Class 7 Science Chapter 5 Question Answers - Physical and Chemical Changes

Q1: What is rust?

Ans : When iron oxide is hydrated, it is known as Rust(Fe₂O₃. XH₂O)

Q2: What is added to steel to make it stainless?

Ans:  Stainless steel is made by mixing iron with carbon and metals like chromium, nickel and manganese.

Q3: What type of change is photosynthesis?

Ans : Chemical change

Q4: What is a chemical change?

Ans : A change, in which a new substance with different properties is formed, is known as a chemical change.

Q5: A change, in which a new substance with different properties is formed, is known as a _________________.

Ans : Chemical change

Q6: ___________is a property by virtue of which a metal can be beaten into thin sheets without breaking.

Ans : Malleability

Q7: Write the chemical formula of blue vitriol.

Ans : Crystals of copper sulphate pent hydrate (CuSO4.5H₂O) are blue in colour. So, it is commonly known as blue vitriol.

Q8: State the two important conditions for rusting.

Ans : Presence of water and presence of oxygen

Q9: What is freezing point of water?

Ans : The freezing point is 0°C at which water freezes.

Q10: What is freezing of water?

Ans : Freezing is the process in which water turns to ice when cold enough (below 0°C temperature).

Q11: What is melting?

Ans : Melting is the process of turning a solid to a liquid.

Q12: Name the process by which water molecules continuously change to vapour.

Ans : Evaporation

Q13: At what temperature water evaporates?

Ans : 100 °C

Q14: Extraction of metal is a chemical process or physical process?

Ans : Chemical process

Q15: A medicine is the end product of the chain of ________________________.

Ans : Chemical reaction

Q16: Condensation of steam is chemical process or physical change?

Ans : physical change

Q17: Cutting of log of wood into pieces is a chemical process or physical change?

Ans : physical change

Q18: Name the two methods by which rusting of iron can be prevented.

Ans : By painting or greasing and by galvanizing with chromium or zinc

Q19: Give an example of chemical change in which heat, light, sound and unpleasant gases are produced.

Ans : Explosion of a firework

Q20: Define galvanisation.

Ans : The process of depositing a layer of zinc on iron is called galvanisation.