Half Yearly Class 7 Science Set 3 (Solutions) | Science (Curiosity) Class 7 - New NCERT PDF Download

Time: 3 Hours
Maximum Marks: 80
General Instructions:
(i) The question paper consists of 34 questions divided into four sections: A, B, C, and D.
(ii) All questions are compulsory.
(iii) Section A: Questions 1 to 15 are multiple-choice questions, carrying 1 mark each. Select the most appropriate response.
(iv) Section B: Questions 16 to 22 are short-answer questions, carrying 2 marks each.
(v) Section C: Questions 23 to 31 are short-answer questions, carrying 4 marks each.
(vi) Section D: Questions 32 to 34 are short-answer questions, carrying 5 marks each.
(vii) Answer the questions in the space provided or on a separate answer sheet as instructed.
(viii) Use of calculators is not permitted.

Section A

Q1. The first step in understanding a natural phenomenon in science is:  (1 mark)
(a) Guessing
(b) Observing
(c) Ignoring
(d) Concluding

Ans: (b)
Sol: Observing is the first step in understanding phenomena, as per The Ever-Evolving World of Science.

Q2. A substance that turns red litmus blue is:  (1 mark)
(a) Acidic
(b) Basic
(c) Neutral
(d) Salty

Ans: (b)
Sol: Basic substances turn red litmus blue, as per Exploring Substances: Acidic, Basic, and Neutral.

Q3. The component that allows or stops current flow in a circuit is:  (1 mark)
(a) Bulb
(b) Battery
(c) Switch
(d) Wire

Ans: (c)
Sol: A switch controls current flow, as per Electricity: Circuits and their Components.

Q4. Which of the following is a non-metal used in disinfectants?  (1 mark)
(a) Iron
(b) Chlorine
(c) Aluminium
(d) Copper

Ans: (b)
Sol: Chlorine is a non-metal used in disinfectants, as per The World of Metals and Non-metals.

Q5. The burning of magnesium ribbon is an example of:  (1 mark)
(a) Physical change
(b) Chemical change
(c) Reversible change
(d) Temporary change

Ans: (b)
Sol: Burning magnesium forms magnesium oxide, a chemical change, as per Changes Around Us: Physical and Chemical.

Q6. Which hormone is responsible for height increase during adolescence?  (1 mark)
(a) Estrogen
(b) Testosterone
(c) Growth hormone
(d) Adrenaline

Ans: (c)
Sol: Growth hormone causes height increase, as per Adolescence: A Stage of Growth and Change.

Q7. The reaction of an acid with a base is called:  (1 mark)
(a) Evaporation
(b) Neutralization
(c) Filtration
(d) Sedimentation

Ans: (b)
Sol: Neutralization is the reaction between an acid and a base, as per Exploring Substances: Acidic, Basic, and Neutral.

Q8. Which component in a circuit produces heat when current flows?  (1 mark)
(a) Switch
(b) Fuse
(c) Heating element
(d) Battery

Ans: (c)
Sol: A heating element produces heat, as per Electricity: Circuits and their Components.

Q9. Which non-metal is used in making pencil leads?  (1 mark)
(a) Sulphur
(b) Carbon (graphite)
(c) Nitrogen
(d) Phosphorus

Ans: (b)
Sol: Graphite is used in pencil leads, as per The World of Metals and Non-metals.

Q10. The process of obtaining pure salt from seawater is an example of:  (1 mark)
(a) Filtration
(b) Evaporation
(c) Distillation
(d) Sedimentation

Ans: (b)
Sol: Evaporation obtains salt from seawater, a physical change, as per Changes Around Us: Physical and Chemical.

Q11. Which of the following is a reversible change?  (1 mark)
(a) Burning of paper
(b) Melting of butter
(c) Cooking of rice
(d) Rusting of iron

Ans: (b)
Sol: Melting butter is a reversible physical change, as per Changes Around Us: Physical and Chemical.

Q12. The development of breasts in girls during adolescence is due to:  (1 mark)
(a) Testosterone
(b) Estrogen
(c) Adrenaline
(d) Growth hormone

Ans: (b)
Sol: Estrogen causes breast development, as per Adolescence: A Stage of Growth and Change.

Q13. Which material is a good conductor of heat?  (1 mark)
(a) Wood
(b) Plastic
(c) Aluminium
(d) Glass

Ans: (c)
Sol: Aluminium conducts heat, as per The World of Metals and Non-metals.

Q14. Which indicator turns pink in a basic solution?  (1 mark)
(a) Litmus
(b) Phenolphthalein
(c) Turmeric
(d) China rose

Ans: (b)
Sol: Phenolphthalein turns pink in basic solutions, as per Exploring Substances: Acidic, Basic, and Neutral.

Q15. Which of the following is a chemical change? (1 mark)
(a) Evaporation of water
(b) Rusting of iron
(c) Cutting of vegetables
(d) Melting of wax

Ans: (b)
Sol: Rusting forms iron oxide, a chemical change, as per Changes Around Us: Physical and Chemical.

Section B

Q16. What role does questioning play in a scientific investigation?  (2 marks)

Ans: Questioning identifies problems and guides observations. 
Example: Why does fruit ripen faster in a closed bag?

