📌 How to Use
First read the questions carefully using the Read section. Use the Details button to understand concepts clearly. After completing reading, click Practice Quiz to test yourself.
📘 Study MCQs
Q1. why does a cut or wound often get covered with a band-aid?
• to stop bleeding
• to make it look nice
• to prevent microbes from entering
• to keep it warm
Answer: to prevent microbes from entering
a band-aid acts as a barrier. it prevents harmful microbes present in the air and on surfaces from entering the wound and causing an infection.
Q2. why do we get a foul smell from a garbage dump?
• due to chemicals
• due to burning plastic
• due to decomposition by microbes
• due to rain
Answer: due to decomposition by microbes
microbes like bacteria and fungi decompose the organic waste in the garbage. this process releases gases that have a foul smell.
Q3. why does dough increase in volume when kept for some time?
• it absorbs water
• yeast produces carbon dioxide gas
• it gets heated
• flour expands
Answer: yeast produces carbon dioxide gas
yeast added to the dough ferments the sugar and produces carbon dioxide gas. the gas fills up and expands the dough, making it rise.
Q4. what is the white cottony layer that appears on moist leather shoes?
• dust
• salt
• fungus
• paint
Answer: fungus
in warm and humid conditions, fungi grow on organic materials like leather. the white cottony layer is a fungal growth.
Q5. why are we advised to drink boiled water during the rainy season?
• it tastes better
• it is colder
• boiling kills harmful microbes
• it has more minerals
Answer: boiling kills harmful microbes
water sources can get contaminated with disease-causing microbes during rains. boiling the water kills these microbes, making it safe to drink.
Q6. why do pickles, jams, and murabba not spoil for months?
• they are kept in dark
• they are made of fruits
• they have high salt or sugar content
• they are cooked
Answer: they have high salt or sugar content
high concentration of salt or sugar draws out water from microbes through osmosis. this prevents their growth and preserves the food.
Q7. why does a mother tell a child to wash hands with soap before eating?
• to make hands smell good
• to remove dirt and germs
• to make hands soft
• to warm the hands
Answer: to remove dirt and germs
our hands touch many surfaces that may have harmful microbes. washing with soap removes these microbes and prevents them from entering our body with food.
Q8. what is the reason behind the sour taste of curd?
• addition of sugar
• addition of lemon
• action of lactobacillus bacteria
• addition of salt
Answer: action of lactobacillus bacteria
lactobacillus bacteria convert the lactose sugar present in milk into lactic acid. this acid gives curd its sour taste.
Q9. why do we see a greenish layer on stagnant water in ponds?
• due to oil
• due to growth of algae
• due to rust
• due to soil
Answer: due to growth of algae
stagnant water and sunlight provide ideal conditions for the growth of microscopic algae, which form a green layer on the surface.
Q10. what causes the ‘holes’ in swiss cheese?
• made by insects
• made with a machine
• carbon dioxide produced by bacteria
• air bubbles during mixing
Answer: carbon dioxide produced by bacteria
a specific bacterium used in cheese-making produces carbon dioxide gas during fermentation. the gas bubbles get trapped, forming holes.
Q11. why are onions and potatoes often stored in a cool, dry place?
• to keep them colorful
• to prevent rotting by microbes
• to make them sweeter
• to keep them hard
Answer: to prevent rotting by microbes
a cool and dry place does not support the growth of microbes (fungi and bacteria) that cause rotting of vegetables.
Q12. why is a person with a cold advised to use a handkerchief while sneezing?
• to look polite
• to keep the nose warm
• to prevent the spread of germs in the air
• to wipe tears
Answer: to prevent the spread of germs in the air
sneezing releases thousands of tiny droplets containing the cold virus. using a handkerchief traps these droplets and prevents the infection from spreading to others.
Q13. why does idli and dosa batter rise when kept overnight?
• it absorbs water
• fermentation by bacteria or yeast
• it gets cooked
• flour expands
Answer: fermentation by bacteria or yeast
microbes present in the batter or from the air ferment the sugars, producing carbon dioxide gas. this gas makes the batter fluffy and rise.
