Page 18 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 1] BASIC CONCEPTS 7

even a nuclear reaction) mass is neither created nor destroyed. Because of the close approximation that the mass
of an object is the quantity of matter it contains (excluding the mass corresponding to its energy) the law of
conservation of mass can be approximated by the law of conservation of matter: During an ordinary chemical
reaction, matter can be neither created nor destroyed.

EXAMPLE 1.3. When a piece of iron is left in moist air, its surface gradually turns brown and the object gains mass.
Explain this phenomenon.
Ans. The brown material is an iron oxide, rust, formed by a reaction of the iron with the oxygen in the air.

Iron + oxygen −→ an iron oxide
The increase in mass is just the mass of the combined oxygen. When a long burns, the ash (which remains) is much
lighter than the original log, but this is not a contradiction of the law of conservation of matter. In addition to the log,
which consists mostly of compounds containing carbon, hydrogen, and oxygen, oxygen from the air is consumed
by the reaction. In addition to the ash, carbon dioxide and water vapor are produced by the reaction.

Log + oxygen −→ ash + carbon dioxide + water vapor
The total mass of the ash plus the carbon dioxide and the water vapor is equal to the total mass of the log plus the
oxygen. As always, the law of conservation of matter is obeyed as precisely as chemists can measure. The law of
conservation of mass is fundamental to the understanding of chemical reactions. Other laws related to the behavior
of matter are equally important, and learning how to apply these laws correctly is a necessary goal of the study of
chemistry.

Solved Problems

1.1. Are elements heterogeneous or homogeneous?
Ans. Homogeneous. They look alike throughout the sample because they are alike throughout the sample.

1.2. How can you tell if the word mixture means any mixture or a heterogeneous mixture?
Ans. You can tell from the context. For example, if a problem asks if a sample is a solution or a mixture, the word
mixture means heterogeneous mixture. If it asks whether the sample is a compound or a mixture, it means
any kind of mixture. Such usage occurs in ordinary English as well as in technical usage. For example, the
word day has two meanings—one is a subdivision of the other. ”How many hours are there in a day? What
is the opposite of night?”

1.3. Are compounds heterogeneous or homogeneous?
Ans. Homogeneous. They look alike throughout the sample because they are alike throughout the sample. Since
there is only one substance present, even if it is a combination of elements, it must be alike throughout.

1.4. A generality states that all compounds containing both carbon and hydrogen burn. Do octane and propane
burn? (Each contains only carbon and hydrogen.)
Ans. Yes, both burn. It is easier to learn that all organic compounds burn than to learn a list of millions of organic
compounds that burn. On an examination, however, a question will probably specify one particular organic
compound. You must learn a generality and be able to respond to a specific example of it.
1.5. Sodium is a very reactive metallic element; for example, it liberates hydrogen gas when treated with
water. Chlorine is a yellow-green, choking gas, used in World War I as a poison gas. Contrast these
properties with those of the compound of sodium and chlorine—sodium chloride—known as table salt.
Ans. Salt does not react with water to liberate hydrogen, is not reactive, and is not poisonous. It is a white solid
and not a silvery metal or a green gas. In short, it has its own set of properties; it is a compound.