NCERT Solutions for Class 8th Science Chapter 3 End-of-chapter questions — Keep the curiosity alive

Communicable: cold and flu, typhoid, chickenpox.   Non-communicable: diabetes, asthma.

(b) (ii) and (iii) — asthma and diabetes.

(i) Immediate actions by the school. Flu spreads through air, so every step must block droplets:

• Inform parents at once, and ask that any child with fever, cough or cold stay at home and rest until well — the chapter notes this both helps recovery and minimises spreading the disease.
• Provide masks for anyone who is coughing or sneezing, and ask everyone to cover the mouth and nose.
• Make soap and water available at every washbasin and set fixed handwashing times — before the mid-day meal and after games.
• Keep classrooms well ventilated, with windows open.
• Stop the sharing of water bottles, tiffins, handkerchiefs and towels.
• Clean frequently touched surfaces — door handles, desks, taps, sports equipment.
• Move a child who falls ill during the day to a separate room and call the parents; postpone crowded assemblies and large gatherings.
• Inform the local health authority if the number of cases keeps rising.

(ii) Responding to your benchmate considerately. The aim is to protect both of you without making your friend feel blamed or shunned.

• Say it quietly and kindly: “You don’t look well — shall we tell Ma’am?” Do not announce it to the class.
• Offer a tissue or a spare mask, and offer to walk with them to the school nurse or the office.
• Suggest sitting a little further apart for the day, and explain why — so that they know it is about the virus, not about them.
• Offer to share your notes and to call them in the evening. Being ill at home is lonely.
• Never move away with a face of disgust, never tease, and never call anyone the “source” of the outbreak. Anyone can catch flu; it says nothing about the person.

(iii) Protecting yourself and others.

• Wash hands with soap and water frequently, and avoid touching your eyes, nose and mouth with unwashed hands.
• Wear a mask in the classroom while the outbreak lasts.
• Cover your own mouth and nose when you cough or sneeze — with a tissue or your sleeve, not your palm.
• Do not share bottles, tiffins or handkerchiefs.
• Keep the windows open; sit in fresh air during breaks.
• Eat a balanced diet and get enough sleep, so your immune system is at its strongest.
• If you develop symptoms, stay at home. That is the single most effective thing you can do for the rest of the class.

Malaria is caused by a protozoa and reaches us only through the bite of a mosquito, which acts as the vector. Every precaution therefore aims at one thing: do not get bitten.

(i) Precautions

(ii) Explaining it to a younger brother or sister. Keep it simple and true:

“Malaria does not spread by sitting next to someone or by breathing the same air. It comes only from the bite of one particular kind of mosquito. That means if the mosquito cannot reach your skin, the disease cannot reach you at all. The net is a wall around your bed and the repellent is a smell the mosquito will not come near — and there is no third way for it to get in.”

Make it a shared job rather than an order: let the child tuck in the net, and let them be the one who spots and empties any standing water in pots, coolers or old tyres.

(iii) If travellers ignore health advisories.

• They are far more likely to be bitten and to develop malaria — high fever, chills and profuse sweating — which can become serious enough to need hospital care.
• Diagnosis is often delayed, because a doctor back home may not think of malaria unless the travel is mentioned. Delay makes the illness worse.
• They can carry the disease home. If a local mosquito bites an infected traveller, it can pass the parasite on to others — so one person’s carelessness becomes a risk to a whole neighbourhood.
• The same neglect — drinking unsafe water, ignoring standing water — also invites dengue, typhoid and cholera on the same trip.

(i) What to say to your uncle. Speak privately, respectfully, and from concern rather than accusation. An elder who feels attacked stops listening.

• Begin with the relationship, not the habit: “Chachaji, I am telling you this only because I want you with us for a long time.”
• Give one or two facts rather than a lecture. Tobacco is named in the chapter among the harmful substances we should say NO to. Smoking damages the lungs and the heart, and it makes infections such as tuberculosis harder to fight.
• Mention that the smoke reaches whoever is in the room — children and elders at home breathe it too.
• Point to the reason he started. Fitting in with friends is a poor bargain if the price is his health, and his friends will not be the ones who suffer it.
• Offer help instead of judgement: go with him to a doctor, or look up the national tobacco quitline together. Quitting is hard because tobacco is addictive, so treat every attempt as progress and never mock a relapse.

(ii) If a friend offers you a cigarette.

• Refuse plainly and without drama: “No thanks, I don’t smoke.” A short answer is stronger than a long explanation, and you owe no one a justification.
• Move on — change the topic, take a drink of water, join another group. If the pressure continues, leave the party.
• Decide your answer before you are in the situation. Peer pressure works on hesitation, and a ready sentence removes it.
• Tell a parent, teacher or another trusted adult if this keeps happening, especially if others in your class are being pushed the same way.

Notice that this is exactly what happened to your uncle. Refusing once, early, is far easier than stopping later.

(iii) How schools can help.