Q14. what is the black powder that appears on mustard leaves?
• dirt
• fungus
• pollen
• insect eggs
Answer: fungus
it is a fungal disease called ‘alternaria blight’. the fungus appears as black spots or powder on the leaves.
Q15. why is it recommended to finish a full course of antibiotics prescribed by a doctor?
• to make the doctor happy
• to ensure all bacteria are killed
• antibiotics are tasty
• to save medicine for later
Answer: to ensure all bacteria are killed
stopping medicine early may leave some of the strongest bacteria alive. these can multiply and cause a relapse, possibly with antibiotic resistance.
Q16. why does wet laundry start smelling if left in a bucket for 2-3 days?
• due to detergent
• due to growth of microbes
• due to color of clothes
• due to hard water
Answer: due to growth of microbes
damp clothes in a warm place provide perfect conditions for bacteria and moulds to grow. their metabolic activity causes the bad smell.
Q17. why are dried fish and dry fruits less likely to spoil than fresh ones?
• they are more expensive
• they have less water content
• they are kept in boxes
• they are treated with chemicals
Answer: they have less water content
microbes need water to grow. the drying process removes most of the water, making it difficult for microbes to survive and spoil the food.
Q18. why are mosquitoes more common around stagnant water?
• they like to swim
• they lay eggs in stagnant water
• they drink stagnant water
• they find food there
Answer: they lay eggs in stagnant water
mosquitoes breed by laying their eggs in stagnant water. the larvae and pupae develop in this water before becoming adult mosquitoes.
Q19. why does a rust-like reddish powder appear on wheat plants?
• due to iron
• due to a fungal disease
• due to pollution
• due to insect dust
Answer: due to a fungal disease
wheat rust is a common fungal disease. the fungus produces reddish-brown spores on the stems and leaves, which look like rust.
Q20. what is the common reason for stomach upset or food poisoning after eating food from a street vendor?
• food is too spicy
• food may be contaminated with harmful bacteria
• food is cheap
• food is cooked in oil
Answer: food may be contaminated with harmful bacteria
sometimes, food prepared in unhygienic conditions can get contaminated with bacteria like salmonella or e. coli, which cause food poisoning.
Q21. why does a doctor clean a wound with a brown-colored liquid (antiseptic) before giving an injection?
• to make the skin brown
• to numb the area
• to kill any microbes on the skin
• to mark the spot
Answer: to kill any microbes on the skin
the brown liquid is an antiseptic like povidone-iodine. it kills microbes on the skin’s surface to prevent them from entering the body through the needle prick.
Q22. why does a gardener sometimes grow pea or bean plants in a field?
• they are pretty
• they grow fast
• they enrich the soil with nitrogen
• they need less water
Answer: they enrich the soil with nitrogen
pea and bean plants have rhizobium bacteria in their root nodules. these bacteria fix atmospheric nitrogen into the soil, making it fertile for the next crop.
Q23. why does the surface of an old loaf of bread become fuzzy and blackish-green?
• it is stale
• mould has grown on it
• it has dried out
• it has absorbed smells
Answer: mould has grown on it
bread mould (rhizopus) spores from the air land on the bread. in moist conditions, they germinate and grow, forming a fuzzy, coloured layer.
Q24. why are vaccination camps organized for children?
• to give them sweets
• to protect them from diseases
• to check their height and weight
• to give them toys
Answer: to protect them from diseases
vaccines given in these camps train the child’s immune system to fight specific diseases like polio, measles, and diphtheria, preventing future illness.
Q25. why does adding a little old curd to warm milk help in setting new curd?
• old curd adds color
• old curd contains lactobacillus starter
• old curd adds taste
• old curd makes milk cold
Answer: old curd contains lactobacillus starter
the old curd contains millions of active lactobacillus bacteria. when added to warm milk, they multiply and convert the milk into curd.
Q26. why are dry neem leaves put in containers storing grains?
