⚡ Quick quiz

Test yourself in under a minute: Class 8 Science Practice Test

Expect a mix of physics, chemistry, and biology here, from friction and reflection to electroplating and puberty hormones.

1. 1. A student tests two beakers with the same circuit and bulb, one with sugar dissolved in water, one with salt dissolved in water. The salt water lights the bulb, but the sugar water lights nothing at all. Why the difference?

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   A) Both liquids conduct exactly the same, the bulb is faultyB) Sugar water is always a better conductor than salt waterC) The bulb only works with clear liquidsD) Salt water releases free ions when it dissolves, sugar water doesn't

2. 2. Two identical candles are lit. One is placed in a sealed jar with a small amount of air, the other burns in the open. Which one goes out sooner, and why?

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   A) Both go out at exactly the same timeB) Neither goes out, candles don't need oxygenC) The one in the open, sealed air burns longerD) The one in the sealed jar, its oxygen runs out

3. 3. A wildlife area allows villagers to graze cattle and collect firewood under certain rules, but bans hunting. Is this more likely a wildlife sanctuary or a national park?

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   A) Wildlife sanctuaryB) National park

4. 4. A student argues that a hollow plastic ball and a solid steel ball of the same size must sink or float the same way, since 'size is what matters for floating.' Is this reasoning correct?

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   A) Yes, but only in salt waterB) No, only the material's colour decides floatingC) Yes, same size always means same floating behaviourD) No, floating depends on density, not size alone

5. 5. A dog reacts to a high-pitched whistle that a person standing right next to it cannot hear at all. What does this tell you about the whistle's frequency compared to the human hearing range?

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6. 6. A student shines a flashlight on a mirror and then on a sheet of paper, from the same distance and angle. Only one surface shows a clear reflected image of the bulb. Which surface, and why doesn't the other one work?

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   A) Both show an equally clear imageB) Neither surface reflects light at allC) Paper, diffused reflection forms a clearer imageD) Mirror, regular reflection keeps rays parallel

7. 7. A farmer adds only chemical fertilizer to a field every year and never adds manure. What long-term problem is this farmer likely to run into with the soil itself, even if crop yields stay high at first?

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   A) The soil will become permanently infertile within a yearB) The soil's texture and water-holding capacity will worsenC) The crop will stop needing water entirelyD) There is no downside, fertilizer fully replaces manure

8. 8. A frog's egg and sperm join and develop into offspring entirely outside the mother's body, in water. What type of fertilization is this, and how does it differ from what happens in humans?

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9. 9. Why doesn't a national park typically need something like a lightning conductor to protect its forest from lightning strikes, the way a building does?

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   A) Lightning only strikes buildings, never open landB) Tall trees already act as natural lightning conductorsC) Forests are never struck by lightningD) Trees are electrical insulators, so lightning avoids them

Force, Pressure & Friction

Tests the SI unit of force, speed calculations, the pressure formula, ways to reduce friction, floating versus sinking, and the difference between static and sliding friction.

1. 1. What is the SI unit of force, and what is it equal to in terms of mass and acceleration?

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   newton (N); 1 N = 1 kg x 1 m/s²🔊

   Force is defined through Newton's second law, F = mass x acceleration. One newton is the force needed to give a 1 kg mass an acceleration of 1 m/s², so the unit itself is built directly from the mass and acceleration units, not a separate invented quantity.

2. 2. A car travels 150 km in 3 hours. Find its average speed.

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   50 km/h🔊

   Average speed = total distance / total time = 150 km / 3 h = 50 km/h. This only gives the average over the whole trip, the car's actual speed could have been higher or lower at different moments along the way.

3. 3. Why does a sharp knife cut vegetables more easily than a blunt knife, even with the same force applied?

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   A sharp knife has a much smaller area of contact, so the same force creates far greater pressure.🔊

   Pressure = Force / Area, so for the same applied force, reducing the area sharply increases the pressure. A sharp blade concentrates the cutting force onto a very thin edge, while a blunt blade spreads the same force over a wider area, producing far less pressure at any single point.

4. 4. State one way to reduce friction between two moving machine parts, and explain why it works.

