Bihar Board Class 10 Science Question Paper 2023 (Code 112 Set-J) Available- Download Here with Solution PDF

Step 1: Definition.

Non-renewable sources of energy are those which cannot be replenished in a short period of time. They take millions of years to form.

Step 2: Analysis of options.

- Sun, Wind, and Flowing water are renewable sources.

- Petroleum is formed from fossils over millions of years and once used cannot be replenished quickly.

Step 3: Conclusion.

Hence, petroleum is a non-renewable source.
Quick Tip: Petroleum, coal, and natural gas are non-renewable sources because they take millions of years to form.

Step 1: Origin of fossil fuels.

Fossil fuels are formed from plants and animals buried under the earth millions of years ago.

Step 2: Energy source.

Plants get their energy through photosynthesis from the sun. That energy is trapped and stored in fossil fuels.

Step 3: Conclusion.

Thus, the Sun is the ultimate source of fossil fuel energy.
Quick Tip: All fossil fuels store ancient solar energy captured by plants.

Step 1: Definition.

A fuel is any substance that produces heat and energy on combustion.

Step 2: Explanation of options.

- Engine uses fuel but is not itself fuel.

- Heater is a device, not a substance.

- Fuel like coal, petrol, LPG gives heat on burning.

Step 3: Conclusion.

Therefore, substances producing heat on combustion are called fuels.
Quick Tip: Petrol, diesel, coal, LPG are common fuels that release energy when burned.

Step 1: Background.

Euro norms are standards for controlling emissions from vehicles.

Step 2: Explanation.

Euro-II was introduced to reduce harmful gases like CO, NOx, SO2 from vehicles. It directly deals with air pollution.

Step 3: Conclusion.

Hence, Euro-II relates to air pollution control.
Quick Tip: Euro norms are emission standards for vehicles to reduce air pollution.

Step 1: Dispersion meaning.

Dispersion is the splitting of white light into seven colours (VIBGYOR).

Step 2: Device responsible.

A prism bends different colours at different angles due to refraction. This causes dispersion.

Step 3: Conclusion.

Hence, prism produces dispersion of light.
Quick Tip: Dispersion is best shown in Newton’s prism experiment.

Step 1: Definition.

A solar cell is a device that directly converts solar energy into electrical energy using photovoltaic effect.

Step 2: Options.

Daniel cell and Leclanche cell are chemical cells, not solar energy devices.

Step 3: Conclusion.

Hence, the correct device is solar cell.
Quick Tip: Solar cells work on the principle of photovoltaic effect.

Step 1: Definition.

Strong bases are those which completely dissociate in water.

Step 2: Explanation.

- NaOH dissociates completely, hence strong base.

- Cu(OH)₂, NH₄OH, Mg(OH)₂ are weak bases because they dissociate partially.

Step 3: Conclusion.

Thus, NaOH is the correct answer.
Quick Tip: Strong bases are generally hydroxides of alkali metals like NaOH and KOH.

Step 1: Definition.

Dry ice is the solid form of carbon dioxide.

Step 2: Explanation.

It is called "dry" because it sublimates directly into gas without becoming liquid.

Step 3: Conclusion.

Thus, formula of dry ice is CO₂.
Quick Tip: Dry ice is widely used for cooling and fog effects in stage shows.

Step 1: Background.

Haemoglobin is a protein present in red blood cells. It binds oxygen and contains iron.

Step 2: Explanation.

The iron in haemoglobin gives blood its characteristic red colour.

Step 3: Conclusion.

Hence, haemoglobin is responsible for red colour.
Quick Tip: Haemoglobin carries oxygen from lungs to tissues.

Step 1: Definition.

Chlorophyll is the green pigment present in plant leaves.

Step 2: Explanation.

It absorbs red and blue light for photosynthesis and reflects green light, making plants look green.

Step 3: Conclusion.

Thus, chlorophyll pigment is green.
Quick Tip: Chlorophyll is essential for photosynthesis as it traps solar energy.

