For Class 8, 9 & 10
Master the basics - and everything after gets easier
Concept-first questions with clear model answers in Physics, Chemistry, Maths and Biology, all NCERT-aligned. Start early, build the habit, and walk into your boards, NEET and JEE already ahead.
Interactive lessons
learn by playingDrag, slide and build - watch each concept come alive, then reveal the answer.
200 interactive lessons
Ohm's law
Class 10 Physics
Slide V & R, watch the bulb glow
Open →pH scale
Class 10 Chemistry
Slide across acids and bases
Open →Atomic number and mass number
Class 9 Chemistry
Add protons & neutrons, build shells
Open →Laws of reflection
Class 8 Physics
Change the angle, watch it bounce
Open →Volume of a sphere
Class 9 Maths
Grow the radius, see the volume
Open →Area of a trapezium
Class 8 Maths
Drag the sides, read the area
Open →Power of a lens
Class 10 Physics
Move the object, trace the rays
Open →Food chain and energy flow
Class 10 Biology
Follow the energy as it flows
Open →Speed
Class 8 Physics
Slide distance & time, watch the speed
Open →Density
Class 9 Physics
Pack mass into volume, float or sink
Open →Work done
Class 9 Physics
Push harder or farther, watch work grow
Open →Kinetic energy
Class 9 Physics
Speed it up - energy grows with the square
Open →Power of a lens
Class 10 Physics
Shorten the focal length, boost the power
Open →Mole concept
Class 9 Chemistry
Weigh out grams, count the moles
Open →Avogadro's number
Class 9 Chemistry
Add moles, count the particles
Open →Microscope magnification
Class 8 Biology
Grow the image, read the magnification
Open →Population density
Class 10 Biology
Add individuals, shrink the land, see crowding
Open →Simple interest
Class 8 Maths
Slide money, rate & time, watch interest
Open →Pythagoras theorem
Class 9 Maths
Stretch the two sides, get the hypotenuse
Open →Probability of an event
Class 10 Maths
Change the outcomes, watch the odds
Open →Newton's second law
Class 9 Physics
Push a mass, pick an acceleration
Open →Momentum
Class 9 Physics
Slide mass & velocity, build momentum
Open →Pressure
Class 8 Physics
Shrink the area, feel the pressure rise
Open →Weight
Class 9 Physics
Change the planet's gravity, watch your weight
Open →Refractive index
Class 10 Physics
Slow light in the medium, raise the index
Open →Resistors in series
Class 10 Physics
Add two resistors in a line
Open →Mass percentage of a solution
Class 9 Chemistry
Dissolve solute, read the strength
Open →Concentration of a solution
Class 9 Chemistry
Pack solute into less liquid
Open →Population change
Class 10 Biology
Balance births against deaths
Open →Compound microscope
Class 8 Biology
Combine eyepiece & objective lenses
Open →Area of a circle
Class 8 Maths
Grow the radius, watch the area square
Open →Volume of a cuboid
Class 8 Maths
Stretch length, breadth & height
Open →Electronic configuration and valency
Class 9 Chemistry
Slide the atomic number, build the atom
Open →Homologous series (alkanes)
Class 10 Chemistry
Add carbons, name the compound
Open →Mass number
Class 9 Chemistry
Add protons & neutrons, get the mass number
Open →Power
Class 9 Physics
More work in less time = more power
Open →Potential energy
Class 9 Physics
Lift a mass higher, store energy
Open →Wave speed
Class 9 Physics
Tune frequency & wavelength, set the speed
Open →Electric current
Class 10 Physics
Push charge per second, get the current
Open →Percentage
Class 8 Maths
Compare part to whole as a %
Open →Electron dot structure
Class 9 Chemistry
Draw valence electrons as dots
Open →Acceleration
Class 9 Physics
Speed up over time, find acceleration
Open →Distance, speed and time
Class 8 Physics
Set speed & time, cover the distance
Open →Frequency and time period
Class 9 Physics
Shorten the period, raise the frequency
Open →Heating effect of current
Class 10 Physics
Raise the current, watch heating soar
Open →Area of a triangle
Class 8 Maths
Set base & height, halve the rectangle
