CBSE Class 7 Science — NCERT Solutions

Given: The answer is 'Just add some milk.'

This is an open-ended creative activity. There is no single correct question. The idea is to think imaginatively about situations where this answer would make sense.

Some possible questions:

(i) 'My porridge has become too thick and lumpy — what should I do?'

(ii) 'How can I make this cup of tea less bitter and more creamy?'

(iii) 'The mashed potatoes are too dry and stiff — how do I fix them?'

(iv) 'My little sister refuses to drink her Horlicks because it tastes too strong — what can I do?'

Note: Any creative, thoughtful question that logically leads to the answer 'Just add some milk' is perfectly acceptable. The purpose of this activity is to practise thinking like a scientist — by asking good questions rather than only finding answers.

Given: The answer is 'Because the cat's teeth were crooked.'

This is an open-ended creative activity. There is no single correct question. Think of funny, imaginative, or even absurd situations where this answer would fit.

Some possible questions:

(i) 'Why did the cat's bite leave such a strange zigzag mark on the butter?'

(ii) 'Why did the dentist open a clinic especially for cats in our neighbourhood?'

(iii) 'Why did the cat keep dropping the fish it caught from the pond?'

(iv) 'Why did the vet say the cat needed braces?'

Note: Any imaginative question that logically or humorously leads to the answer 'Because the cat's teeth were crooked' is perfectly valid. The goal is to practise creative and scientific thinking.

Correct Option: (iii) Vinegar

Given: A solution turns red litmus to blue, which means the solution is basic in nature.

Concept: A basic solution turns red litmus blue. To reverse this change (i.e., turn the blue litmus back to red), we need to add an acidic substance, because acids neutralise bases.

Reasoning:

• Lime water → basic (would not reverse the change)
• Baking soda → basic (would not reverse the change)
• Vinegar → acidic (would neutralise the base and turn the solution acidic, reversing the blue colour back to red)
• Common salt solution → neutral (would not reverse the change)

Conclusion: Excess addition of vinegar (an acid) would neutralise the basic solution and reverse the colour change of litmus from blue back to red.

Correct Option: (iii) Basic, basic, and acidic

Given:

• Solution A turns red litmus solution to blue.
• Solution B turns turmeric solution to red (reddish-brown).
• Solution C turns red rose extract to green.

Concept and Analysis:

Re-checking Solution C: Red rose extract turns green in a basic solution and red in an acidic solution. So solution C turning the extract green means C is basic.

Re-evaluating all options: A = Basic, B = Basic, C = Basic → Option (iv).

Wait — careful re-reading: The question states red rose extract turns green for solution C. From the chapter: red rose extract gives red colour in acidic solution and green colour in basic solution. Therefore C is basic.

So the sequence is: Basic, Basic, Basic → Option (iv) Basic, basic, and basic.

Correct Option: (iv) Basic, basic, and basic

• Solution A: Red litmus turns blue → Basic
• Solution B: Turmeric turns red/reddish-brown → Basic
• Solution C: Red rose extract turns green → Basic

Correct Answer: (i) A switch is the source of electric current in a circuit.

Justification: A switch is NOT a source of electric current. The source of electric current in a circuit is the electric cell or battery. A switch is simply a device that either completes (closes) or breaks (opens) the circuit, thereby controlling the flow of current. Statements (ii), (iii), and (iv) are all correct descriptions of a switch.

Given: A circuit with a lamp, a cell, and two free ends A and B where different materials can be connected.

Concept Used: Materials that do not allow electric current to pass through them are called insulators. When an insulator is connected between A and B, the circuit remains open (broken) and the lamp does not glow.

Answer: The lamp will not glow when an insulating material is connected between ends A and B.

Examples of such materials include: rubber, plastic, glass, wood, paper, wax (candle), cork, and eraser.

Reason: These materials are poor conductors (insulators) of electricity. They do not allow electric current to flow through them, so the circuit remains incomplete and the lamp does not glow.

(Note: The specific options in Fig. 3.16 are not fully visible in the OCR, but the answer is whichever option is an insulating material such as rubber, plastic, or wood.)

Correct Option: (i) Aluminium

Justification: Aluminium is cheap, lightweight, and highly malleable — meaning it can be beaten into very thin sheets (aluminium foil) that can be folded easily into any shape. It is therefore widely used for food packaging materials such as foil wraps and containers.

Given: A list of changes to observe and record.

Concept: Changes can be observed through our senses — sight, smell, touch, hearing, and taste. They may involve changes in size, shape, smell, colour, state, or other properties.