Q17. Name two basic substances used at home and their applications.  (2 marks)

Ans: 

• Soap: Used for cleaning dishes.
• Baking soda: Used in baking and odor removal.

Q18. Draw a circuit diagram with a battery, two bulbs, and a fuse in series.  (2 marks)

Ans: Draw a battery, two bulbs, a fuse, and connecting wires in a single loop.

Q19. State one property of a metal and one property of a non-metal.  (2 marks)

Ans: 

• Metal: Malleable, e.g., aluminium.
• Non-metal: Brittle, e.g., sulphur.

Q20. Give one example each of a physical change and an irreversible change.  (2 marks)

Ans: 

• Physical change: Boiling water.
• Irreversible change: Burning wood.

Q21. Name two physical changes observed during adolescence in boys.  (2 marks)

Ans: 

• Growth of facial hair
• Increase in shoulder width

Q22. Why are non-metals generally poor conductors of electricity?  (2 marks)

Ans: Non-metals lack free electrons, unlike metals, which have free electrons to carry current.

Section C

Q23. Describe how to test a substance using phenolphthalein as an indicator.  (4 marks)

Ans: 

• Add a few drops of phenolphthalein to the substance.
• Observe: Colorless in acidic or neutral solutions.
• Pink in basic solutions.
• Record the result to identify the substance’s nature.

Q24. Explain the role of a fuse and a wire in an electric circuit with a diagram.  (4 marks)

Ans: 

• Fuse: Breaks the circuit if current is too high, preventing damage.
• Wire: Conducts electricity between components.
• Diagram: Show a battery, bulb, fuse, and wires in a circuit.

Q25. List two physical and two chemical properties of metals with examples.  (4 marks)

Ans: 
Physical properties:

• Lustrous: E.g., gold shines.
• Malleable: E.g., aluminium can be beaten into sheets.

Chemical properties:

• Form basic oxides: E.g., copper forms CuO.
• React with acids: E.g., magnesium produces hydrogen with HCl.

Q26. Explain the chemical change that occurs during the rusting of iron.  (4 marks)

Ans: Rusting is a chemical change where iron reacts with oxygen and moisture to form iron oxide (rust). New substances are formed, and the process is irreversible, weakening the iron.

Q27. Describe three physical changes and one emotional change during adolescence in boys.  (4 marks)

Ans: 
Physical changes:

• Deepening of voice
• Growth of pubic hair
• Increase in height

Emotional change: Increased sensitivity to peer opinions.

Q28. What is a neutralization reaction? Write the chemical equation for the reaction between hydrochloric acid and calcium hydroxide.  (4 marks)

Ans: Neutralization: A reaction between an acid and a base forming salt and water.
Equation: 2HCl + Ca(OH)₂ → CaCl₂ + 2H₂O

Q29. Explain why parallel circuits are preferred in household wiring with diagrams.  (4 marks)

Ans: 
Advantages:

• Each appliance works independently; if one fails, others function.
• Appliances receive full voltage, ensuring efficiency.

Diagrams:

• Series: Battery → Bulb 1 → Bulb 2 → Battery.
• Parallel: Battery with two branches, each with a bulb.

Q30. Why are metals used to make wires? Give two examples of metals and their uses in wiring.  (4 marks)

Ans: Importance: Metals are good conductors of electricity and ductile, allowing them to be drawn into wires.
Examples:

• Copper: Used in household wiring.
• Aluminium: Used in power lines.

Q31. Describe an activity to demonstrate a chemical change by burning a piece of paper.  (4 marks)

Ans: 

• Take a piece of paper and light it with a matchstick.
• Observe: Paper burns, producing ash, smoke, and heat.
• Explanation: Burning forms new substances (ash, carbon dioxide), indicating a chemical change.
• Note: The process is irreversible.

Section D

Q32. (a) Design an activity to observe the effect of air on the burning of a candle.  (3 marks)
(b) Identify the physical and chemical changes in the activity.  (2 marks)

Ans: (a) Activity: Light a candle and cover it with a glass jar. Observe the flame extinguishing after some time due to lack of air. Remove the jar, relight, and compare. 
(b) Changes:

• Physical: Wax melting (reversible, state change).
• Chemical: Wax burning to produce carbon dioxide and water (irreversible, new substances).

Q33. (a) Describe an activity to demonstrate a neutralization reaction using lemon juice and baking soda.  (3 marks)
(b) Explain how neutralization can be used to treat soil acidity.  (2 marks)

Ans: (a) Activity: Mix lemon juice (acid) with baking soda (base) in a container. Observe bubbles of carbon dioxide and test the resulting solution with litmus (neutral).
(b) Application: Neutralization treats acidic soil by adding lime (calcium hydroxide), which reacts with soil acids to form neutral salts, improving soil fertility. 

Q34. (a) Explain the importance of balanced diet and personal hygiene during adolescence.  (2 marks)
(b) Describe an activity to show the effect of a balanced diet on physical growth.  (3 marks)

Ans: (a) Importance: A balanced diet provides nutrients for growth (e.g., height, muscle development), and personal hygiene prevents infections due to increased sweating.
(b) Activity: Compare the height and weight of two groups of students—one following a balanced diet, one not—over a month. Record and compare growth differences.