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SSC-Standard General Studies Foundation · Chapter 8

Class-10 Physics and Chemistry: Quick Revision

What to remember

  • Light: reflection and refraction follow fixed laws. Concave mirrors and convex lenses can converge light; convex mirrors and concave lenses diverge it.
  • Electricity and magnetism: Ohm's law is V = IR. A current produces a magnetic field, and a changing magnetic field produces a current (electromagnetic induction).
  • Chemistry: acids turn blue litmus red, bases turn red litmus blue. Reactive metals displace less reactive ones. Carbon forms millions of compounds because of catenation and strong covalent bonds.

Light

Reflection: the angle of incidence equals the angle of reflection, and the incident ray, reflected ray and normal lie in one plane.

Mirror / lensNatureUses
Plane mirrorVirtual, same size, laterally inverted imageEveryday mirror
Concave mirrorConverges light; can form real or virtual imagesShaving mirror, torch and headlight reflector, dentist's mirror
Convex mirrorDiverges light; image always virtual, small, erectVehicle rear-view mirror (wide field of view)
Convex lensConverges lightMagnifying glass, camera, hypermetropia correction
Concave lensDiverges lightCorrection of myopia (short sight)

Mirror formula: 1/v + 1/u = 1/f. The focal length f = R/2, where R is the radius of curvature. Magnification m = image height/object height = -v/u. Lens formula: 1/v - 1/u = 1/f. Power of a lens P = 1/f (f in metres); its unit is the dioptre (D). A convex lens has positive power; a concave lens has negative power.

Refraction is the bending of light when it goes from one medium to another because its speed changes. Light bends toward the normal when it goes from a rarer to a denser medium. Refractive index n = speed of light in vacuum/speed of light in the medium. Snell's law: sin i / sin r = constant. Glass slab refraction gives a sideways shift of the emerging ray, but it is parallel to the incident ray.

Total internal reflection occurs when light goes from a denser to a rarer medium at an angle greater than the critical angle. It is used in optical fibres, prisms and diamonds' sparkle. Mirage also involves it.

Eye defects: myopia (near-sighted; corrected by concave lens), hypermetropia (far-sighted; corrected by convex lens), presbyopia (old-age; bifocal lens) and cataract (cloudy lens, treated by surgery). Dispersion is the splitting of white light into seven colours (VIBGYOR) by a prism. A rainbow is caused by dispersion, refraction and total internal reflection in raindrops. The sky looks blue because of the scattering of light; the sun looks red at sunrise and sunset because blue light is scattered away most.

Electricity

  • Electric current I = Q/t. The SI unit is the ampere. Potential difference V is measured in volts. Resistance R is in ohms.
  • Ohm's law: V = IR at constant temperature.
  • Resistivity depends on the material, not on the length or area of the wire. Resistance is directly proportional to length and inversely proportional to area of cross-section.
  • Series circuit: the same current passes through all; total resistance is the sum. Parallel circuit: voltage is the same across each branch; the total resistance is less than the smallest one. House wiring is parallel.
  • Electric power P = VI = I²R. The unit is the watt. Energy is measured in kilowatt-hour (kWh), which is one "unit" of electricity (1 kWh = 3.6 × 10⁶ joule).
  • Heating effect: H = I²Rt (Joule's law). Used in heaters and electric irons. The filament of a bulb is made of tungsten because of its high melting point. A fuse is made of a low melting point wire and breaks the circuit when too much current flows.
  • An ammeter is connected in series; a voltmeter is connected in parallel.

Magnetism and electromagnetism

  • A magnet has two poles. Like poles repel; unlike poles attract. Magnetic field lines go from the north pole to the south pole outside the magnet and never cross.
  • Oersted found that a current-carrying wire gives a magnetic field. The direction is given by the right-hand thumb rule.
  • A solenoid is a long coil that acts like a bar magnet. An electromagnet has a soft iron core inside a solenoid.
  • Force on a current-carrying conductor in a magnetic field is given by Fleming's left-hand rule. It is the principle of an electric motor, which changes electrical energy to mechanical energy.
  • Electromagnetic induction (Faraday): a changing magnetic field makes a current in a coil. Direction is given by Fleming's right-hand rule. This is the principle of the electric generator, which changes mechanical energy to electrical energy.
  • AC and DC: AC changes direction regularly (in India, 50 Hz); DC flows one way. Earth wire provides safety in a household circuit; the colour of the earth wire is green.

