Class Notes NCERT Solutions for CBSE Students
Question: What are slow and fast changes? Give examples.
Ans: The changes which take place in a long period of time are called slow changes whereas that changes which take place in a short period of time are called fast changes.
Question: What is a physical change? Explain with example.
Answer: The changes in which only physical properties of substances are changed and no new substance is formed is called physical change. It is a reversible change. Example: Boiling and freezing of water.
Question: What is a chemical change? Explain with example.
Answer: The changes in which new substance with new chemical properties are formed are called chemical changes. Example: Reaction between washing soda and lemon juice in which C02 and other substances are formed.
Question: What happens when sugar is heated?
Answer: When sugar is heated continuously then a black powdery substance is formed. This is a chemical change.
Question: Explain how a metal rim slightly smaller than a wooden wheel can be fixed on it.
Answer: The metal rim is always made slightly smaller than the wooden wheel. The metal rim is heated. On heating, the rim expands and fit onto the wheel. Cold water is then poured over the rim. Due to cooling the metal rim contracts and fits tightly onto the wheel.
Question: How does curd being set? Is this change reversible?
Answer: A small quantity of curd is added to warm milk. The milk is stirred and is set aside undisturbed for a few hours at a warm place. In a few hours, the milk changes into curd.
Curd formed from milk cannot be changed into milk again. So, this an irreversible (cannot be reversed) change.