Open →Area of a rectangle
Class 8 Maths
Set length & breadth, fill the area
Open →Mean (average)
Class 9 Maths
Share the total equally across items
Open →Discount
Class 8 Maths
Slide price & % off, see the saving
Open →Heart rate
Class 10 Biology
Set heart rate & time, count the beats
Open →Resistors in parallel
Class 10 Physics
Wire two resistors side by side
Open →Electric charge
Class 10 Physics
Flow current over time, collect charge
Open →Electrical energy and units
Class 10 Physics
Run appliances, add up the units
Open →Kelvin temperature scale
Class 9 Chemistry
Slide Celsius, read the Kelvin
Open →Moles from number of particles
Class 9 Chemistry
Divide particles by Avogadro's number
Open →Ten percent law
Class 10 Biology
See 10% of energy reach the next level
Open →Area of a square
Class 8 Maths
Grow the side, square the area
Open →Volume of a cube
Class 8 Maths
Grow the edge, cube the volume
Open →Circumference of a circle
Class 8 Maths
Grow the radius, roll out the rim
Open →Surface area of a cube
Class 9 Maths
Grow the edge, cover six faces
Open →Potential difference
Class 10 Physics
Share work across charge, get volts
Open →Resistance from Ohm's law
Class 10 Physics
Divide voltage by current, get resistance
Open →Echo and SONAR
Class 9 Physics
Time the echo, find the distance
Open →Mass from moles
Class 9 Chemistry
Multiply moles by molar mass
Open →Breathing rate
Class 10 Biology
Set breathing rate & time
Open →Volume of a cylinder
Class 10 Maths
Set radius & height, fill the can
Open →Compound interest
Class 8 Maths
Compound money over years
Open →Profit and loss percentage
Class 8 Maths
Set cost & selling price, see profit %
Open →Perimeter of a rectangle
Class 8 Maths
Set length & breadth, walk the border
Open →Surface area of a sphere
Class 10 Maths
Grow the radius, wrap the ball
Open →Time period
Class 9 Physics
Raise the frequency, shrink the period
Open →Relative velocity
Class 9 Physics
Two objects approach - add their speeds
Open →Average velocity
Class 9 Physics
Average the start and end speeds
Open →Equations of motion (v = u + at)
Class 9 Physics
Accelerate from u for a time t
Open →Kelvin to Celsius
Class 9 Chemistry
Slide Kelvin, read the Celsius
Open →Population growth rate
Class 10 Biology
Balance births vs deaths per population
Open →Perimeter of a square
Class 8 Maths
Grow the side, walk four edges
Open →Perimeter of a triangle
Class 8 Maths
Add the three sides
Open →Area of a parallelogram
Class 8 Maths
Set base & height, slide the shape
Open →Area of a rhombus
Class 8 Maths
Set the two diagonals
Open →Equations of motion (distance)
Class 9 Physics
Start, accelerate, cover ground
Open →Joule's law of heating
Class 10 Physics
Raise current, resistance or time
Open →Electric power (P = VI)
Class 10 Physics
Multiply voltage by current
Open →Average atomic mass of isotopes
Class 9 Chemistry
Mix two isotopes by abundance
Open →Seed germination percentage
Class 9 Biology
Count sprouted seeds out of the total
Open →Volume of a cone
Class 9 Maths
Set radius & height, fill the cone
Open →Surface area of a cylinder
Class 9 Maths
Wrap the side and both ends
Open →Surface area of a cuboid
Class 9 Maths
Cover all six rectangular faces
Open →nth term of an AP
Class 10 Maths
Step from the first term by d
Open →Sum of an AP
Class 10 Maths
Add up the first n terms
Open →Focal length of a mirror
Class 10 Physics
Halve the radius to find the focus
Open →Speed of light in a medium
Class 10 Physics
Raise the index, slow the light
Open →Percentage purity
Class 9 Chemistry
Weigh the pure part of a sample
Open →Slope of a line
Class 10 Maths
Rise over run gives the steepness
Open →Percentage change
Class 8 Maths
Compare a new value to the old
Open →Volume of a hemisphere
Class 9 Maths
Grow the radius of half a ball
Open →Area of a sector
Class 10 Maths
Cut a slice of angle from a circle
Open →Length of an arc
Class 10 Maths
