Class 12 practice
Class 12 science questions.
Science questions for Class 12, at the level of students aged 15–17, each with its answer and a short explanation. Try each one before opening the answer.
A researcher crosses a pea plant heterozygous for round yellow seeds with a homozygous recessive plant exhibiting wrinkled green seeds. What phenotypic ratio is observed in the resulting generation?
Level 4 of 5Show the answer
Answer: 1:1:1:1
Crossing a dihybrid (RrYy) with a homozygous recessive individual (rryy) represents a dihybrid test cross. This cross produces four distinct phenotypes in an equal 1:1:1:1 ratio.
During strenuous exercise in humans, active muscle tissues produce lactic acid. Which physiological change directly promotes the unloading of oxygen from haemoglobin in these active tissues?
Level 4 of 5Show the answer
Answer: A decrease in local blood pH
Accumulation of lactic acid lowers blood pH. Under the Bohr effect, increased acidity reduces haemoglobin's affinity for oxygen, shifting the dissociation curve to the right and releasing more oxygen to tissues.
For the gas-phase dissociation reaction N2O4(g) ⇌ 2NO2(g), the degree of dissociation is α at total equilibrium pressure P. Which mathematical expression correctly gives the equilibrium constant Kp?
Level 4 of 5Show the answer
Answer: 4α²P / (1 - α²)
Equilibrium amounts are 1 - α of N2O4 and 2α of NO2, giving 1 + α total moles. Substituting partial pressures into Kp = [p(NO2)]² / p(N2O4) yields 4α²P / (1 - α²).
An ideal heat engine operates between a heat source at 327 degrees Celsius and a heat sink at 27 degrees Celsius. What is the maximum theoretical efficiency achievable by this engine?
Level 4 of 5Show the answer
Answer: 50%
Carnot efficiency equals 1 - (T_cold / T_hot) in Kelvin. Here, T_cold is 300 K and T_hot is 600 K. Efficiency equals 1 - (300/600) = 0.50 or 50%.
In a classic dihybrid cross between two pea plants heterozygous for both round seeds and yellow seeds, what proportion of the offspring is expected to be homozygous for round seeds and heterozygous for yellow seeds?
Level 4 of 5Show the answer
Answer: 2/16
The probability of inheriting the homozygous dominant genotype for seed shape is 1/4. The probability of inheriting the heterozygous genotype for seed colour is 1/2. Multiplying these gives 2/16.
In the human nephron, where does the majority of water and essential solute reabsorption take place under normal physiological conditions?
Level 4 of 5Show the answer
Answer: Proximal convoluted tubule
The proximal convoluted tubule reabsorbs approximately 70 to 80 percent of filtered electrolytes and water, along with almost all glucose and amino acids, back into peritubular capillaries.
Earth's thermosphere reaches temperatures exceeding 1000 degrees Celsius, yet a satellite in this layer radiates heat away and cools rapidly. What physical property of the thermosphere explains this phenomenon?
Level 4 of 5Show the answer
Answer: Extremely low gas density
Temperature measures the average kinetic energy of molecules, which is very high. However, the particle density is so sparse that collisions occur rarely, transferring negligible heat to macroscopic objects.
A chemical reaction is endothermic and has a positive change in standard entropy. Under what thermal conditions will this reaction proceed spontaneously?
Level 4 of 5Show the answer
Answer: Only at sufficiently high temperatures
For spontaneity, the Gibbs free energy change (ΔG = ΔH - TΔS) must be negative. Because both ΔH and ΔS are positive, ΔG becomes negative only when the temperature is high enough for TΔS to exceed ΔH.
- True or false?
In a photoelectric experiment, doubling the intensity of the incident monochromatic beam doubles the maximum kinetic energy of the emitted photoelectrons.
Level 4 of 5Show the answer
Answer: False
Beam intensity determines photon arrival rate, so doubling it doubles the saturation photocurrent. The maximum kinetic energy depends solely on photon frequency and the metal's characteristic work function.
- True or false?
During semi-conservative DNA replication in eukaryotic cells, both leading and lagging strands require RNA primers to initiate synthesis.
Level 4 of 5Show the answer
Answer: True
DNA polymerases cannot initiate synthesis de novo because they require an existing 3-prime hydroxyl group. Therefore, primase must generate an RNA primer on both the leading and lagging strands.