Step 1: Fact.

Enamel is a highly mineralised layer (mainly hydroxyapatite) covering the crown of the tooth.

Step 2: Comparison.

Dentine lies below enamel and is softer; pulp cavity contains nerves and blood vessels; canine is a type of tooth, not a tissue.

Step 3: Conclusion.

Therefore, enamel is the hardest part of the tooth.
Quick Tip: Enamel is the hardest substance in the human body—harder than bone.

Step 1: Key idea.

Thyroxine (T₄) and triiodothyronine (T₃) are iodinated hormones of the thyroid gland.

Step 2: Application.

Iodine deficiency leads to reduced thyroxine, causing goitre and cretinism.

Step 3: Conclusion.

Hence iodine is essential for thyroxine synthesis.
Quick Tip: Use iodised salt to prevent iodine-deficiency disorders.

Step 1: Definition.

Decomposers break down dead organic matter into simpler substances, recycling nutrients.

Step 2: Analysis.

Fungi (and many bacteria) are classic decomposers; cow and tiger are consumers; grass is a producer.

Step 3: Conclusion.

Therefore, fungus is a decomposer.
Quick Tip: Decomposers close the nutrient cycle by converting complex organics back to soil nutrients.

Step 1: Core concept.

Almost all natural energy on Earth originates from the Sun—directly (solar energy) or indirectly (wind, water cycle, biomass, fossil fuels).

Step 2: Conclusion.

Hence, the Sun is the ultimate/main source of energy on Earth.
Quick Tip: Even fossil fuels store ancient solar energy captured by plants.

Step 1: Understanding environment.

The environment includes air (atmosphere), water bodies (hydrosphere), and land/rocks (lithosphere), along with the biosphere.

Step 2: Conclusion.

Therefore, all listed spheres are components of the environment.
Quick Tip: Remember: Atmosphere + Hydrosphere + Lithosphere + Biosphere = Environment.

Step 1: Concept.

Typical forest food chains include: producers (plants), primary consumers (herbivores), secondary consumers (small carnivores), and tertiary consumers (top carnivores).

Step 2: Conclusion.

Thus, commonly we consider four trophic levels in a forest ecosystem.
Quick Tip: Energy decreases by about 90% at each successive trophic level (10% law).

Step 1: Definitions.

Double displacement reactions involve mutual exchange of ions between two compounds (AB + CD \(\rightarrow\) AD + CB).

Step 2: Note.

Precipitation is a type of double displacement where an insoluble solid forms.

Step 3: Conclusion.

Hence the general name is double displacement.
Quick Tip: If two ionic solutions react to form a precipitate, think “double displacement.”

Step 1: Count atoms on each side.

LHS: Pb=1, N=2, O=6.

RHS: Pb=1, N=1 (in NO₂), O=5 (1 in PbO, 2 in NO₂, 2 in O₂).

Step 2: Observation.

N and O counts differ, so the equation is not balanced. Others are balanced.

Step 3: Conclusion.

Thus option (4) is not balanced.
Quick Tip: Always tally each element’s atoms on LHS and RHS to check balance.

Step 1: Concept.

Neutral solutions have equal concentrations of H⁺ and OH^-- ions.

Step 2: Definition.

On the pH scale (0–14), neutrality corresponds to pH = 7 at 25\(^\circ\)C.

Step 3: Conclusion.

Hence, neutral solutions have pH 7.
Quick Tip: pH < 7 acidic; pH > 7 basic; pH = 7 neutral.

Step 1: Idea.

Natural indicators are obtained from plants (e.g., litmus, red cabbage, turmeric).

Step 2: Analysis.

Phenolphthalein and methyl orange are synthetic indicators; turmeric is natural and turns reddish-brown in bases.

Step 3: Conclusion.

Therefore, turmeric is a natural indicator.
Quick Tip: Turmeric stains become reddish-brown with soap (basic), a quick home-test for bases.

