Half Yearly Class 7 Science Set 2 (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 process of carefully watching natural phenomena in science is called:  (1 mark)
(a) Guessing
(b) Observation
(c) Prediction
(d) Assumption

Ans: (b)
Sol: Observation involves carefully watching natural phenomena, as per The Ever-Evolving World of Science.

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

Ans: (a)
Sol: Turmeric turns red in basic substances, as per Exploring Substances: Acidic, Basic, and Neutral.

Q3. Which component in a circuit stores electrical energy? (1 mark)
(a) Bulb
(b) Switch
(c) Battery
(d) Wire

Ans: (c)
Sol: A battery stores and provides electrical energy, as per Electricity: Circuits and their Components.

Q4. Which of the following is a metal used in electrical wiring? (1 mark)
(a) Carbon
(b) Sulphur
(c) Copper
(d) Phosphorus

Ans: (c)
Sol: Copper is a metal used in wiring due to its conductivity, as per The World of Metals and Non-metals.

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

Ans: (b)
Sol: Food spoilage forms new substances, a chemical change, as per Changes Around Us: Physical and Chemical.

Q6. Which hormone causes the growth of facial hair in boys during adolescence? (1 mark)
(a) Estrogen
(b) Testosterone
(c) Adrenaline
(d) Insulin

Ans: (b)
Sol: Testosterone causes facial hair growth in boys, as per Adolescence: A Stage of Growth and Change.

Q7. The product of a neutralization reaction includes: (1 mark)
(a) Sugar and water
(b) Salt and water
(c) Acid and base
(d) Oxygen and salt

Ans: (b)
Sol: Neutralization produces salt and water, as per Exploring Substances: Acidic, Basic, and Neutral.

Q8. The component that breaks a circuit if current is too high is: (1 mark)
(a) Bulb
(b) Fuse
(c) Wire
(d) Switch

Ans: (b)
Sol: A fuse breaks the circuit to prevent damage, as per Electricity: Circuits and their Components.

Q9. Which non-metal is used in making fertilizers? (1 mark)
(a) Oxygen
(b) Nitrogen
(c) Chlorine
(d) Sulphur

Ans: (b)
Sol: Nitrogen is used in fertilizers, 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 is used to obtain salt from seawater, a physical change, as per Changes Around Us: Physical and Chemical.

Q11. Which of the following is a physical change? (1 mark)
(a) Burning of charcoal
(b) Freezing of water
(c) Curdling of milk
(d) Rusting of iron

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

Q12. The increase in muscle mass in boys during adolescence is due to: (1 mark)
(a) Estrogen
(b) Testosterone
(c) Growth hormone
(d) Adrenaline

Ans: (b)
Sol: Testosterone increases muscle mass, as per Adolescence: A Stage of Growth and Change.

Q13. Which material is used to insulate electric wires? (1 mark)
(a) Copper
(b) Aluminium
(c) Plastic
(d) Iron

Ans: (c)
Sol: Plastic is an insulator for wires, as per Electricity: Circuits and their Components.

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

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

Q15. Which of the following is a chemical change? (1 mark)
(a) Dissolving sugar in water
(b) Burning of kerosene
(c) Boiling of water
(d) Cutting of paper

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

Section B

Q16. What is a scientific investigation? Give one example from daily life.  (2 marks)

Ans: A scientific investigation involves observing, questioning, and testing to understand phenomena. 
Example: Observing why plants grow better with fertilizer.

Q17. Name two acidic substances used in cooking and their sources.  (2 marks)

Ans: 

• Lemon juice: From lemons, used in salads.
• Vinegar: From fermented fruits, used in pickling.

Q18. Draw a circuit diagram showing a battery, a bulb, and a switch in a closed circuit.  (2 marks)

Ans: Draw a battery, bulb, switch, and connecting wires.

Q19. State one use of a metal and one use of a non-metal in daily life.  (2 marks)

Ans: 

• Metal: Copper – used in electrical wires.
• Non-metal: Oxygen – used for breathing.

Q20. Differentiate between a physical change and a chemical change with one example each.  (2 marks)

Ans: 

• Physical change: No new substances formed, reversible. 
  Example: Melting ice.
• Chemical change: New substances formed, irreversible. 
  Example: Cooking food.