Energy

  • Work W = F × d. The unit is the joule. Power is work per unit time. The unit is the watt.
  • Kinetic energy = 1/2 mv². Potential energy = mgh.
  • Law of conservation of energy: energy is neither created nor destroyed; it changes form.
  • Sources: renewable (solar, wind, hydro, biomass, tidal, geothermal) and non-renewable (coal, oil). A solar cell changes light to electricity. Nuclear fission splits heavy nuclei (uranium); nuclear fusion joins light nuclei (the Sun's energy source).
  • Gobar gas (biogas) has methane as the chief part.

Chemical reactions

Types of reactions

TypeMeaningExample
CombinationTwo or more substances form oneBurning of magnesium; quicklime and water
DecompositionOne substance breaks into simpler onesHeating of limestone
DisplacementA more reactive element replaces a less reactive oneIron nail in copper sulphate
Double displacementExchange of ions between two compoundsSodium sulphate and barium chloride
OxidationGain of oxygen or loss of electronsRusting
ReductionLoss of oxygen or gain of electronsCopper oxide with hydrogen

A reaction in which oxidation and reduction occur together is a redox reaction. Exothermic reactions release heat; endothermic reactions absorb heat. Rusting is the oxidation of iron in the presence of air and moisture; painting, galvanising and oiling prevent it. A balanced equation obeys the law of conservation of mass.

Acids, bases and salts

  • Acids: sour, turn blue litmus red; give H⁺ ions. Examples: HCl, H₂SO₄, HNO₃, citric acid (lemon), acetic acid (vinegar), lactic acid (curd), tartaric acid (tamarind).
  • Bases: bitter, soapy; turn red litmus blue; give OH⁻ ions. Soluble bases are called alkalis (NaOH, KOH).
  • pH scale: 0 to 14. pH below 7 is acidic; 7 is neutral; above 7 is basic. Blood pH is about 7.4. Tooth decay begins when pH falls below about 5.5.
  • Neutralisation: acid + base gives salt + water. Antacids (such as milk of magnesia) treat acidity; baking soda treats bee sting; an acid is used for wasp sting.
CompoundFormulaUse
Baking sodaNaHCO₃Antacid, baking
Washing sodaNa₂CO₃·10H₂OCleaning
Bleaching powderCaOCl₂Disinfecting water, bleaching
Plaster of ParisCaSO₄·½H₂OCasts, statues
Common saltNaClRaw material for NaOH, chlorine, baking soda

Metals and non-metals

  • Metals are lustrous, malleable, ductile and good conductors. Mercury is the liquid metal; gold is the most malleable and ductile. Sodium and potassium are stored in kerosene, as they react with air.
  • Non-metals are dull, brittle and poor conductors. Exceptions: graphite conducts electricity; iodine is lustrous; bromine is a liquid non-metal.
  • Reactivity series (high to low): K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Ag, Au.
  • Ionic compounds have high melting points and conduct electricity when molten or in solution.
  • Extraction: low-activity metals (Hg) by heating the ore; middle metals (Zn, Fe) by reduction with carbon; high-activity metals (Na, Al) by electrolysis.
  • Alloys are mixtures of metals: brass (copper and zinc), bronze (copper and tin), steel (iron and carbon), solder (lead and tin). Amalgam has mercury.