• Teach the facts clearly in class, and invite a doctor to speak and answer questions.
• Keep the school and its surroundings tobacco-free, and take up with the authorities any shop selling tobacco to minors near the gate.
• Run poster competitions, debates, street plays and health campaigns so that students explain it to one another — a message from a classmate carries further than one from a notice board.
• Provide a counsellor whom students can talk to privately and without fear of punishment, as the school in Activity 3.1 did.
• Offer sports, music, clubs and NSS-type activity, so that students have a group to belong to without needing a harmful habit as the entry ticket.
• Involve parents, so that the same message is heard at home and at school.

Vinita should ask questions that make Saniya check her own claim against what she already knows.

• “Which germs do antibiotics actually kill? Do they work against the viruses that cause colds, flu, measles, chickenpox and dengue, or against the protozoa that cause malaria?”
• “If antibiotics could cure any infection, why does the doctor send us home with rest and fluids when we have a cold, instead of a tablet?”
• “Do you know what antibiotic resistance is? What happens to the bacteria that survive when someone takes an antibiotic that was not needed, or stops the course halfway?”
• “Why do doctors insist on the full course if the medicine works so completely?”
• “Are diabetes and asthma treated with antibiotics? What about diseases that are not infections at all?”
• “If antibiotics settled everything, why does the whole world still need vaccines, clean water and toilets?”

The correct position. Antibiotics work only against bacterial infections, because they target parts of bacterial cells that are different from human or other animal cells. They do not work against viruses or diseases caused by protozoa, and they have nothing to do with non-communicable diseases at all. Moreover, their indiscriminate use has already led to a decline in their effectiveness. So the sentence is wrong twice over — antibiotics cannot cure any infection, and relying on them makes them less useful for everyone.

(i) Highest: July, August and September — 65 cases in each of the three months.

(ii) Lowest: January — 10 cases.

(iii) Natural and environmental factors in the peak months. July to September is the monsoon and the weeks just after it.

• Rainwater collects and stands — in old tyres, pots, coolers, tanks, discarded containers, blocked drains and potholes. The dengue mosquito breeds in exactly this kind of clean, still water, and the chapter’s own preventive measure is to avoid areas with still water.
• Warm and humid weather lets the mosquito complete its life cycle faster and lets the adults live longer, so far more of them are biting.
• Waterlogging and poor drainage after heavy rain create breeding sites everywhere at once.
• There is a lag between the rain and the cases: eggs must hatch, mosquitos must mature and bite, and the illness must develop. That is why the count climbs through June (40) and only peaks from July.
• By October the rain stops, the water dries up and the temperature falls — and the count halves, from 65 to 30.

(iv) Preventive steps before the peak season. The value of the graph is that it shows the peak arriving at the same time every year, so it can be prepared for.

• Clean and desilt drains, and fill in potholes and low ground, before the monsoon.
• Run a weekly “dry day” campaign — every household empties and scrubs coolers, tanks, pots and tyres once a week, which destroys the larvae before they become adults.
• Fogging and larvicide spraying in the localities that reported cases last year.
• Release larvivorous fish in ponds, tanks and ornamental pools.
• Distribute or subsidise mosquito nets and repellents, and encourage full-sleeved clothing.
• Awareness drives through schools, health workers, radio and posters at the start of the rains.
• Ask health centres to report suspected cases early, so that a cluster is spotted before it becomes an outbreak, and ensure hospitals are stocked and staffed for the peak weeks.

A campaign works when each message is short, specific and something a student can do the same day. I would run it under one slogan — “Clean hands, clean surroundings, healthy habits” — with two sets of messages, because the two kinds of disease are stopped in two different ways.

How I would run it: a health board updated weekly by students; a two-minute demonstration of correct handwashing in each class; a “dry day” every Friday when each class checks its own corridor and garden for standing water; a poster and street-play competition; a doctor invited to answer questions; and a simple monthly count — of absences due to illness — so the school can see whether the campaign is working.

Because antibiotics work only against bacterial infections, and a cold, cough or flu is caused by a virus. The medicine has nothing in the patient it can act on.

Taking one anyway is not merely useless — it does harm:

• It kills the good bacteria that protect the body from infection (Fig. 3.5b), leaving room for the resistant ones to grow and take over.
• It adds to antibiotic resistance, so that when a real bacterial infection comes later, the medicine may no longer work — for you or for anyone else.
• It can cause side effects, and it delays proper care by giving the false comfort that something is being done.

For a viral cold or flu the right treatment is what the chapter recommends: rest at home, fluids, and covering the mouth and nose. The immune system clears the virus itself in a few days.

Hepatitis A, poliomyelitis and cholera.

Because after the first encounter the immune system remembers that pathogen. The second response is not a fresh start — it is a trained one.

This is the entire principle of vaccination. A vaccine gives the immune system a safe first exposure — using dead or weakened pathogens, or an inactive or harmless part of the pathogen — so that the memory is built without the illness. When the real pathogen arrives, the body treats it as a second exposure and defeats it quickly.