- True or false?
Multiplying all stoichiometric coefficients of a balanced electrochemical cell reaction by two doubles the standard electromotive force of that cell.
Level 4 of 5Show the answer
Answer: False
Standard cell potential is an intensive property and does not depend on the quantity of species reacting. While standard Gibbs free energy doubles, the cell potential remains unchanged.
- Fill in the blank.
The anaerobic sequence of enzymatic reactions in the cell cytoplasm that converts glucose into pyruvate is known as ___ [Clue: starts with 'g'].
Level 4 of 5Show the answer
Answer: glycolysis
Glycolysis is the universal initial step of cellular respiration occurring in the cytosol. It splits one six-carbon glucose molecule into two three-carbon pyruvate molecules, yielding ATP and NADH.
- Fill in the blank.
In C4 plants such as maize and sugarcane, the wreath-like radial arrangement of bundle-sheath cells surrounding the vascular bundles is termed ___ anatomy (first letter: k).
Level 4 of 5Show the answer
Answer: kranz
Kranz anatomy, from the German word for wreath, concentrates carbon dioxide in bundle-sheath cells, effectively minimizing photorespiration and enhancing photosynthetic efficiency in C4 plants.
Which optical phenomenon is the odd one out regarding the nature of light?
Level 4 of 5Show the answer
Answer: Photoelectric effect
Interference, diffraction, and polarisation demonstrate the wave nature of light. The photoelectric effect provides direct experimental proof for the particle or photon nature of light.
In a randomly mating diploid population obeying Hardy-Weinberg equilibrium, nine percent of individuals express an autosomal recessive trait. What proportion of the population are heterozygous carriers?
Level 5 of 5Show the answer
Answer: 0.42
The recessive genotype frequency q squared is 0.09, so the recessive allele frequency q is 0.3. The dominant allele frequency p is 1 minus 0.3, which equals 0.7. Carrier frequency is 2pq = 2(0.7)(0.3) = 0.42.
A uniform solid sphere and a thin hollow spherical shell of identical mass and radius roll purely down an inclined plane. Both start from rest at the same height. What is the ratio of the linear acceleration of the solid sphere to that of the hollow shell?
Level 5 of 5Show the answer
Answer: 25 : 21
Linear acceleration equals (g sinθ) / (1 + I/mR²). For a solid sphere, a1 = (5/7)g sinθ. For a hollow shell, a2 = (3/5)g sinθ. Their ratio a1 : a2 simplifies to 25 : 21.
The octahedral complex [Co(NH3)6]3+ is diamagnetic, whereas [CoF6]3- is paramagnetic. Which electronic configuration of the cobalt 3d subshell explains the diamagnetism of [Co(NH3)6]3+?
Level 5 of 5Show the answer
Answer: t2g^6 eg^0
Ammonia is a strong-field ligand that produces crystal field splitting greater than the pairing energy. This forces all six 3d electrons of Co(III) to pair in the lower-energy t2g orbitals.
A uniform solid disc and a thin hoop of identical mass and radius roll down an inclined plane from rest without slipping. Neglecting air resistance, what is the ratio of the linear acceleration of the disc to that of the hoop?
Level 5 of 5Show the answer
Answer: 4 : 3
Acceleration for rolling without slipping is a = g sin(theta) / (1 + I / mR^2). For a disc, a = 2/3 g sin(theta). For a hoop, a = 1/2 g sin(theta). The ratio is (2/3) / (1/2) = 4/3.
- Fill in the blank.
During the primary carbon fixation reaction of C4 photosynthesis in maize leaves, phosphoenolpyruvate carboxylase converts phosphoenolpyruvate into the four-carbon compound ___ (clue: begins with o).
Level 5 of 5Show the answer
Answer: oxaloacetate
In mesophyll cells of C4 plants, atmospheric carbon dioxide is fixed by phosphoenolpyruvate carboxylase to yield the initial four-carbon product oxaloacetate.
Based on their magnetic response in an external magnetic field, which is the odd one out?
Level 5 of 5Show the answer
Answer: Aluminium
Bismuth, copper, and water are diamagnetic substances that are weakly repelled by an external magnetic field. Aluminium is paramagnetic and is weakly attracted by a magnetic field.
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