Step 1: Identification.

Washing soda is the decahydrate form of sodium carbonate.

Step 2: Formula.

Its correct chemical formula is Na₂CO₃.10H₂O.

Step 3: Conclusion.

Thus, option (3) is correct.
Quick Tip: Washing soda is used for cleaning and water softening because it removes hardness.

Step 1: Concept.

Heterogeneous compounds are those in which the composition is not uniform throughout.

Step 2: Explanation.

Marble (CaCO₃ with impurities) has non-uniform distribution of minerals, making it heterogeneous.

Step 3: Conclusion.

Thus, marble is the heterogeneous compound among the given options.
Quick Tip: Impurities in marble cause its different colours and make it heterogeneous.

Step 1: Fact.

Diamond is made of pure carbon atoms arranged in a rigid tetrahedral structure.

Step 2: Hardness.

This structure makes diamond the hardest natural substance on Mohs hardness scale.

Step 3: Conclusion.

Hence, diamond is the hardest natural substance.
Quick Tip: Diamond is used in cutting tools due to its extreme hardness.

Step 1: Recall.

Red litmus turns blue in alkaline/basic medium.

Step 2: Explanation.

OH^-- ions are responsible for basic nature. Thus, they turn red litmus paper blue.

Step 3: Conclusion.

So, the correct answer is OH^--.
Quick Tip: Acids release H⁺ (turn blue litmus red), bases release OH^-- (turn red litmus blue).

Step 1: Recall.

Ethylene is a gaseous plant hormone responsible for ripening of fruits like bananas and mangoes.

Step 2: Other hormones.

Auxins promote growth, gibberellins stimulate elongation, cytokinins promote cell division.

Step 3: Conclusion.

Thus, the fruit ripening hormone is ethylene.
Quick Tip: Ethylene gas is often used commercially to ripen fruits quickly.

Step 1: Recall.

Mushrooms feed on dead and decaying organic matter.

Step 2: Explanation.

This mode of nutrition is called saprophytic. Mushrooms cannot make their own food.

Step 3: Conclusion.

Hence, mushrooms are saprophytes.
Quick Tip: Fungi like mushrooms decompose organic matter and recycle nutrients.

Step 1: Recall.

Ozone is an allotrope of oxygen formed by three oxygen atoms.

Step 2: Properties.

It forms the ozone layer which protects Earth from harmful UV radiation.

Step 3: Conclusion.

Thus, formula of ozone is O₃.
Quick Tip: Ozone is less stable than O₂ and has a sharp smell.

Step 1: Definition.

Saprophytic nutrition means obtaining food from dead organic matter.

Step 2: Explanation.

Fungi cannot photosynthesise; they release enzymes and absorb nutrients.

Step 3: Conclusion.

Thus, fungi show saprophytic nutrition.
Quick Tip: Fungi recycle nutrients back to the ecosystem via saprophytic nutrition.

Step 1: Recall.

Energy enters the ecosystem through sunlight (producers) and flows to herbivores, carnivores, and decomposers.

Step 2: Concept.

Unlike matter, energy cannot be recycled. It flows in one direction only.

Step 3: Conclusion.

Thus, energy flow in an ecosystem is unidirectional.
Quick Tip: Matter cycles in ecosystem, but energy flows in a single direction and eventually dissipates as heat.

Step 1: Recall.

The human heart has four chambers: two atria (upper) and two ventricles (lower).

Step 2: Importance.

This separation allows efficient double circulation: oxygenated and deoxygenated blood remain separate.

Step 3: Conclusion.

Hence, human heart has 4 chambers.
Quick Tip: Four-chambered hearts are found in birds and mammals for efficient oxygen supply.

Step 1: Concept.

Humans share about 98–99% of DNA with chimpanzees, making them our closest relatives.

Step 2: Analysis of options.

Spiders are arthropods, bacteria are prokaryotes, orangutans are close but less similar than chimpanzees.