• for good smell
• to add color
• neem leaves keep insects and microbes away
• to fill empty space
Answer: neem leaves keep insects and microbes away
neem has natural anti-microbial and insect-repelling properties. the leaves help protect the grains from fungal growth and insect damage.
Q27. what causes the ‘fizz’ or bubbles in soft drinks?
• shaking the bottle
• dissolved carbon dioxide gas
• dissolved oxygen
• dissolved sugar
Answer: dissolved carbon dioxide gas
carbon dioxide gas is dissolved under pressure in soft drinks. when the bottle is opened, the pressure is released, and the gas escapes in the form of bubbles.
Q28. why does a vaccinated person not get a disease even if exposed to the germs?
• the germs avoid them
• their body has ready-made fighters (antibodies)
• they take vitamins
• they wear a mask
Answer: their body has ready-made fighters (antibodies)
the vaccine teaches the body to make specific antibodies against a disease. if the real germ attacks later, these antibodies quickly recognize and destroy it.
Q29. why does a farmer sometimes plough the stubble (leftover plant parts) into the soil after harvest?
• to clear the field
• microbes decompose it to form manure
• to make the soil hard
• to bury weeds
Answer: microbes decompose it to form manure
soil microbes decompose the plant remains. this process adds organic matter and nutrients back to the soil, acting as natural manure.
Q30. why do we sometimes see a slippery green layer on steps of a public bathroom?
• due to soap
• due to growth of algae
• due to paint
• due to oil
Answer: due to growth of algae
the constantly wet and humid surface of bathroom steps provides an ideal environment for microscopic algae to grow, creating a slippery layer.
Q31. why does a mother insist on covering cooked food?
• to keep it hot
• to prevent dust and flies from settling on it
• to make it cook more
• to keep smells in
Answer: to prevent dust and flies from settling on it
dust carries microbes, and flies sit on garbage and carry germs on their legs. covering food prevents these carriers from contaminating it.
Q32. why does vinegar act as a good preservative for pickles?
• it is sour
• it is acidic
• it is liquid
• it is cheap
Answer: it is acidic
most bacteria cannot survive in an acidic environment. the acetic acid in vinegar creates such an environment, preventing microbial growth.
Q33. why does a cut apple turn brown after some time?
• due to paint
• due to rust
• due to air (oxidation) and microbial action
• due to loss of water
Answer: due to air (oxidation) and microbial action
when cut, the apple’s flesh is exposed to air and microbes. a chemical reaction (oxidation) and early microbial activity cause the browning.
Q34. why are pulses (dal) and grains often sun-dried before storage?
• to make them crunchy
• to reduce their moisture content
• to change their color
• to make them warm
Answer: to reduce their moisture content
sun-drying removes moisture. with low moisture, fungi and bacteria cannot grow, preventing spoilage during storage.
Q35. what is the white layer sometimes seen on the surface of salty pickles or very salty fish?
• salt crystals
• fungal growth
• fat
• wax
Answer: fungal growth
if the salt concentration is not high enough, certain salt-tolerant fungi can still grow on the surface, forming a white layer.
Q36. why does a jar of honey last for years without spoiling?
• it is very sweet
• it has natural preservatives and low water
• it is kept sealed
• all of the above
Answer: all of the above
honey has high sugar content, natural anti-microbial properties, and very low moisture. a tight seal prevents new microbes from entering.
Q37. why do we feel body ache and fever during a viral infection like flu?
• virus produces pain
• it is our body’s defense response
• due to lack of food
• due to cold weather
Answer: it is our body’s defense response
fever and aches are signs that our immune system is fighting the virus. the higher temperature can help slow down the virus’s growth.
Q38. why does a deep puncture wound from a rusty nail risk causing tetanus?
• rust is poisonous
• the bacterium clostridium tetani lives in soil and rust
• nail causes bleeding
• iron causes infection
Answer: the bacterium clostridium tetani lives in soil and rust
the tetanus bacteria are commonly found in soil, dust, and on rusty metals. a deep wound provides an oxygen-free environment where they can grow and produce toxin.
Q39. why do we add baking soda (eno) or yeast while making cakes?
