CBSE Class 9 Science Question Paper Set 36 with Solutions

Answer: (b) SO₄²⁻. The sulphate ion has one sulphur and four oxygen atoms and carries a charge of 2−.

Teacher's Note:
1. Learn the common polyatomic ions with their charges as a set, for example NH₄⁺, OH⁻, NO₃⁻, CO₃²⁻, SO₄²⁻.
2. SO₂ is a neutral molecule (sulphur dioxide), not an ion.

Answer: (b) 40 u. Na + O + H = 23 + 16 + 1 = 40 u.

Teacher's Note:
1. Formula unit mass is the sum of the atomic masses of all atoms in the formula of an ionic compound.
2. Do not forget the hydrogen atom in OH.

Answer: (a) Starch solution. Starch particles are 1 nm to 1000 nm in size, so they stay spread through the water and scatter light.

Teacher's Note:
1. CuSO₄ in water is a true solution, and chalk powder in water is a suspension that settles.
2. Kerosene and water are immiscible and separate into layers.

Answer: (b) 7 atoms. 2 phosphorus atoms + 5 oxygen atoms = 7 atoms.

Teacher's Note:
1. The subscript after each symbol tells how many atoms of that element are present.
2. Add all subscripts to get the total number of atoms in one molecule.

Answer: (a) Collenchyma. Its cells have thickened corners, giving mechanical support while still allowing bending.

Teacher's Note:
1. Sclerenchyma is hard and stiff because of lignin, so it gives strength but not flexibility.
2. Remember: collenchyma means flexible support, sclerenchyma means rigid support.

Answer: (d) Adipose tissue. It is a type of connective tissue found below the skin and around organs, and it stores fat.

Teacher's Note:
1. Adipose tissue also acts as an insulator and cushions internal organs.
2. Phloem is a plant tissue, so it cannot be the answer for the human body.

Answer: (b) Mitochondria. They release energy from food in the form of ATP.

Teacher's Note:
1. ATP is called the energy currency of the cell.
2. Plastids are found only in plant cells, while mitochondria are found in nearly all cells.

Answer: (d) (D). For a body at rest the distance does not change with time, so the graph is a horizontal line.

Teacher's Note:
1. A rising straight line means uniform speed, and a curve means changing speed.
2. The slope of a distance-time graph gives the speed, and zero slope means zero speed.

Answer: (a) 6 m/s. Starting from rest, u = 0, so v = u + at = 0 + 2 × 3 = 6 m/s.

Teacher's Note:
1. "After the start" means the initial velocity is zero.
2. Check that all quantities are in SI units before substituting.

Answer: (a) move forwards. The oil has inertia of motion, so it tends to continue moving forward when the tanker suddenly stops.

Teacher's Note:
1. This is the law of inertia: bodies resist a change in their state of motion.
2. The same idea explains why passengers lurch forward when a bus brakes suddenly.

Answer: (a) h = v²/2g. Setting mgh = ½mv² and cancelling m gives h = v²/(2g).

Teacher's Note:
1. Equate the two energy expressions and cancel the common mass.
2. Check units: v²/g has units of m²/s² ÷ m/s² = m, so it is a length.

Answer: (d) A is false but R is true. Camphor burns almost completely and leaves hardly any residue, while it is true that camphor sublimes on heating.

Teacher's Note:
1. Judge the assertion and the reason separately before checking whether one explains the other.
2. Sublimation is a physical change, whereas burning is a chemical change.

Answer: (b) Both A and R are correct, but R is not the correct explanation for A. An atom is electrically neutral because the positive charge of protons equals the negative charge of electrons; merely being made of sub-atomic particles does not explain this.

Teacher's Note:
1. Ask whether the reason answers "why" the assertion is true.
2. Electrical neutrality of the atom is the real explanation here.

Answer: 1. Milk in water is a colloid. Its particles are between 1 nm and 1000 nm, big enough to scatter a beam of light, so the path of the beam becomes visible (Tyndall effect).
2. Copper sulphate in water is a true solution. Its particles are smaller than 1 nm, too small to scatter light, so the beam passes through without being seen.

Teacher's Note:
1. The key point is the size of the dispersed particles.
2. Always compare the particle size with the wavelength of visible light when explaining scattering.

Answer: Radius r = 10 cm, and half the circle is covered.
(a) Distance = half the circumference = πr = 3.14 × 10 = 31.4 cm.
(b) Displacement = shortest straight-line distance between the two points = diameter = 2r = 20 cm, directed from the starting point to the opposite point.