Other changes in surroundings: Rusting of iron, curdling of milk, germination of seeds, dissolving salt in water, melting of butter, etc.

Given: Ramesh is 11 years old and has developed pimples on his face. His mother attributes this to biological changes in his body.

(i) Possible reasons for pimples:

During adolescence, the body undergoes hormonal changes. These hormones stimulate the oil (sebaceous) glands in the skin to produce more oil (sebum). When excess oil, dead skin cells, and bacteria block the pores of the skin, pimples (acne) develop. Since Ramesh is 11 years old, he is entering adolescence, and the increased hormonal activity is the primary reason for the pimples on his face.

(ii) Ways to get relief from pimples:

• Wash the face regularly with a mild soap or face wash at least twice a day to remove excess oil and dirt.
• Avoid touching or squeezing the pimples, as this can spread bacteria and cause more pimples or scarring.
• Drink plenty of water to keep the skin hydrated and flush out toxins.
• Eat a balanced diet rich in fruits, vegetables, and whole grains; avoid oily and junk food.
• Keep hair clean and away from the face, as oily hair can worsen pimples.
• Consult a doctor if pimples are severe or causing discomfort, as a doctor may prescribe appropriate medication.

Conclusion: Pimples during adolescence are a normal part of growing up caused by hormonal changes. Good personal hygiene and a healthy lifestyle can provide significant relief.

Correct Option: (c) A is a good conductor and B is a poor conductor of heat

Justification: In a saucepan, the body (A) is made of metal (such as steel or aluminium) which is a good conductor of heat so that heat passes easily from the flame to the food. The handle (B) is made of a material like plastic or wood which is a poor conductor (insulator) of heat so that the person holding the handle does not get burnt.

Given:

• Distance = 150 m
• Time = 10 s

Formula:
$$\text{Speed} = \frac{\text{Distance}}{\text{Time}}$$

Step 1: Find speed in m/s
$$\text{Speed} = \frac{150 \text{ m}}{10 \text{ s}} = 15 \text{ m/s}$$

Step 2: Convert m/s to km/h
$$1 \text{ m/s} = \frac{3600}{1000} \text{ km/h} = 3.6 \text{ km/h}$$
$$\text{Speed} = 15 \times 3.6 = 54 \text{ km/h}$$

Answer: The speed of the car is $\mathbf{54 \text{ km/h}}$.

Given: The alimentary canal is the pathway through which food travels during digestion.

Concept: The alimentary canal consists of the mouth, oesophagus, stomach, small intestine, large intestine, and anus, in that order.

Answer:

$$\text{Food} \rightarrow \text{Mouth} \rightarrow \textbf{Oesophagus} \rightarrow \text{Stomach} \rightarrow \textbf{Small Intestine} \rightarrow \textbf{Large Intestine} \rightarrow \text{Anus}$$

Explanation of each part:

• Oesophagus: A muscular tube that carries food from the mouth to the stomach.
• Small Intestine: Where most of the digestion and absorption of nutrients takes place.
• Large Intestine: Absorbs most of the remaining water and some salts from undigested food before it is expelled.

Given: A list of objects — Mars, Moon, Pole Star, Sun, Venus, Mirror.

Concept: Luminous objects are those that emit their own light.

Working:

• Mars — a planet; it only reflects sunlight. Not luminous.
• Moon — a natural satellite; it only reflects sunlight. Not luminous.
• Pole Star — a star; it produces its own light by nuclear fusion. Luminous.
• Sun — a star; it produces its own light. Luminous.
• Venus — a planet; it only reflects sunlight. Not luminous.
• Mirror — reflects light falling on it. Not luminous.

Answer: The luminous objects from the given list are Pole Star and Sun.

Given: Fig. 12.17 shows the Earth with sunlight falling on it during a particular season (based on the chapter context, this refers to a solstice position where one pole is tilted toward the Sun and the other away).

Concept: The Earth completes one rotation in 24 hours. During a solstice, one pole is continuously in sunlight (facing the Sun) and the other is continuously in darkness.

Answer:

• The North Pole receives 24 hours of sunlight (continuous daylight — the Sun does not set) during the summer solstice in the Northern Hemisphere.
• The South Pole receives 0 hours of sunlight (continuous darkness — the Sun does not rise) during the same period.

(Note: At the opposite solstice, these values are reversed — the South Pole gets 24 hours of sunlight and the North Pole gets 0 hours.)