Measure the curved edge of a slice
Open →Slant height of a cone
Class 9 Maths
Combine radius & height for the slant
Open →Unit conversion (km/h to m/s)
Class 9 Physics
Convert km/h into m/s
Open →Equations of motion (v^2 = u^2 + 2as)
Class 9 Physics
Accelerate over a distance, find v
Open →Impulse
Class 9 Physics
Hit harder or longer, change momentum
Open →Wavelength
Class 9 Physics
Speed over frequency gives wavelength
Open →Oscillations
Class 9 Physics
Vibrate at a frequency for a time
Open →Cost of electricity
Class 10 Physics
Units times rate gives the bill
Open →Number of neutrons
Class 9 Chemistry
Take protons away from the mass number
Open →Curved surface area of a cone
Class 9 Maths
Wrap the slanted side of a cone
Open →Total surface area of a cone
Class 9 Maths
Add the base circle to the cone's side
Open →Curved surface area of a hemisphere
Class 9 Maths
Cover the dome of a hemisphere
Open →Diagonal of a square
Class 9 Maths
Cross a square corner to corner
Open →Unit conversion (m/s to km/h)
Class 9 Physics
Convert m/s into km/h
Open →Distance from velocities
Class 9 Physics
From two speeds, find the distance
Open →Diagonal of a rectangle
Class 9 Maths
Cross a rectangle corner to corner
Open →Diagonal of a cuboid
Class 9 Maths
The longest rod that fits in a box
Open →Area by Heron's formula
Class 9 Maths
Area from just the three sides
Open →Interior angle sum of a polygon
Class 8 Maths
Add up a polygon's inside angles
Open →Exterior angle of a regular polygon
Class 8 Maths
Share 360 among a polygon's corners
Open →Number of diagonals of a polygon
Class 8 Maths
Count the diagonals of a polygon
Open →Discriminant
Class 10 Maths
Test how many roots a quadratic has
Open →Sum of roots
Class 10 Maths
Sum of a quadratic's roots
Open →Punnett square (monohybrid cross)
Class 10 Biology
Cross two parents, predict the offspring
Open →Balancing chemical equations
Class 10 Chemistry
Slide coefficients until atoms balance
Open →Writing chemical formulae (valency)
Class 9 Chemistry
Criss-cross valencies into a formula
Open →Current from power
Class 10 Physics
Divide power by voltage for current
Open →Power (P = V^2 / R)
Class 10 Physics
Voltage squared over resistance
Open →Sine ratio
Class 10 Maths
Opposite over hypotenuse
Open →Cosine ratio
Class 10 Maths
Adjacent over hypotenuse
Open →Tangent ratio
Class 10 Maths
Opposite over adjacent
Open →Area of an equilateral triangle
Class 9 Maths
Area of an equilateral triangle
Open →Curved surface area of a cylinder
Class 9 Maths
Wrap only the curved side
Open →Loss percentage
Class 8 Maths
Sell below cost, find the loss %
Open →Amount with simple interest
Class 8 Maths
Principal plus its simple interest
Open →Distance formula
Class 10 Maths
Straight distance between two points
Open →States of matter
Class 9 Chemistry
Heat particles solid → liquid → gas
Open →Parts of a plant cell
Class 8 Biology
Tap a cell part to see its job
Open →Diagonal of a cube
Class 9 Maths
Longest diagonal through a cube
Open →Total surface area of a hemisphere
Class 9 Maths
Dome plus its flat circle
Open →Sum of first n natural numbers
Class 10 Maths
Add 1 + 2 + ... + n instantly
Open →Range of data
Class 9 Maths
Spread from smallest to largest
Open →Class mark
Class 9 Maths
Midpoint of a class interval
Open →Selling price from profit percent
Class 8 Maths
Mark up cost by a profit %
Open →Perimeter of a sector
Class 10 Maths
Two radii plus the curved arc
Open →Circumference from diameter
Class 8 Maths
Circumference straight from diameter
Open →Power (P = F x v)
Class 9 Physics
Force times velocity gives power
Open →Percentage of a number
Class 8 Maths
Find a percentage of a number
Open →Series and parallel circuits
Class 10 Physics
Break a bulb in series vs parallel
Open →Symbols of elements
Class 9 Chemistry
Match each element to its symbol
Open →Free fall (velocity)
Class 9 Physics
Drop from a height, hit this speed
Open →Free fall (time)
