• Change is the law of nature. 

  • We observe various types of changes around us. 

  • The growth of seed in to a plant,

  • The burning of wood in a fireplace, 

  • The rusting of iron articles in moist air, 

  • The rotting of food and 

  • The evaporation of liquids are some of the changes that we see taking place around us.

  Scientists categorize changes as 

  • physical and 

  • chemical changes. 

A physical change is a change that does not involve the formation of a new substance with  a new   chemical composition.

Examples of physical changes are:- 

  • evaporation of liquids, 

  • powdering of sugar and

  •  melting of ice. 

Chemical changes occur due to chemical reactions between substances.

  • In general, a change is called a chemical reaction if it exhibits all or part of the following characteristics.

i. Formation of new substances

ii. Production of heat or light or both

iii. Change in color

iv. Change in temperature and etc.


  • Why are the growth of plants, ripening of fruits, photosynthesis of plants, and combustion of glucose in the metabolic process are considered chemical changes?

Solution

Because in all there are formation of new substance with New composition. 

  6CO₂ + 6H₂O light → C₆H₁₂O₆ + 6O₂

  • Is the process in which reacting substances, called reactants, are converted into new substances, called products. 

  • The characteristics of the products are completely different from those of the reactants.

  • For example, if you burn sulphur with oxygen, the sulfur and oxygen are completely converted to sulphur dioxide

                                       

  • Sulphur dioxide is a colorless gas with a pungent odour.

  • A chemical reaction is represented by a shorthand notation called chemical equation as:


                                       

  • A chemical equation uses chemical symbols to show what happens during a chemical reaction. 

  • A balanced chemical equation can be used to describe the relationships between the amounts of reactants and products. 

  • The quantitative study of reactants and products in a chemical reaction is called reaction stoichiometry. 

A. What is a chemical reaction?

  • Chemical reaction is the process in which reacting substances, called reactants, are converted into new substances, called products.

B. Explain the meanings of chemical and physical changes using examples.

Physical change is a change in which the substance doesn’t lose identity and no new substance is formed.

Examples:

  • Evaporation of a liquid

  • Powdering of sugar,

  • Dissolution of salt.

Chemical change: is the change in which the identity of the original

substance is changed and a new substance is formed.

Examples:

  • Ripening of fruits,

  • growth of plants,

  • souring of milk,

  • digestion of food.

C. Give  two examples of  evidences for chemical reactions not mentioned in the text.

Color change

  • Example when iron rust it change from metallic gray to reddish-brown due to the formation of iron oxide.

Gas production:

  • The formation of gas bubbles is a clear sign of a chemical reaction

Last modified: Roobii, 1 Onkoloolessa 2025, 8:41 AM