Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Desirable Changes: Changes that are beneficial to human beings and nature are called desirable changes. Examples include the ripening of fruits, the growth of a seedling into a tree, and the formation of curd from milk.
Undesirable Changes: Changes that are harmful to us, the environment, or our property are termed undesirable changes. Examples include the rusting of iron, the spoiling of food, and forest fires.
Context-Dependent Changes: A change can be desirable in one situation and undesirable in another. For example, rain is a desirable change for a farmer as it helps in irrigation, but it may be an undesirable change if it causes floods.
Chemical Nature of Changes: Many undesirable changes occur due to chemical reactions with the atmosphere. For instance, rusting happens when iron reacts with oxygen () and moisture () to form a layer of hydrated iron oxide.
Prevention of Undesirable Changes: We can often prevent or slow down undesirable changes. For example, food can be refrigerated to prevent the growth of bacteria, or iron can be painted to prevent contact with and .
📐Formulae
💡Examples
Problem 1:
Identify the type of change: The spoiling of cooked food left outside during summer.
Solution:
Undesirable Change.
Explanation:
The spoiling of food is caused by the growth of microorganisms which produce toxins. This makes the food unfit for consumption and is therefore harmful to human health.
Problem 2:
Is the burning of fuel () in a kitchen stove a desirable or undesirable change?
Solution:
Desirable Change.
Explanation:
The combustion of methane () provides the thermal energy required to cook food, which is a beneficial process for humans.
Problem 3:
Classify the following: A sudden earthquake occurring in a residential area.
Solution:
Undesirable Change.
Explanation:
Earthquakes cause massive destruction to life and property (), making them highly harmful and unwanted events in nature.