Chapter U1.7, Problem 1E

Interpretation Introduction

Interpretation:

A chemical reaction and some possible signs of a chemical reaction have to be explained.

Concept introduction:

Chemists are always on a hunt to transform substances into new substances. Every substance has some intensive properties such as color, hardness, density and melting point which do not change with change in the amount of matter. Sometimes matter is changed into new physical forms and sometimes it is changed into a new substance. Some changes are temporary while some changes are permanent.

Answer to Problem 1E

When a substance is changed or converted into another substance, a chemical change occurs. This chemical change is represented by a chemical reaction.

There are some possible signs or evidence which suggest that a chemical reaction has occurred. These include:

1. Formation of new substances.
2. Production of a gas.
3. Some color changes.
4. Formation of a precipitate in solution.

Explanation of Solution

Change is a part of nature. A matter is made of elements and compounds. A matter can be changed into new substances. Such change which produces an entirely different substance is called a chemical change.

For example, when nitric acid is added to brown colored copper, a blue-colored solution is formed. This blue solution is a new compound called copper nitrate. So here a chemical reaction has taken place as a new substance is formed and there is a color change due to a reaction between copper and nitric acid.

Some possible signs prove that a chemical reaction has taken place. These are:

1. Formation of new substances - A chemical reaction always produces a new substance that has properties entirely different from the properties of the original substance. In the mentioned reaction, the properties of the new substance (copper nitrate) are different from the starting substances; copper and nitric acid.
2. Production of a gas - Some chemical reactions are accompanied by the evolution of a gas. For example, when a solid magnesium ribbon is placed in a test tube containing hydrochloric acid, bubbles of gas are formed. Hydrogen gas is released and a new substance magnesium chloride is formed. Production of a gas proves that a chemical reaction has occurred.
3. Some color-change - When nitric acid is added to brown colored copper, a blue-colored solution is formed. There is a color change during the reaction. This suggests that a chemical reaction has occurred.
4. Formation of a precipitate in solution - In some reactions, a precipitate is formed. For example, when sodium hydroxide is added to blue colored copper nitrate solution a blue solid deposits at the bottom. This is copper hydroxide which is insoluble and separates. This also proves that a chemical reaction has occurred.

Conclusion

Thus, a chemical reaction is a change in which a substance is converted into a new substance. A chemical reaction may be accompanied by the evolution of a gas, some color change, or formation of a precipitate in solution.