Question

Define physical change and chemical change. State which type of change occurs in each of the following statements: (a) Passing an electric current through molten magnesium chloride yields molten magnesium and gaseous chlorine. (b) The iron in discarded automobiles slowly forms reddish brown, crumbly rust.

Short answer

A physical change alters the form, not the substance. A chemical change forms new substances. Both statements (a) and (b) describe chemical changes.

Step-by-step solution

- Definition of Physical Change

A physical change is a type of change in which the form of matter is altered, but its identity remains the same. Examples include changes in state, shape, or size without altering the substance's chemical composition.

- Definition of Chemical Change

A chemical change is a change that results in the formation of new chemical substances. During a chemical change, atoms are rearranged, and chemical bonds are broken or formed, resulting in substances with different properties.

- Analyze Statement (a)

Passing an electric current through molten magnesium chloride yields molten magnesium and gaseous chlorine. This process produces new substances (magnesium and chlorine), indicating a chemical change.

- Analyze Statement (b)

The iron in discarded automobiles slowly forms reddish brown, crumbly rust. Rust formation is a chemical reaction between iron and oxygen, resulting in a new substance (iron oxide), indicating a chemical change.

Key concepts

Physical Change

A physical change involves a transformation in the appearance or state of a substance without changing its chemical composition. For instance, melting ice, tearing paper, or dissolving sugar in water are all physical changes. In these scenarios, the substance itself remains the same even though it undergoes a visible transformation. Physical changes are usually reversible since no new substances are created.

Chemical Change

A chemical change results in the formation of one or more new substances with new chemical properties. This happens because the molecules of the original substances are restructured to form new compounds. Examples include burning wood, rusting iron, or digesting food. Chemical changes are often irreversible under normal conditions. During a chemical change, you might notice evidence such as color change, gas production, temperature change, or the formation of a precipitate.

Matter Transformation

Matter transformation involves the process of change that matter undergoes. There are two primary types: physical and chemical transformations. Physical transformations, like freezing or boiling, affect the form or state of matter without altering its composition. Chemical transformations, on the other hand, involve a change in the molecular structure of the matter, resulting in new substances.

Understanding the type of transformation helps in predicting the behavior and properties of substances during different processes.

Chemical Reactions

Chemical reactions are processes where substances, called reactants, are converted into different substances, known as products. This involves breaking and forming chemical bonds. For example, in the reaction where magnesium chloride is decomposed into magnesium and chlorine gas, a chemical reaction is responsible. Similarly, the rusting of iron in automobiles, where iron reacts with oxygen to form iron oxide, is another classic example.

• Reactions often require certain conditions, such as heat, light, or catalysts.
• They can be represented by chemical equations that show the reactants and products.
• Understanding chemical reactions is fundamental in fields like chemistry, biology, and environmental science.