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   Applying oil or grease (a lubricant) between the two surfaces.🔊

   A lubricant fills in the microscopic bumps and grooves on both surfaces, letting them slide past each other more smoothly. This reduces the force needed to keep the parts moving and also cuts down on the heat and wear that friction would otherwise cause.

5. 5. A ship made of iron floats on water, but a solid iron nail sinks. Explain why, using the idea of density.

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   The ship's hollow, wide shape gives it a lower overall density than water, while the solid nail's density is higher than water.🔊

   Floating depends on the object's overall (average) density compared to the liquid, not on what material it's made of. The ship's iron hull encloses a large volume of air, spreading the same mass of iron over far more space, so its average density drops below that of water. The solid nail has no such hollow space, so its density stays equal to iron's actual density, which is higher than water's.

6. 6. Which type of friction acts on an object that is already sliding smoothly over a surface, and how does it generally compare to the maximum static friction for the same surfaces?

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   Sliding friction; it is usually smaller than the maximum static friction for the same surfaces.🔊

   Static friction resists an object starting to move and increases up to a maximum value right before motion begins. Once the object is actually sliding, kinetic (sliding) friction takes over, and it is generally a bit smaller than that maximum static value, which is why a heavy box is harder to start moving than to keep moving.

Sound & Natural Phenomena

Covers the human hearing range, how the ear detects sound, and how lightning forms and is safely discharged during a storm.

1. 1. What is the approximate frequency range of sound audible to the human ear?

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   About 20 Hz to 20,000 Hz (20 kHz)🔊

   This range is specific to human hearing, sounds below 20 Hz (infrasound) and above 20,000 Hz (ultrasound) exist and are used by some animals and instruments, but the human ear cannot detect them.

2. 2. Name the part of the human ear that vibrates when sound waves strike it.

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   The eardrum (tympanic membrane)🔊

   The eardrum is a thin, stretched membrane at the end of the ear canal. Incoming sound waves make it vibrate, and those vibrations are then passed along through tiny bones in the middle ear before reaching the nerve signals the brain interprets as sound.

3. 3. What causes lightning during a thunderstorm, in terms of electric charge?

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   A build-up and sudden transfer (discharge) of electric charge between clouds, or between a cloud and the ground.🔊

   Inside a storm cloud, collisions between ice particles and water droplets build up separated positive and negative charges. When the charge difference becomes large enough, it discharges suddenly as a bright flash, either between two clouds or between a cloud and the ground, which is what we see as lightning.

4. 4. What is the purpose of a lightning conductor fitted on top of tall buildings?

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   It gives lightning's electric charge a safe, low-resistance path directly into the ground.🔊

   A lightning conductor is a metal rod connected by a thick wire to a metal plate buried in the ground. If lightning strikes the building, the conductor offers the charge an easier path to travel than through the building's structure, carrying it safely into the earth instead of through walls or wiring.

5. 5. Why should you avoid standing under a tall, isolated tree during a lightning storm?

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   A tall, isolated object is more likely to be struck, since it offers the shortest path to the ground.🔊

   Lightning tends to strike the tallest, most exposed point in an area because that point offers the least resistance path between the charged cloud and the ground. Standing under an isolated tree during a storm puts a person directly in that path if the tree is struck.

Light

Tests the laws of reflection, regular versus diffused reflection, the retina's role in vision, the Braille system, and repeated reflections between two mirrors.

1. 1. State the two laws of reflection of light.

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   (1) The angle of incidence equals the angle of reflection. (2) The incident ray, the reflected ray, and the normal all lie in the same plane.🔊

   These two laws hold for every reflecting surface, not just flat mirrors. The first law is what lets us predict exactly where a reflected ray will go once we know the angle it arrived at, measured from the normal (an imaginary line perpendicular to the surface).

2. 2. What is the difference between regular (specular) and diffused (irregular) reflection?

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   Regular reflection keeps parallel rays parallel after reflecting off a smooth surface; diffused reflection scatters parallel rays in many directions off a rough surface.🔊

   Both types of reflection still obey the same two laws of reflection, the difference is only in the surface. A mirror's surface is smooth enough that every ray reflects at a predictable, matching angle, producing a clear image. A rough surface, like paper or an unpolished wall, reflects each ray at a slightly different angle because the surface itself isn't flat at a microscopic level, so no clear image forms.