Class 9 Physics
How long a drop takes
Open →Free fall (distance)
Class 9 Physics
Distance fallen in a given time
Open →Complement of an event
Class 10 Maths
Chance an event does NOT happen
Open →Product of roots
Class 10 Maths
Product of a quadratic's roots
Open →Exterior angle theorem
Class 9 Maths
Exterior angle = sum of remote interiors
Open →Complementary angles
Class 10 Maths
What adds to 90 degrees
Open →Supplementary angles
Class 9 Maths
What adds to 180 degrees
Open →Perimeter of a semicircle
Class 10 Maths
Curved half plus the diameter
Open →Area of a semicircle
Class 10 Maths
Half the area of a circle
Open →Turning effect (moments)
Class 9 Physics
Balance the see-saw with moments
Open →Reflex arc
Class 10 Biology
Step through a reflex, stimulus to action
Open →Buoyant force (upthrust)
Class 9 Physics
Displace liquid, feel the upthrust
Open →Relative density
Class 9 Physics
Compare a density to water's
Open →Power in lifting a load
Class 9 Physics
Lift a load, faster needs more power
Open →Cosecant ratio
Class 10 Maths
Hypotenuse over opposite
Open →Secant ratio
Class 10 Maths
Hypotenuse over adjacent
Open →Cotangent ratio
Class 10 Maths
Adjacent over opposite
Open →Height from angle of elevation
Class 10 Maths
Height from an angle of elevation
Open →Area of a quadrant
Class 10 Maths
A quarter of a circle's area
Open →Interior angle of a regular polygon
Class 8 Maths
One inside angle of a regular polygon
Open →Sum of first n odd numbers
Class 10 Maths
Add the first n odd numbers
Open →Sum of first n even numbers
Class 10 Maths
Add the first n even numbers
Open →Quadratic formula (a root)
Class 10 Maths
Larger root of a quadratic
Open →LCM from HCF
Class 10 Maths
LCM from the product and HCF
Open →Depreciation
Class 8 Maths
Value drops by a % each year
Open →Cost price from selling price
Class 8 Maths
Work back to the cost price
Open →Downstream speed
Class 8 Maths
Row with the current
Open →Upstream speed
Class 8 Maths
Row against the current
Open →Average speed for a round trip
Class 8 Maths
Average speed there and back
Open →Sales tax / GST
Class 8 Maths
Tax added on a price
Open →Area of a ring (annulus)
Class 10 Maths
Area of a ring between two circles
Open →Edge of a cube from volume
Class 9 Maths
Edge back from the volume
Open →Radius from area
Class 10 Maths
Radius back from a circle's area
Open →Side from area of a square
Class 8 Maths
Side back from a square's area
Open →Height of a triangle from area
Class 9 Maths
Height back from area and base
Open →Rate from simple interest
Class 8 Maths
Rate back from the interest
Open →Time from simple interest
Class 8 Maths
Time back from the interest
Open →Principal from simple interest
Class 8 Maths
Principal back from the interest
Open →Mean proportional
Class 10 Maths
Geometric mean of two numbers
Open →Fourth proportional
Class 8 Maths
Complete the proportion a : b = c : ?
Open →Marked price from selling price
Class 8 Maths
Marked price back from the sale price
Open →Chambers of the human heart
Class 10 Biology
Tap a heart chamber to see its job
Open →Equation of a line (y = mx + c)
Class 9 Maths
Read y off a straight line
Open →Average term of an AP
Class 10 Maths
Average of first and last term
Open →Number of terms in an AP
Class 10 Maths
How many terms in an AP
Open →Midpoint of two points
Class 10 Maths
x-coordinate of a midpoint
Open →Empirical mode
Class 10 Maths
Estimate the mode from mean & median
Open →Length of a shadow
Class 10 Maths
Shadow from height and sun angle
Open →Train crossing a pole
Class 8 Maths
Speed to cross a pole
Open →Time and work
Class 8 Maths
More workers, fewer days
Open →Dividing in a ratio
Class 8 Maths
Split a total in a ratio
Open →Unitary method
Class 8 Maths
Cost of a single item
Open →Your progress — Foundation - Class 8 to 10
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Showing 9 questions for Class 9. Tap a card to reveal the answer.