Q21. Name two secondary sexual characteristics observed in girls during adolescence.  (2 marks)

Ans: 

• Development of breasts
• Growth of pubic hair

Q22. Why do metals conduct electricity better than non-metals?  (2 marks)

Ans: Metals conduct electricity better because they have free electrons that carry current, while non-metals lack free electrons.

Section C

Q23. Explain how to use a China rose indicator to identify acidic and basic substances.  (4 marks)

Ans: 

• Soak China rose petals in water to prepare the indicator.
• Add a few drops to the substance.
• Dark pink color indicates an acidic substance.
• Green color indicates a basic substance.

Q24. Describe the role of a battery and a bulb in an electric circuit with a labelled diagram.  (4 marks)

Ans: 

• Battery: Provides electrical energy to drive the circuit.
• Bulb: Converts electrical energy into light.
• Diagram: Show a battery, bulb, switch, and wires in a closed circuit.

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

Ans: 

Physical properties:

• Dull appearance: E.g., sulphur is non-lustrous.
• Poor conductor of electricity: E.g., carbon (except graphite).

Chemical properties:

• Form acidic oxides: E.g., sulphur forms SO₂.
• Non-reactive with acids: E.g., nitrogen does not react with HCl.

Q26. Explain the chemical change that occurs when food is cooked.  (4 marks)

Ans: Cooking food involves chemical changes where heat causes reactions, forming new substances (e.g., proteins denature, starches break down). The process is irreversible, and the taste, texture, and composition change.

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

Ans: 
Physical changes:

• Breast development
• Widening of hips
• Growth of underarm hair

Emotional change: Mood swings due to hormonal changes.

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

Ans: Neutralization: A reaction between an acid and a base to form salt and water.
Equation: CH₃COOH + NaOH → CH₃COONa + H₂O

Q29. Explain the difference between a series and a parallel circuit with diagrams.  (4 marks)

Ans: 

• Series circuit: Components are connected in one path; current flows sequentially.
• Parallel circuit: Components are in multiple paths; current splits.

Diagrams:

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

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

Ans: Importance: Metals are durable, good heat conductors, and can be shaped easily.
Examples:

• Stainless steel: Used in cooking pots.
• Aluminium: Used in frying pans.

Q31. Describe an activity to demonstrate a physical change by dissolving sugar in water.  (4 marks)

Ans: 

• Take a glass of water and add a teaspoon of sugar.
• Stir until the sugar disappears.
• Observe: Sugar dissolves, forming a solution without forming a new substance.
  Explanation: This is a physical change as sugar can be recovered by evaporating water.

Section D

Q32. (a) Describe an activity to test whether a substance conducts electricity.  (3 marks)
(b) Explain why some materials conduct electricity while others do not.  (2 marks)

Ans: (a) Activity: Connect a substance (e.g., copper, plastic) in a circuit with a battery, bulb, and switch. Close the switch and observe if the bulb glows. If it glows, the substance conducts electricity.
(b) Explanation: Conductors (e.g., metals) have free electrons that allow current flow. Insulators (e.g., plastic) lack free electrons, preventing current flow. 

Q33. (a) Explain how hormones affect physical changes during adolescence.  (2 marks)
(b) Describe an activity to demonstrate a physical change using ice and water.  (3 marks)

Ans: (a) Hormones: Growth hormone causes height increase, testosterone leads to facial hair and voice changes in boys, and estrogen causes breast development and hip widening in girls.
(b) Activity: Place ice in a container, heat it gently, and observe it melting into water. This is a physical change as only the state changes, reversible by freezing.

Q34. (a) Design an activity to compare the reaction of an acid with a metal and a non-metal.  (3 marks)
(b) Explain why the reaction differs between the two.  (2 marks)

Ans: (a) Activity: Add dilute hydrochloric acid to zinc (metal) and sulphur (non-metal) in separate test tubes. Observe: Zinc produces hydrogen gas (bubbles); sulphur shows no reaction.
(b) Explanation: Metals react with acids to form hydrogen gas due to their ability to lose electrons. Non-metals lack this property and do not react with acids.