3. 3. Name the light-sensitive layer at the back of the human eye that receives the image formed by the eye's lens.

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   Retina🔊

   Light entering the eye is focused by the lens onto the retina, a layer packed with light-sensitive cells that convert the image into electrical signals. These signals travel through the optic nerve to the brain, where the actual 'seeing' happens.

4. 4. What system allows visually impaired people to read and write using raised dots representing letters and numbers?

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   The Braille system🔊

   Braille uses patterns of raised dots that can be felt with the fingertips, with each pattern representing a letter, number, or punctuation mark. It lets visually impaired people read and write without relying on sight at all.

5. 5. Why do you see multiple, progressively fainter images of yourself when standing between two mirrors facing each other?

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   Each mirror reflects the image the other mirror has already formed, over and over, losing some light with each additional reflection.🔊

   When two mirrors face each other, the first image formed by one mirror becomes an object for the second mirror to reflect again, and that new image becomes an object for the first mirror once more. This chain continues, but each reflection loses a little light to absorption, so the repeated images get progressively fainter.

Chemical Effects of Electric Current

Tests why pure water conducts poorly, what electroplating is, what happens at each electrode during electrolysis, and how conductivity relates to bulb brightness.

1. 1. Does pure, distilled water conduct electricity well? What is generally added to water to make it conduct better?

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   No, pure water is a poor conductor; dissolving a salt, acid, or base in it makes it conduct much better.🔊

   Electric current in a liquid is carried by ions, charged particles that can move freely through the liquid. Pure water has very few free ions, so it barely conducts. Dissolving a salt, acid, or base releases many ions into the water, giving the current a way to flow much more easily.

2. 2. What is electroplating, and give one everyday example of where it is used.

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   Electroplating is coating one metal with a thin layer of another metal using an electric current passed through a solution containing that metal's ions. Example: chromium plating on bicycle handlebars or car parts.🔊

   The object to be plated is connected as one electrode, and the current pulls metal ions out of the solution and deposits them as a thin, even layer onto that object's surface. This is used to make cheaper metals look shinier, resist rust, or resist scratches, without using an expensive metal throughout the whole object.

3. 3. When an electric current passes through copper sulphate solution using copper electrodes, what happens at each electrode?

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   Copper deposits form on the negative electrode, while the positive electrode dissolves into the solution.🔊

   Passing current through the copper sulphate solution makes positively charged copper ions move toward the negative electrode and deposit there as solid copper. At the same time, the positive copper electrode loses copper atoms into the solution as ions, which is what keeps the amount of copper in the solution roughly constant during the process.

4. 4. If a bulb connected in a circuit with a liquid, like salt water, glows only dimly, what does that tell you about the liquid?

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   The liquid is a weak conductor, meaning it contains only a small number of free ions.🔊

   A dim glow means only a small current is flowing through the circuit, which happens when the liquid has relatively few free ions available to carry charge. A strong conductor, with many free ions, would let more current through and make the bulb glow brightly.

Combustion, Flame, Coal & Petroleum

Covers the conditions needed for combustion, ignition temperature, why water shouldn't be used on oil or electrical fires, the zones of a flame, and fossil fuel formation.

1. 1. Which gas in air is necessary to support combustion?

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   Oxygen🔊

   Combustion is a chemical reaction between a substance and oxygen that releases heat and light. Without enough oxygen present, a fuel cannot sustain burning, which is exactly why smothering a fire (cutting off its oxygen supply) is one of the main ways to put it out.

2. 2. Define ignition temperature.

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   The lowest temperature at which a substance catches fire and starts burning on its own.🔊

   Every combustible substance has its own ignition temperature. Below this temperature, the substance won't catch fire even if oxygen is present, which is why some materials need to be heated first (like striking a matchstick) before they'll burn.

3. 3. Why is water not used to extinguish a fire caused by burning oil or an electrical short circuit?