ChemistryMatter in Our SurroundingsmediumWhy does evaporation cause cooling?
Reveal answer ↓
What it is
When the fastest particles escape a liquid's surface, the particles left behind are cooler.
Answer
During evaporation the fastest-moving (most energetic) particles at the surface of a liquid escape into the air. This lowers the average kinetic energy of the remaining particles, so the temperature of the liquid falls. The escaping particles also absorb latent heat from the surroundings, which is why evaporation produces a cooling effect - for example, sweating cools our body.
- •Fast (energetic) particles leave the surface
- •Remaining particles have lower average energy
- •Latent heat absorbed from surroundings
- •Example: sweating cools the body
Why learn this
It's why sweating, earthen pots (matka) and desert coolers keep things cold without a fridge.
💡 Memory trick
The 'hot-heads' leave first, so what's left behind is cooler.
ChemistryMatter in Our SurroundingseasyConvert 25 C to the Kelvin scale. Slide the temperature to explore.
Reveal answer ↓
What it is
The Kelvin (absolute) temperature is the Celsius temperature plus 273.
Answer
Kelvin = Celsius + 273 = 25 + 273 = 298 K. The lowest possible temperature, absolute zero, is 0 K, which equals -273 C; that is why the Kelvin scale never goes negative.
K = C + 273
- •K = C + 273
- •0 K (absolute zero) = -273 C
- •The Kelvin scale has no negative values
Why learn this
Gas laws and scientific work use the Kelvin scale, which has no negative values.
💡 Memory trick
K = C + 273. To go back, C = K - 273.
ChemistryMatter in Our SurroundingseasyConvert 300 K to the Celsius scale. Slide the temperature to explore.
Reveal answer ↓
What it is
To convert a Kelvin temperature to Celsius, subtract 273.
Answer
Celsius = Kelvin - 273 = 300 - 273 = 27 C. So 300 K is a comfortable 27 C. This is simply the reverse of the rule K = C + 273.
C = K - 273
- •C = K - 273
- •Reverse of K = C + 273
- •300 K is about 27 C
Why learn this
Everyday temperatures are in Celsius, so we often convert back from Kelvin.
💡 Memory trick
C = K - 273. The reverse of K = C + 273.
ChemistryMatter in Our SurroundingseasyHow do the particles change as a solid is heated into a liquid and then a gas? Slide the heat to explore.
Reveal answer ↓
What it is
Matter exists as solid, liquid or gas depending on how much energy its particles have and how tightly they are held.
Particles are packed in a fixed pattern and only vibrate. A solid keeps its shape and volume.
Answer
In a solid, particles are packed in a fixed pattern and only vibrate, so it has a fixed shape and volume. Adding heat lets them break free into a liquid, where they stay close but can slide, keeping the volume but taking the container's shape. More heat spreads them far apart into a gas, which moves freely and fills all the space.
- •Solid: packed, fixed shape and volume
- •Liquid: close but flowing, fixed volume
- •Gas: far apart, fills all space
- •Heating increases particle energy
Why learn this
It explains melting, boiling and why gases fill any container.
💡 Memory trick
Add heat -> particles gain energy -> solid to liquid to gas.
ChemistryMatter in Our SurroundingseasyCompare the three states of matter in terms of shape, volume and compressibility.
Reveal answer ↓
What it is
Matter exists mainly in three states - solid, liquid and gas - that differ in shape, volume and compressibility.
Answer
A solid has a fixed shape and a fixed volume and is almost incompressible, because its particles are closely packed with strong forces. A liquid has a fixed volume but no fixed shape (it takes the shape of its container) and is only slightly compressible. A gas has neither a fixed shape nor a fixed volume and is highly compressible, because its particles are far apart with very weak forces.
