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1.3 Properties of Matter

Different types of matter have different distinguishing characteristics that we can use to tell them apart. These characteristics are called physical properties and chemical properties. Physical and chemical properties may be intensive or extensive. Intensive properties such as density, color, and boiling point do not depend on the size of the sample of matter and can be used to identify substances. Extensive properties such as mass and volume do depend on the quantity of the sample.

Physical properties are those that we can determine without changing the identity of the substance we are studying. For instance, we can observe or measure the physical properties of sodium metal. It is a soft, lustrous, silver-colored metal with a relatively low melting point and low density. Hardness, color, melting point and density are all physical properties. Figure 7.19 shows a chunk of metallic sodium, which is soft enough to be cut with a knife.


Figure 7.19

 

Chemical properties describe the way a substance can change or react to form other substances. These properties, then, must be determined using a process that changes the identity of the substance of interest. One of the chemical properties of alkali metals such as sodium and potassium is that they react with water. To determine this, though, we would have to combine an alkali metal with water and observe what happens.

Sodium and Potassium in Water


Sodium metal (Na) reacts rather vigorously with water to produce sodium hydroxide (NaOH) and hydrogen gas (H2). After the reaction has occurred, although we now have evidence of one of sodium metal's chemical properties, we no longer have sodium metal. Potassium reacts even more vigorously with water to produce potassium hydroxide (KOH) and hydrogen gas. As with sodium, once we have determined a chemical property of potassium metal, we no longer have potassium metal. To determine the chemical properties of a substance, it is necessary to change the substance's chemical identity.

The changes undergone by sodium and potassium when they react with water are chemical changes, also known as chemical reactions. Matter can also undergo physical changes in which the chemical identity of the matter does not change. One example of a physical change is the melting of a solid. When ice melts, it changes from a solid state to a liquid state, but its chemical identity (H2O) is unchanged. All changes of state are physical changes.

Questions

1
A freshly cut surface of sodium quickly becomes dull with a film of white sodium oxide when it is exposed to air. Is this a physical change or a chemical change?
Physical
Chemical

2
Another property of sodium is that it conducts electricity, as do all metals. Is electrical conductivity an intensive or extensive property of sodium metal?
Intensive
Extensive