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   For oil fires, water sinks below the oil and can splash it, spreading the fire further. For electrical fires, water conducts electricity and can cause a severe shock.🔊

   Water is denser than oil, so pouring water onto a burning oil fire causes the water to sink beneath the oil, pushing the burning oil upward and outward, often spreading the flames rather than putting them out. Separately, water conducts electricity well, so using it near live electrical wiring risks passing a dangerous shock back through the water stream to whoever is holding it.

4. 4. Name the three zones of a candle flame from innermost to outermost, and identify which is the hottest.

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   Innermost dark zone, middle luminous zone, outer non-luminous zone; the outer zone is the hottest.🔊

   The innermost zone contains unburnt wax vapour and doesn't glow. The middle zone has partial combustion and produces the flame's visible yellow light, but burns at a lower temperature. The outer zone gets the most oxygen and burns completely, giving off the least light but reaching the highest temperature, which is why the tip of a flame, not its glowing middle, is used for the hottest, most efficient heating.

5. 5. Name the fossil fuel formed from the remains of dead plants buried underground over millions of years, and name one product obtained from it.

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   Coal; one product obtained from it is coal tar (or coke, or coal gas).🔊

   Coal formed over millions of years from dead plant matter buried under layers of earth, compressed and slowly transformed by heat and pressure. Heating coal in the absence of air (destructive distillation) breaks it down into several useful products, including coal gas (used as fuel), coal tar (used in dyes and medicines), and coke (used in metal extraction).

6. 6. Why are CNG and LPG considered cleaner fuels compared to coal?

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   CNG and LPG burn more completely and produce far less smoke, soot, and harmful residue than coal.🔊

   Coal contains impurities like sulfur that release harmful gases and leave behind ash when burned, along with visible smoke and soot from incomplete combustion. CNG and LPG are cleaner-burning gases that combust more completely, releasing mostly carbon dioxide and water vapour with far less pollution.

Crop Production & Microorganisms

Tests the difference between manure and fertilizer, weeding, fermentation in bread-making, pasteurization, nitrogen fixation, and why drinking water is boiled.

1. 1. What is the main difference between manure and chemical fertilizers in what they provide to soil?

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   Manure improves the soil's texture and water-holding capacity slowly by adding organic matter; fertilizers supply specific nutrients quickly but don't improve the soil's structure.🔊

   Manure is made from decomposed organic (plant or animal) waste, so it adds bulk and organic content that helps soil retain water and nutrients over time. Fertilizers are manufactured chemical compounds that deliver a concentrated dose of a specific nutrient (like nitrogen) quickly, which boosts plant growth fast but does nothing to improve the physical quality of the soil itself.

2. 2. Name the process by which farmers remove unwanted, wild plants growing along with the main crop.

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   Weeding🔊

   Weeds compete with the main crop for water, nutrients, sunlight, and space, reducing the crop's yield if left unchecked. Weeding removes these unwanted plants, usually before they can flower and spread further, so the main crop gets the full benefit of the field's resources.

3. 3. Name the microorganism responsible for the fermentation that makes bread dough rise.

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   Yeast🔊

   Yeast is a microorganism that respires sugars in the dough and releases carbon dioxide gas as a byproduct. This gas gets trapped in the dough as tiny bubbles, causing it to rise and become soft and airy once baked.

4. 4. What is pasteurization, and why is it done to milk?

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   Heating milk to a specific temperature for a fixed time, then rapidly cooling it, to kill most harmful microorganisms without spoiling the milk.🔊

   Pasteurization is named after Louis Pasteur, who discovered that controlled heating could kill disease-causing microbes in a liquid without needing to boil it, which would otherwise change the milk's taste and nutritional value. The rapid cooling afterward also helps prevent any surviving microbes from multiplying quickly.

5. 5. Name the process by which certain bacteria convert atmospheric nitrogen into a form plants can use, and where in a plant this mainly happens.

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   Nitrogen fixation, mainly carried out by Rhizobium bacteria living in root nodules of leguminous plants (like beans and peas).🔊

   Plants cannot use the nitrogen gas that makes up most of the atmosphere directly, they need it in a chemically combined form. Rhizobium bacteria living inside small swellings (nodules) on the roots of leguminous plants convert atmospheric nitrogen into compounds the plant can absorb and use, in exchange for nutrients from the plant.