- •Solid: fixed shape and volume, incompressible
- •Liquid: fixed volume, no fixed shape
- •Gas: no fixed shape or volume, highly compressible
- •Particle spacing and forces decide the state
Why learn this
It explains why ice is rigid, water flows and steam fills any container.
💡 Memory trick
Solid = fixed shape and volume; Liquid = fixed volume, no shape; Gas = neither fixed.
ChemistryMatter in Our SurroundingsmediumState the main characteristics of the particles of matter.
Reveal answer ↓
What it is
Particles of matter are very small, have spaces between them, are continuously moving and attract each other.
Answer
The particles of matter are extremely small in size. They have spaces (intermolecular gaps) between them, which is why substances like sugar dissolve in water. They are continuously moving, that is, they possess kinetic energy that increases with temperature, which causes diffusion. They also attract each other with a force of attraction that holds them together, being strongest in solids and weakest in gases.
- •Particles are very small
- •Have spaces between them
- •Are continuously moving (kinetic energy)
- •Attract each other (strongest in solids)
Why learn this
These ideas explain diffusion, dissolving and the states of matter.
💡 Memory trick
Tiny, spaced, moving, attracting - the four key facts about particles.
ChemistryMatter in Our SurroundingshardWhat is latent heat of fusion? Why does the temperature stay constant during melting?
Reveal answer ↓
What it is
Latent heat is the heat absorbed or released during a change of state at constant temperature.
Answer
The latent heat of fusion is the amount of heat required to change 1 kg of a solid into liquid at its melting point without any change in temperature. During melting, the heat supplied is used to overcome the forces of attraction between the particles and to increase their spacing, rather than to raise their kinetic energy, so the temperature remains constant until all the solid has melted.
Heat = mass x latent heat (Q = mL)
- •Latent heat = heat for change of state, no temperature change
- •Fusion: solid to liquid at melting point
- •Heat overcomes forces between particles
- •Temperature is constant during the change
Why learn this
It is why ice at 0 degrees Celsius cools a drink better than water at 0 degrees Celsius.
💡 Memory trick
Latent heat is 'hidden' heat - the temperature stays constant while the state changes.
ChemistryMatter in Our SurroundingsmediumWhat is evaporation? State the factors that affect its rate.
Reveal answer ↓
What it is
Evaporation is the change of a liquid into vapour at any temperature below its boiling point, occurring at the surface.
Answer
Evaporation is the process by which a liquid changes into its vapour at any temperature below its boiling point; it takes place only from the surface of the liquid. The rate of evaporation increases with an increase in surface area, an increase in temperature, an increase in wind speed, and a decrease in humidity. Evaporation causes cooling because the fastest particles escape, taking energy with them and lowering the temperature of the remaining liquid.
- •Liquid to vapour below boiling point, at the surface
- •Faster with more surface area and higher temperature
- •Faster with more wind, less humidity
- •Evaporation causes cooling
Why learn this
It explains why wet clothes dry and why sweating cools the body.
💡 Memory trick
Evaporation increases with more surface area, higher temperature, more wind, lower humidity.
ChemistryMatter in Our SurroundingsmediumHow do temperature and pressure affect the state of matter?
Reveal answer ↓
What it is
Raising temperature or lowering pressure can change a solid to liquid to gas; the reverse can liquefy gases.
Answer
Increasing the temperature gives particles more kinetic energy, so they move apart and a solid can change to a liquid (melting) and a liquid to a gas (boiling). Decreasing the temperature does the reverse. Increasing the pressure pushes gas particles closer together and can turn a gas into a liquid, while decreasing pressure allows particles to move apart. This is why gases such as LPG are stored as liquids under high pressure.
- •High temperature: solid -> liquid -> gas
- •Low temperature: reverse changes
- •High pressure can liquefy a gas
- •LPG stored as liquid under pressure
Why learn this
It is how gases like LPG are stored as liquids under pressure.
💡 Memory trick
Heat -> particles move apart (solid to gas). Compress + cool -> gas turns to liquid.
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