Teacher's Note:
1. Distance is the length of the path taken, while displacement is the straight line from start to end.
2. Draw a quick sketch of the half circle and its diameter to avoid mixing them up.

Answer: Free fall is the motion of a body falling towards the earth under the influence of gravity alone, with no other force (such as air resistance) acting on it; its acceleration is g = 9.8 m/s².
Mass of the object = 10 kg (same everywhere). g_moon = 9.8/6 ≈ 1.63 m/s².
Weight on the moon = m × g_moon = 10 × 1.63 ≈ 16.3 N (about one-sixth of its weight on the earth, 98 N).

OR
1. Mass is the quantity of matter in a body; weight is the force with which the earth (or another body) attracts it.
2. Mass is a scalar; weight is a vector (acts towards the centre).
3. Mass is constant everywhere; weight changes from place to place.
4. Mass is measured in kg using a beam balance; weight is measured in newton using a spring balance.

Teacher's Note:
1. Mass stays the same on the moon but weight becomes one-sixth because g is smaller there.
2. Remember weight = mass × g, and give the answer in newton, not kg.

Answer: (a) Ammonium NH₄⁺ (charge 1) and sulphate SO₄²⁻ (charge 2). Criss-crossing the charges gives (NH₄)₂SO₄.
(b) Calcium Ca²⁺ and carbonate CO₃²⁻. The charges are equal and cancel, so the formula is CaCO₃.
(c) Sodium Na⁺ and sulphate SO₄²⁻. Criss-crossing gives Na₂SO₄.

Teacher's Note:
1. Write the symbols with charges, swap the numerical charges as subscripts, and reduce to the simplest ratio.
2. Put a polyatomic ion in brackets when its subscript is more than 1.

Answer: 1. A beam of fast-moving alpha (α) particles from a radioactive source was directed at a very thin gold foil in an evacuated chamber.
2. A circular fluorescent zinc sulphide screen around the foil was used to detect the deflected and undeflected particles by the flashes they produced.
3. Observations: most α-particles passed straight through, a few were deflected through small angles, and about 1 in 12000 bounced back almost 180°. From this he concluded that an atom has a tiny, dense, positively charged nucleus with mostly empty space around it.

OR
Bohr's model (postulates):
1. Electrons revolve around the nucleus only in certain fixed circular paths called orbits or energy levels (shells), K, L, M, N.
2. While an electron stays in a particular orbit it does not radiate energy, so these orbits are called stationary or discrete orbits.
3. Energy is absorbed or emitted only when an electron jumps from one orbit to another.
4. The maximum number of electrons in a shell is 2n² (n = shell number).

Teacher's Note:
1. Use the sequence: set-up, detection, observation to organise the answer.
2. For Bohr's model, the key word is stable (non-radiating) orbits, which fixed the flaw in Rutherford's model.

Answer: Second law: The rate of change of momentum of a body is directly proportional to the applied unbalanced force and takes place in the direction of the force.
Derivation: Let a body of mass m change its velocity from u to v in time t. Initial momentum = mu, final momentum = mv. Rate of change of momentum = (mv − mu)/t = m(v − u)/t = ma. So F ∝ ma, i.e. F = kma. With the unit of force chosen so that k = 1, F = ma.
Numerical: The velocity is constant, so a = 0. F = ma = 2 × 0 = 0 N. No force is required because the table is frictionless and the object keeps moving by inertia.

Teacher's Note:
1. Write momentum as p = mv and use a = (v − u)/t to reach F = ma.
2. Constant velocity means zero acceleration, which means zero net force, and a common mistake is to use F = mv.

Answer: Property: Reflection of sound from a hard surface such as a wall or cliff.
Conditions:
1. The time gap between the original sound and the reflected sound must be at least 0.1 s, because the sensation of sound persists in the brain for about 0.1 s.
2. The reflecting surface must be at least 17.2 m away from the source (speed of sound in air about 344 m/s at 22 °C, so the sound travels 2 × 17.2 m in 0.1 s). The reflector should also be large and hard.
Curved ceilings and walls: A curved surface reflects sound towards all parts of the hall, so it reaches the audience evenly without losing intensity.

OR
Ultrasonic waves are sound waves of frequency higher than 20 000 Hz, which humans cannot hear.
Uses:
1. Cleaning parts that are hard to reach, such as spiral tubes and electronic components, by placing them in a cleaning solution and passing ultrasound; dust and grease get detached.
2. Detecting cracks and flaws in metal blocks used in bridges, machines and buildings, since ultrasonic waves are reflected by the defect.

