrations.  That amounts to a new substance registration every few seconds!   Source: Chemical Abstracts Service Registry Image URL: http://www.cas.org/bar2.gif

Why are we are interested in the chemical and physical properties of substances? For many reasons:

• Knowing the properties of a substance helps us recognize it when it appears in a new place.
  • Example: the melting point of a compound can identify it when it is found somewhere in nature.
       
• Properties are a guide to the composition and structure of a substance.
  • Example: A property such as its spectrum in ultraviolet or infrared light greatly narrows the possibilities for what it could be.
       
• When we find a new substance, its properties help us guess what other substances the new substance may be like.
  • Example: its solubility in different substances, its spectrum, and other properties place each new substance within a class of existing compounds.

• Objects have many observable properties, including size, shape, color, temperature, and the ability to react with other substances. Those properties can be measured using tools.
     
• Objects can be described by the properties of the materials from which they are made, and those properties can be used to separate or sort a group of objects or materials.
     
• Materials can exist in different states--solid, liquid, and gas. Some common materials, such as water, can be changed from one state to another by heating or cooling.

What do we mean by "property"? Pay attention to both parts of the definition:

Both definitions imply that a property (in the chemical sense) is the way a substance behaves in a particular condition.  For example, the substance water always boils when the temperature is 100^o C.   That is a property of water.  Or, when sodium bicarbonate (baking soda) is mixed with an acid solution (for example vinegar), bubbles of carbon dioxide (CO₂) are released.  That is a property of sodium bicarbonate.

What is the difference between chemical properties and physical properties? How do we tell them apart?

This is, in our opinion, a very tricky matter.  Textbooks often define these terms in circular ways.  For example, a chemical property is often defined as a property that involves chemical change.  But what is chemical change?   Chemical change can be defined as a process that changes the chemical properties of a substance.  We're right back where we started!  Likewise with physical change.  

What is worse, there are cases where even chemists disagree on whether something is a physical or chemical change.  Often a long and difficult history of painstaking discovery is covered up by simply declaring that a given change is chemical or physical.

In our opinion, it is too early, conceptually, to try to have 3rd-graders make this distinction.

For teachers, let us work with the following definitions, and not be concerned right now with how we recognize chemical and physical changes.

Chemical change

A change that causes a change in how atoms are bonded together.   A chemical change results in the creation of new substance(s) and/or the destruction of existing substance(s).

Physical Change

A process that changes only the way in which existing substances are organized.  The most common examples of physical change are:

• Melting and freezing
• Boiling and condensing
• Dissolving and coming out of solution
• Forming a mixture

Why does it matter?  The distinction matters only if you are interested in whether a new substance has been created.  We think that for 3rd-graders the distinction is largely lost.  Or (what is worse) if learned it will be learned only as rote definitions without any real meaning.  We urge that if concentrating on this distinction, make it real.

How are the properties of a substance related to its chemical composition?  Simple: the chemical composition of a substance completely determines both its physical and chemical properties.  That's why properties are so important in identifying a substance.  Properties are directly observable by the senses, chemical composition is not.  Chemical composition has to be deduced from properties.