Test: Physical and Chemical Changes - Class 7 MCQ

Sublimation is the process where a solid changes directly into vapor without passing through the liquid state, as seen with substances like dry ice and iodine. This unique transition highlights the diverse behaviors of matter under varying conditions.

A chemical change is characterized by the formation of one or more new substances with different properties from the original materials. This transformation often involves breaking and forming chemical bonds, making it distinct from physical changes.

The growth of plants is considered a desirable change as it contributes positively to the environment and supports ecosystems. In contrast, pollution and rusting have harmful effects, while burning fossil fuels can lead to undesirable environmental changes.

Rusting of iron is a chemical change because it involves a reaction between iron and oxygen in the presence of moisture to form rust, a new substance. In contrast, melting or boiling water are physical changes where no new substances are formed.

Sublimation is exemplified by iodine crystals turning directly into vapor when heated, bypassing the liquid phase. This unique phase transition is important in various applications, including freeze-drying and certain purification processes.

The change of seasons is an example of a natural change that occurs without human intervention. It results from Earth's axial tilt and orbit around the sun, leading to predictable and cyclical transformations in weather and ecology.

Both physical and chemical changes can be classified as reversible or irreversible. For example, water freezing is reversible, while cooking food is generally irreversible. Understanding this classification helps in analyzing various processes in nature and industry.

Exothermic changes release heat into the environment, such as combustion, while endothermic changes absorb heat, like the melting of ice. Understanding this difference is crucial in thermodynamics and various chemical reactions.

During condensation, a gas transitions into a liquid state when cooled. This process is commonly observed when water vapor in the air forms droplets on a cold surface, such as a glass of ice water.

Baking a cake is an irreversible change because the ingredients undergo a chemical reaction to create a new substance that cannot revert back to its original components. In contrast, melting snow or freezing water are reversible physical changes.

Desirable changes are those that have positive outcomes and are beneficial, such as the growth of plants or the production of food. These changes contribute positively to our environment and daily lives, distinguishing them from undesirable changes.

A physical change is characterized by the fact that no new substance is formed and the chemical composition remains the same. Examples include melting ice or dissolving sugar in water, where the original substances retain their properties.

Boiling is the process where a liquid turns into vapor at a specific temperature. This occurs when the liquid reaches its boiling point, allowing molecules to gain sufficient energy to transition into the gaseous state.

A key indicator of a chemical change is the formation of one or more new substances with different properties from the original materials. This is often accompanied by other signs such as color change, gas production, or heat release.

Endothermic changes absorb heat from their surroundings. An example is the melting of ice, where heat is taken in, allowing the solid to transition into a liquid state. This absorption of heat is critical in various physical and chemical processes.