6. 6. Why should drinking water be boiled in areas where microbial contamination is a concern?

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   Boiling kills most disease-causing microorganisms present in the water, making it safe to drink.🔊

   Many microorganisms that cause waterborne diseases cannot survive the high temperature of boiling water. Boiling is a simple, reliable way to make contaminated water safer to drink when other purification methods aren't available.

Conservation of Plants and Animals

Covers endemic species, the Red Data Book, the difference between a wildlife sanctuary and a national park, and the effects of deforestation.

1. 1. What term describes a species of plant or animal found naturally in only one specific geographic area and nowhere else?

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   Endemic species🔊

   An endemic species has evolved and exists naturally only within one particular region or habitat, and is not found anywhere else in the world. This makes such species especially vulnerable, since damage to their single habitat can threaten their entire population at once.

2. 2. Name the record that lists species at risk of extinction, used to guide conservation priorities.

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   The Red Data Book🔊

   The Red Data Book maintains records of species that are rare, threatened, or at risk of extinction. Governments and conservation organizations use this information to decide which species most urgently need protection.

3. 3. What is the main difference between a wildlife sanctuary and a national park in terms of allowed human activity?

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   A wildlife sanctuary may still allow some limited human activity, like grazing or timber collection, while a national park enforces much stricter protection with essentially no human activity permitted inside.🔊

   Both are protected areas meant to conserve wildlife and habitats, but a national park's rules are generally tighter, restricting or banning activities like grazing, farming, and forestry that a sanctuary might still permit to some degree.

4. 4. Name one major consequence of large-scale deforestation on local rainfall patterns.

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   Reduced local rainfall.🔊

   Trees release large amounts of water vapour into the air through transpiration, and this water vapour contributes to cloud formation and rainfall in the surrounding region. Large-scale deforestation removes this natural contribution to the water cycle, which can reduce rainfall in that area over time.

Reproduction in Animals & Reaching the Age of Adolescence

Tests sexual versus asexual reproduction, budding in Hydra, the hormonal trigger for puberty, the pituitary gland, and hygiene during adolescence.

1. 1. What is the main difference between sexual and asexual reproduction in animals?

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   Sexual reproduction combines a male and female gamete to form offspring with a mix of both parents' traits; asexual reproduction produces offspring from a single parent, genetically identical to it, with no fertilization involved.🔊

   Sexual reproduction requires the fusion of two special sex cells (gametes), which is why the offspring shows a combination of both parents' characteristics. Asexual reproduction, like budding or binary fission, involves only one parent and no fusion of gametes, so the resulting offspring is essentially a genetic copy of that single parent.

2. 2. Name the simple form of asexual reproduction seen in Hydra, where a new individual grows out from the parent's body.

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   Budding🔊

   In budding, a small outgrowth (bud) forms on the parent Hydra's body, gradually develops the structures of a complete organism, and eventually detaches to live independently. No fertilization or second parent is involved at any point.

3. 3. What triggers the onset of puberty in both boys and girls, in terms of body chemistry?

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   The release of specific hormones, like testosterone in boys and estrogen in girls, from glands in the body.🔊

   Hormones are chemical messengers released into the bloodstream that trigger changes in specific organs and tissues even though they're produced elsewhere in the body. During puberty, rising hormone levels are what trigger the physical and reproductive changes associated with adolescence.

4. 4. Name the gland often called the 'master gland' because it controls the release of many other hormones, including those responsible for puberty.

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   The pituitary gland🔊

   The pituitary gland is often called the master gland because it releases hormones that in turn control the activity of several other glands in the body, including those responsible for triggering the onset of puberty.

5. 5. Why is personal hygiene especially important during adolescence?

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   Increased activity of sweat and oil (sebaceous) glands during puberty makes the body more prone to body odor and skin issues like acne, so regular cleanliness helps manage these changes.🔊

   Hormonal changes during adolescence increase the activity of sweat and sebaceous glands, which can lead to more body odor and clogged pores that cause acne. Regular washing and grooming habits help manage these new changes rather than letting them build up.