Carbon and its compounds

  • Carbon forms covalent bonds by sharing electrons. It shows catenation (links with itself) and tetravalency (four bonds).
  • Allotropes: diamond (hardest; each carbon bonded to four others), graphite (soft; conducts), fullerene (C-60).
  • Hydrocarbons: saturated (alkanes; single bonds; methane CH₄, ethane C₂H₆), unsaturated (alkenes with a double bond, such as ethene; alkynes with a triple bond, such as ethyne).
  • Functional groups: alcohol (-OH), aldehyde (-CHO), ketone (>C=O), carboxylic acid (-COOH).
  • Ethanol is made by fermentation of sugars. Ethanoic acid (acetic acid) 5-8 per cent in water is vinegar. Ethanol with ethanoic acid gives an ester (sweet smell); this is esterification.
  • Soap is a sodium or potassium salt of long-chain fatty acid. It forms micelles to clean oily dirt. In hard water, soap forms scum; detergents do not.
  • Combustion of carbon compounds gives carbon dioxide and water. Saturated hydrocarbons burn with a clean blue flame; unsaturated ones with a sooty yellow flame.

Exam traps

  • Concave mirror can form real or virtual images; convex mirror always forms a virtual image.
  • Concave lens corrects myopia; convex lens corrects hypermetropia.
  • Series: the same current; parallel: the same voltage.
  • A motor uses Fleming's left-hand rule; a generator uses Fleming's right-hand rule.
  • Motor converts electrical to mechanical energy; generator does the opposite.
  • Oxidation is gain of oxygen; reduction is loss of oxygen.
  • Diamond is hard and does not conduct; graphite is soft and conducts.
  • Baking soda is NaHCO₃; washing soda is Na₂CO₃.

One-liners

  • 1. Focal length equals half the radius of curvature.
  • 2. The SI unit of lens power is the dioptre.
  • 3. Total internal reflection is used in optical fibres.
  • 4. White light splits into VIBGYOR in a prism.
  • 5. Ohm's law is V = IR.
  • 6. Household circuits are connected in parallel.
  • 7. One kilowatt-hour is one unit of electricity.
  • 8. A fuse has a low melting point.
  • 9. An electric generator works on electromagnetic induction.
  • 10. The pH of a neutral solution is 7.
  • 11. Rusting is the oxidation of iron.
  • 12. Alkanes have only single bonds between carbon atoms.

Practice questions

  1. The focal length of a spherical mirror is equal to

    1. One-third of the radius of curvature
    2. Half of the radius of curvature
    3. Twice the radius of curvature
    4. The radius of curvature
    Answer

    B. Half of the radius of curvature

    f = R/2.

  2. A mirror used as a vehicle rear-view mirror is

    1. Concave
    2. Plane only
    3. Parabolic
    4. Convex
    Answer

    D. Convex

    A convex mirror gives a wide field of view and an erect image.

  3. The image formed by a convex mirror is always

    1. Real and enlarged
    2. Real and inverted
    3. Virtual, erect and diminished
    4. Virtual and enlarged
    Answer

    C. Virtual, erect and diminished

    A convex mirror cannot form a real image of a real object.

  4. Myopia is corrected using a

    1. Convex lens
    2. Concave lens
    3. Plane mirror
    4. Bifocal prism
    Answer

    B. Concave lens

    A concave lens diverges rays before they enter the eye.

  5. Hypermetropia is corrected using a

    1. Cylindrical mirror
    2. Prism
    3. Concave lens
    4. Convex lens
    Answer

    D. Convex lens

    A convex lens converges light on the retina.

  6. The SI unit of power of a lens is

    1. Candela
    2. Dioptre
    3. Metre
    4. Watt
    Answer

    B. Dioptre

    P = 1/f with f in metres.

  7. The power of a lens of focal length 0.5 m is

    1. +5 D
    2. -2 D
    3. +2 D
    4. +0.5 D
    Answer

    C. +2 D

    P = 1/f = 1/0.5 = 2 D for a convex lens.

  8. The principle behind optical fibres is

    1. Dispersion
    2. Total internal reflection
    3. Scattering
    4. Diffraction
    Answer

    B. Total internal reflection

    Light stays inside the fibre by repeated total reflection.

  9. Splitting of white light into colours by a prism is

    1. Scattering
    2. Interference
    3. Dispersion
    4. Reflection
    Answer

    C. Dispersion

    Different colours bend by different amounts.

  10. The sky appears blue mainly due to

    1. Refraction in the ozone layer
    2. Reflection by clouds
    3. Dispersion by dust
    4. Scattering of light
    Answer

    D. Scattering of light

    Short blue waves are scattered the most.

