Chapter 2, Problem 89A

Interpretation Introduction

Interpretation: Three chemical changes helpful and harmful each to human or the environment needs to be stated.

Concept Introduction: When a substance changes its appearance or change in the state takes place and there is no change in its chemical composition, the change is said to be physical. On the other hand, when there is a change in the chemical composition of a substance or a new substance is formed after a chemical reaction, it is known as a chemical change.

Explanation of Solution

In a chemical change, a new substance is formed and a chemical reaction takes place. These changes are irreversible in nature.

Three chemical changes helpful to humans or the environment are as follows:

1. Digestion of food: Food is digested in the body through several chemical reactions. In digestion, the breakdown of food takes place into glucose, and energy is also released. This reaction is known as hydrolysis.
2. Photosynthesis: the process by which plants make their own food. In this process, plants consume carbon dioxide and water from air and soil to form glucose and oxygen. The overall reaction is represented as follows:

${\text{6CO}}_{\text{2}}+{\text{6H}}_{\text{2}}\text{O}\to {\text{C}}_{\text{6}}{\text{H}}_{\text{12}}{\text{O}}_{\text{6}}+{\text{6O}}_{\text{2}}$

3. Cooking food: It is a chemical change because raw ingredients are cooked by heating. It is a reversible reaction.

Three chemical changes harmful to humans or the environment are as follows:

1. Rusting of iron: When the iron surface is exposed to air. The moisture in the air reacts with iron to form iron oxide. Here, oxidation of iron takes place and iron oxide so formed is known as rust, and the process is known as rusting of iron.
2. Burning of coal: Harmful and acidic gases are produced when coal is burned in the environment. It is a chemical change because when coal is burned, carbon dioxide gas is released.
3. Formation of acid rain: when harmful gases such as sulfur dioxide and nitrogen dioxide react with water in the atmosphere, various acidic compounds are formed. This results in acid rain.