• for sour taste
• to make it heavy
• to produce gas and make it fluffy
• for white color
Answer: to produce gas and make it fluffy
baking soda reacts with acids to produce carbon dioxide. yeast ferments sugar to produce carbon dioxide. this gas creates bubbles, making the cake soft and spongy.
Q40. why does a person with tuberculosis often cough for a long time?
• due to sore throat
• due to allergy
• the bacteria damage the lungs
• due to cold drink
Answer: the bacteria damage the lungs
mycobacterium tuberculosis bacteria attack the lungs, causing inflammation and damage. the body tries to clear this through persistent coughing.
Q41. why should we avoid storing cooked food in copper or brass containers?
• they are expensive
• they can react with acids and salts, producing toxic compounds
• they keep food too cold
• they break easily
Answer: they can react with acids and salts, producing toxic compounds
especially with sour or salty food, chemicals from the metal can dissolve into the food. this can promote microbial growth or cause poisoning.
Q42. why does buttermilk feel cooling in summer and also help in digestion?
• it is served cold
• it contains lactobacillus which is good for gut
• it is yellow in color
• it is cheap
Answer: it contains lactobacillus which is good for gut
the lactobacillus in buttermilk are probiotics. they help maintain a healthy balance of gut bacteria, aiding digestion and providing a cooling effect.
Q43. what causes the spongy, bad-smelling patches on citrus fruits like oranges?
• insect bites
• fungal rot
• sunburn
• overripe fruit
Answer: fungal rot
fungi like penicillium infect the fruit, especially if the skin is bruised. they decompose the fruit, creating soft, smelly, spongy patches.
Q44. why are children given deworming medicine periodically?
• to increase hunger
• to kill protozoan or worm parasites in the gut
• to help them sleep
• to make them tall
Answer: to kill protozoan or worm parasites in the gut
children can get infected with intestinal worms or protozoa through contaminated food or water. deworming medicines kill these parasites.
Q45. why does a person often get a recurring infection if they have a weak immune system?
• they eat less
• their body cannot produce enough antibodies to fight germs
• they stay indoors
• they take baths
Answer: their body cannot produce enough antibodies to fight germs
a strong immune system makes antibodies to fight infections. a weak system cannot make enough, allowing the same germs to cause illness again.
Q46. why do we add oil to the surface of pickles?
• for taste
• oil floats and seals out air, preventing microbial growth
• to make it shiny
• to increase volume
Answer: oil floats and seals out air, preventing microbial growth
oil forms a layer on top, preventing oxygen from reaching the pickles. many harmful microbes need oxygen to grow, so this helps in preservation.
Q47. why does a compost pit in the garden not smell bad if managed properly?
• it is covered with soil
• good microbes decompose waste aerobically without much foul smell
• perfume is added
• it has no garbage
Answer: good microbes decompose waste aerobically without much foul smell
when turned regularly, air helps aerobic bacteria decompose the waste efficiently. this process produces less foul-smelling gases compared to anaerobic decomposition.
Q48. why does a small piece of raw mango make the whole jar of pickles safe from spoilage?
• it adds flavor
• it is a fruit
• it has antimicrobial properties
• it is sour
Answer: it has antimicrobial properties
raw mango contains certain acids and compounds that inhibit the growth of spoilage-causing microbes, acting as a natural preservative.
Q49. why does a vaccine sometimes cause mild fever or pain at the injection site?
• the needle is dirty
• it is a sign the body is reacting and building immunity
• the vaccine is expired
• the person is allergic
Answer: it is a sign the body is reacting and building immunity
the mild symptoms show that the immune system has recognized the vaccine and is starting to build its defense, which is a normal response.
Q50. why does water from an old tank or cooler sometimes cause itching or ear infection?
• water is cold
• water contains harmful bacteria or amoeba
• water has chlorine
• water is hot
Answer: water contains harmful bacteria or amoeba
stagnant water in tanks or coolers can become a breeding ground for microbes like bacteria and amoeba, which can cause skin or ear infections upon contact.