  11. The SI unit of electric current is

    1. Volt
    2. Ohm
    3. Watt
    4. Ampere
    Answer

    D. Ampere

    Current is charge flowing per second.

  12. According to Ohm's law, at constant temperature,

    1. V is proportional to I
    2. V is proportional to 1/I
    3. R is proportional to I
    4. V does not depend on I
    Answer

    A. V is proportional to I

    V = IR.

  13. When resistors are connected in parallel, the total resistance is

    1. Equal to the largest one
    2. Equal to the sum
    3. Less than the smallest resistance
    4. Always zero
    Answer

    C. Less than the smallest resistance

    Parallel paths lower the net resistance.

  14. Domestic wiring is connected in

    1. Series
    2. A single loop
    3. Parallel
    4. Delta only
    Answer

    C. Parallel

    Each appliance gets full voltage and works independently.

  15. One kilowatt-hour is equal to

    1. 3.6 × 10^3 joule
    2. 3.6 × 10^6 joule
    3. 1000 joule
    4. 36 joule
    Answer

    B. 3.6 × 10^6 joule

    1000 W × 3600 s = 3.6 million joule.

  16. A fuse wire is made of a material with

    1. High resistance and no heating
    2. Low melting point
    3. High melting point
    4. Magnetic properties
    Answer

    B. Low melting point

    It melts to break the circuit during overload.

  17. An ammeter is connected in

    1. Series
    2. Parallel
    3. Either way
    4. Across the battery only
    Answer

    A. Series

    Current must pass through the ammeter.

  18. An electric motor works on the force on

    1. A fuse
    2. A static charge
    3. A battery
    4. A current-carrying conductor in a magnetic field
    Answer

    D. A current-carrying conductor in a magnetic field

    Fleming's left-hand rule gives the direction.

  19. An electric generator works on

    1. Electromagnetic induction
    2. Chemical effect
    3. Total reflection
    4. Heating effect
    Answer

    A. Electromagnetic induction

    Faraday discovered induction.

  20. The direction of induced current is given by

    1. Right-hand thumb rule
    2. Snell's law
    3. Fleming's left-hand rule
    4. Fleming's right-hand rule
    Answer

    D. Fleming's right-hand rule

    The right-hand rule applies to generators.

  21. The frequency of AC supply in India is

    1. 100 Hz
    2. 60 Hz
    3. 50 Hz
    4. 25 Hz
    Answer

    C. 50 Hz

    AC reverses direction 100 times per second.

  22. The Sun's energy comes from

    1. Burning of coal
    2. Nuclear fusion
    3. Chemical reactions
    4. Nuclear fission
    Answer

    B. Nuclear fusion

    Hydrogen nuclei fuse to form helium.

  23. The main component of biogas is

    1. Oxygen
    2. Methane
    3. Hydrogen
    4. Carbon monoxide
    Answer

    B. Methane

    Biogas comes from decaying organic waste.

  24. Rusting of iron is an example of

    1. Oxidation
    2. Decomposition
    3. Neutralisation
    4. Reduction
    Answer

    A. Oxidation

    Iron combines with oxygen and moisture.

  25. Placing an iron nail in copper sulphate solution is a

    1. Combination reaction
    2. Neutralisation
    3. Displacement reaction
    4. Decomposition reaction
    Answer

    C. Displacement reaction

    Iron is more reactive and displaces copper.

  26. The pH of a neutral solution is

    1. 0
    2. 7
    3. 14
    4. 1
    Answer

    B. 7

    Below 7 is acidic, above 7 is basic.

  27. Which acid is present in curd?

    1. Tartaric acid
    2. Acetic acid
    3. Citric acid
    4. Lactic acid
    Answer

    D. Lactic acid

    Curd contains lactic acid.

  28. The chemical formula of baking soda is

    1. NaOH
    2. Na2CO3
    3. CaOCl2
    4. NaHCO3
    Answer

    D. NaHCO3

    Washing soda is Na2CO3.

  29. Bleaching powder is

    1. CaOCl2
    2. NaCl
    3. CaCO3
    4. CaSO4
    Answer

    A. CaOCl2

    It is used to disinfect water.

