Physical and Chemical Properties

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I. Physical Properties

A physical property of a pure substance is anything that can be observed without changing the identity (that is, the chemical nature) of the substance. The obervations usually consist of some type of numerical measurement, although sometimes there is a more qualitative (non-numerical) description of the property. There are many physical properties and each textbook will have a different list of examples. Here are some of the more common ones:

melting pointelectrical conductivitycolordensity
boiling pointthermal conductivityodorhardness

There are others which are not mentioned as often. Examples include:

refractive indexatomic radiusductility
ionization energyallotropesmalleability

There are more which have not be mentioned. There is no single, definitive list of physical properties. A few example properties are cited, there is some discussion and the author moves on.

Groups of similar elements or compounds can be characterized by commonality in their physical properties. Metals have a whole bunch of physical properties that are similar. For example, metals are very ductile and very malleable. All easily conduct electricity and heat and all have a bright luster. These all reflect a commonality of structure.

However, the similarities in a group do not extend to every property. Both tantalum and sodium are metals. Tantalum's melting and boiling points are 2996 °C and 5425 °C. Sodium? 98 °C and 883 °C. However, they are both considered metals and no one in the scientific world disputes this. The reason is that both exhibit the characteristic arrangement of atoms and electrons all metals have. (This arrangement will be taught later in the course.) The wide disparity in the melting and boiling points between tantalum and sodium simply highlight the wide range that exists within the common structure all metals have.

I guess human beings would be a good example for the above point. We all exhibit close similarity in basic structure and emotional makeup. However, we range across a wide spectrum in many areas. Mozart was composing music by the age of four and, more than 200 years later, people are still listening to his music. His father took him and his sister all over Europe, showing off their musical skills. You and me, when we play an instrument, dogs begin to howl, the sky clouds over and mothers rush to bring their little children in the house. When you and I try to play an instrument, SWAT teams from 4 counties show up and the guy on the bullhorn says "Come out slowly. Show your hands. NOW!!"

II. Chemical Properties

This one is more difficult. Here is one way to define "chemical property:"

characteristics which are exhibited as one substance is chemically transformed into another.

Here are some examples.

(1) iron rusting. When iron (an element, symbol = Fe) rusts, it combines in a complex fashion with oxygen to form a reddish-colored compound called ferric oxide (formula = Fe2O3). Not all substances rust.

(2) glucose, mixed with yeast, ferments to make alcohol. Glucose (C6H12O6) is a chemical compound which enzymes in yeast can use to make ethyl alcohol (C2H5OH). Not all substances ferment.

(3) trinitrotoluene (TNT) reacts very, very fast when it is ignited. Among other products, it makes LOTS of nitrogen gas and LOTS of heat. Inside the proper container, it can cause an explosion. Not all substances can make an explosion.

There really isn't a set of chemical properties in the same way there is, more or less, a set of physical properties. That's because the chemical properties are tied to the change, whereas a given substance has a property (such as melting point) all to itself.

Another textbook I consulted defined "chemical property" this way:

chemical properties describe the way a substance may change or react to form other substances.

One example was given: flammability - the ability of a substance to burn in the presence of oxygen. Some substances (wood, alcohol) are very flammable, others are not. Iron (see above) reacts with oxygen, but so slowly we do not say the iron burns, but that it rusts.

Here's an example of a question involving chemical properties:

Which of the following is a chemical property?
(a) the ability to burn in air
(b) the hardness of a metal
(c) the melting point of a pure substance
(d) the color of a solid

The key factor to remember is that chemical properties involve the formation of new chemical substances. Burning in air is the only answer choice that has a production of new chemical substances. Therefore, the ability to burn in air is a chemical property.

Just to hammer away at the point, look at the other three answers and realize that no new chemical substances are being produced. The other three answers all describe physical properties.

Generally speaking, information about physical properties is clearly laid out and chemical properties is harder to pin down. That's just the way it is sometimes.

Years after I wrote the above, I came across this:

The ability of a substance to form new substances is a chemical property.

It was coupled with this question:

Physical properties describe the ability of a substance to form new substances. T F

The answer, of course, is false.

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