Test: Changes Around Us: Physical and Chemical - Class 7 MCQ

Combustion, or fire, needs three things: fuel (something to burn), oxygen (from the air), and heat (to start the fire). If any one is missing, the fire can’t start.

When carbon dioxide mixes with lime water, it forms calcium carbonate, making the water look cloudy or milky. It’s a simple chemical change.

Irreversible changes permanently change the substance, like burning paper or cooking food, which cannot go back to their original form.

• Definition: Ignition temperature is the minimum temperature required for a substance to ignite and start burning.
• Importance: Understanding ignition temperature helps in fire safety, determining how easily materials can catch fire.
• Examples: Different materials have different ignition temperatures; for instance, paper ignites at a lower temperature than gasoline.
• Applications: Knowledge of ignition temperature is crucial in industries for handling and storing flammable substances safely.

A reversible change is one where the substance can go back to its original form, like ice melting and then freezing again.

In a chemical change, new substances are formed. For example, burning wood creates ash and gases, which are different from the original wood.

In a physical change, the appearance changes (like ice turning to water) but the substance stays the same.

Oxygen is crucial for fire because it supports the burning process. Without oxygen, the fire goes out.

Burning wood is a chemical change because it turns into new substances, like ash, which can’t be turned back into wood.

Knowing about physical and chemical changes helps us understand what happens during daily activities like cooking or cleaning, making sure we stay safe.

Rusting is a chemical change where iron reacts with air and moisture, weakening the metal and causing damage over time.

When a candle burns, physical changes (like melting wax) and chemical changes (like creating new gases) happen at the same time.

Cooking food is a desirable change because it makes food tastier, safer to eat, and easier to digest.

Baking bread is a chemical change because the ingredients react to form new substances, such as carbon dioxide gas, which makes the bread rise.

Rusting is a chemical change where iron reacts with oxygen and moisture to form iron oxide, or rust.

Erosion is when rocks and soil are broken down and carried away by forces like wind or water.

Some changes can be good or bad depending on the situation. For example, food spoilage is bad, but it can be good for composting.

Chopping vegetables is irreversible because once they are cut, they cannot go back to their original whole form.

Water changes from solid (ice) to liquid (water) to gas (steam) by absorbing heat. These are physical changes, not chemical ones.