  30. Plaster of Paris is

    1. Sodium carbonate
    2. Calcium chloride
    3. Calcium sulphate hemihydrate
    4. Calcium carbonate
    Answer

    C. Calcium sulphate hemihydrate

    It is CaSO4·1/2 H2O and is used for casts.

  31. The liquid metal at room temperature is

    1. Bromine
    2. Mercury
    3. Sodium
    4. Potassium
    Answer

    B. Mercury

    Bromine is a liquid non-metal.

  32. Sodium is stored under kerosene because it

    1. Is radioactive
    2. Dissolves in oil
    3. Is a gas
    4. Reacts vigorously with air and water
    Answer

    D. Reacts vigorously with air and water

    It is highly reactive.

  33. An alloy of copper and zinc is

    1. Steel
    2. Bronze
    3. Brass
    4. Solder
    Answer

    C. Brass

    Bronze is copper and tin.

  34. Which allotrope of carbon conducts electricity?

    1. Graphite
    2. Quartz
    3. Silica glass
    4. Diamond
    Answer

    A. Graphite

    Graphite has free electrons between layers.

  35. The functional group of carboxylic acids is

    1. -OH
    2. -CHO
    3. -COOH
    4. -NH2
    Answer

    C. -COOH

    Ethanoic acid has -COOH.

  36. Consider the statements about mirrors and lenses. 1. A concave mirror can form both real and virtual images. 2. A concave lens converges light.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    A concave lens diverges light.

  37. Consider the statements about electric circuits. 1. In a series circuit, the same current flows through all components. 2. In a parallel circuit, the current is the same in all branches.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    In parallel circuits, the voltage is the same, not the current.

  38. Consider the statements about acids and bases. 1. Acids turn blue litmus red. 2. Bases give H+ ions in water.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Bases give OH- ions.

  39. Consider the statements about metals. 1. Gold is highly malleable and ductile. 2. All non-metals are poor conductors without exception.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Graphite conducts electricity.

  40. Consider the statements about carbon compounds. 1. Alkanes contain only single bonds. 2. Soap forms scum with hard water.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

  41. Consider the statements about motors and generators. 1. A motor changes electrical energy into mechanical energy. 2. A generator changes electrical energy into mechanical energy.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    A generator does the opposite.

  42. Consider the statements about reactions. 1. Burning of magnesium is a combination reaction. 2. In a redox reaction only oxidation takes place.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Oxidation and reduction occur together.

  43. Consider the statements about the refractive index. 1. It is the ratio of the speed of light in vacuum to its speed in the medium. 2. A ray bends away from the normal when entering a denser medium.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    It bends toward the normal in a denser medium.

  44. Match the device (left) with the principle (right). P. Motor Q. Generator R. Optical fibre S. Fuse

    1. P-force on conductor in magnetic field, Q-electromagnetic induction, R-heating effect, S-total internal reflection
    2. P-force on conductor in magnetic field, Q-electromagnetic induction, R-total internal reflection, S-heating effect
    3. P-electromagnetic induction, Q-force on conductor in magnetic field, R-total internal reflection, S-heating effect
    4. P-total internal reflection, Q-electromagnetic induction, R-force on conductor in magnetic field, S-heating effect
    Answer

    B. P-force on conductor in magnetic field, Q-electromagnetic induction, R-total internal reflection, S-heating effect

    Each device matches its working principle in option A.

  45. Match the substance (left) with its use (right). P. Baking soda Q. Bleaching powder R. Plaster of Paris S. Washing soda

    1. P-setting broken bones, Q-disinfecting water, R-antacid, S-cleaning clothes
    2. P-disinfecting water, Q-antacid, R-setting broken bones, S-cleaning clothes
    3. P-antacid, Q-disinfecting water, R-cleaning clothes, S-setting broken bones
    4. P-antacid, Q-disinfecting water, R-setting broken bones, S-cleaning clothes
    Answer

    D. P-antacid, Q-disinfecting water, R-setting broken bones, S-cleaning clothes

    These are standard uses.

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