Teacher's Note:
1. The minimum distance 17.2 m comes from d = (344 × 0.1)/2, so remember the half.
2. Curved reflectors spread or focus sound, and the 20 kHz limit is the dividing line for ultrasound.

Answer: (a) C₁₂H₂₂O₁₁ = 12 × 12 + 22 × 1 + 11 × 16 = 144 + 22 + 176 = 342 u.
(b) K₂CO₃ = 2 × 39 + 12 + 3 × 16 = 78 + 12 + 48 = 138 u.

OR
(a) Baking powder (sodium hydrogencarbonate, NaHCO₃): sodium, hydrogen, carbon, oxygen.
(b) Hydrogen sulphide (H₂S): hydrogen, sulphur.
(c) Calcium chloride (CaCl₂): calcium, chlorine.
(d) Potassium hydroxide (KOH): potassium, oxygen, hydrogen.
(e) Aluminium oxide (Al₂O₃): aluminium, oxygen.

Teacher's Note:
1. Multiply each atomic mass by the number of atoms of that element and then add.
2. In the OR part, write the formula first, because the symbols in it tell you the elements.

Answer: Definition: Kinetic energy is the energy possessed by a body due to its motion.
Derivation: Let a body of mass m at rest (u = 0) be acted on by a constant force F so that it acquires velocity v after covering displacement s with acceleration a.
1. From the third equation of motion: v² = u² + 2as, so with u = 0, s = v²/(2a).
2. Work done W = F × s = ma × v²/(2a) = ½mv².
3. This work is stored as kinetic energy, so KE = ½mv².
Ratio: KE₁ = ½mv², KE₂ = ½m(3v)² = 9 × ½mv².
KE₁ : KE₂ = ½mv² : 9 × ½mv² = 1 : 9.

Teacher's Note:
1. Link the work done by the force to the energy gained by the body, W = KE.
2. Kinetic energy depends on the square of velocity, so tripling the velocity makes the energy 9 times.

Answer: (a) A, D and E. Their outermost shells are complete (duplet for A; octet for D and E), so they are helium, neon and argon, which are noble gases.
(b) E (2, 8, 8) has three shells (K, L, M).
(c) C has 2 + 4 = 6 electrons, so it is carbon; E has 2 + 8 + 8 = 18 electrons, so it is argon.
Diagram: C: nucleus (6 protons, 6 neutrons) at the centre with 2 electrons in the K shell and 4 electrons in the L shell. E: nucleus (18 protons, 22 neutrons) at the centre with 2 electrons in the K shell, 8 in the L shell and 8 in the M shell.

Teacher's Note:
1. Neutral atoms have equal protons and electrons, so adding the electrons gives the atomic number.
2. A full outer shell (2 for the first shell, 8 for others) means the element is unreactive.

Answer: (a) Leucoplast. It stores food materials such as starch, oils and proteins.
(b) Plastid (for example chloroplast, which contains chlorophyll for photosynthesis).
(c) Ribosomes are synthesised in the nucleolus, a dense region inside the nucleus. The nucleolus makes the ribosomal RNA and proteins, which assemble into ribosomal units that then move out through the nuclear pores into the cytoplasm.

Teacher's Note:
1. Chromoplasts are coloured and chloroplasts are green, while leucoplasts have no pigment.
2. Ribosomes themselves have no membrane, but they are made inside the nucleus.

Answer: (a) F ∝ 1/d². If d is halved, the force becomes four times as large.
(b) It explains the motion of the moon around the earth, the planets around the sun, the tides, and the weight of bodies, because gravity acts between all objects in the universe.
(c) By the third law the forces are equal and opposite, but acceleration a = F/m. The apple has a very small mass so it gets a large, visible acceleration, whereas the earth's mass (6 × 10²⁴ kg) is so huge that its acceleration is negligible.
OR
(d) F = GMm/R² = (6.67 × 10⁻¹¹ × 6 × 10²⁴ × 1)/(6 × 10⁶)² = (4.002 × 10¹⁴)/(3.6 × 10¹³) ≈ 11.1 N.

Teacher's Note:
1. The gravitational force varies inversely as the square of the distance, so halving the distance gives 2² = 4 times the force.
2. For the apple and earth, equal forces do not mean equal accelerations, because the masses are very different.