Step 3: Conclusion.

Thus, humans are most similar to chimpanzees.
Quick Tip: Chimpanzees are considered our evolutionary cousins.

Step 1: Process.

Curd forms when lactic acid bacteria ferment lactose sugar in milk.

Step 2: Explanation.

This is a fermentation process, not dissociation, excretion, or photosynthesis.

Step 3: Conclusion.

Hence, curd formation is due to fermentation.
Quick Tip: Lactobacillus bacteria ferment milk to form curd.

Step 1: Recall.

Concave lenses spread out light rays (diverge them).

Step 2: Explanation.

Convex lens is converging, bifocal has both convex + concave parts.

Step 3: Conclusion.

Thus, concave lens is called a diverging lens.
Quick Tip: Concave lens is used in spectacles for myopia (short-sightedness).

Step 1: Spectrum.

Visible spectrum: Violet (shortest wavelength, 400 nm) to Red (longest wavelength, 700 nm).

Step 2: Conclusion.

Hence, red light has maximum wavelength.
Quick Tip: That’s why red is used in danger signals—it scatters the least.

Step 1: Concept.

In series: \(R_{eq} = R_1 + R_2 + R_3 ...\) (greater than individual).

In parallel: \(R_{eq} < R_{smallest}\).

Step 2: Conclusion.

So, to increase resistance, use series.
Quick Tip: Series = more resistance, Parallel = less resistance.

Step 1: Explanation.

Voltmeter and ammeter have polarity signs; electric cell has positive and negative terminals.

Step 2: Note.

A coil is just a wire wound in loops, without polarity signs.

Step 3: Conclusion.

Thus, coil has no + or – sign.
Quick Tip: Only devices that measure or supply current/voltage need polarity marking.

Step 1: Law of reflection.

Plane mirror simply reflects rays without changing their convergence/divergence.

Step 2: Application.

If a converging beam strikes a plane mirror, the reflected rays will remain converging.

Step 3: Conclusion.

Thus, the answer is converging.
Quick Tip: Plane mirrors change direction but not convergence/divergence of rays.

Step 1: Recall.

Magnetic flux \(\Phi = B \cdot A\) (Tesla \(\times\) m\(^2\)).

Step 2: SI unit.

Unit is weber (Wb).

Step 3: Conclusion.

Hence, the SI unit of magnetic flux is weber.
Quick Tip: 1 weber = magnetic flux producing 1 volt when reduced to zero in 1 second.

Step 1: Formula.

Least count = Range / Number of divisions.

Step 2: Substitution.

Range = 2A, divisions = 200 (20 major × 10 small).

So least count = 2 / 200 = 0.01 A.

Step 3: Conclusion.

Thus, least count is 0.01 A.
Quick Tip: Least count is the smallest value an instrument can measure.

Step 1: Formula.

For parallel: \(\frac{1}{R_{eq}} = \frac{1}{R_1} + \frac{1}{R_2} + ...\)

Step 2: Property.
\(R_{eq}\) is always less than the smallest individual resistance.

Step 3: Conclusion.

Hence, equivalent resistance in parallel is less.
Quick Tip: Parallel connections reduce resistance, series increases it.

Step 1: Structure.

Each stoma (pore) in a leaf epidermis is bordered by two kidney-shaped guard cells.

Step 2: Function.

Guard cells control opening and closing of stomata by changing turgor pressure, thus regulating gas exchange and transpiration.

Step 3: Conclusion.

Hence, stomatal pores are surrounded by guard cells.
Quick Tip: Lenticels are pores on woody stems, not on leaf epidermis.

Step 1: Concept.

In aerobic respiration, glucose is oxidised using oxygen to form carbon dioxide and water with release of energy.

Step 2: Equation.

C₆H₁₂O₆ + 6O₂ \(\rightarrow\) 6CO₂ + 6H₂O + Energy.

Step 3: Conclusion.

Therefore, CO₂ is liberated.
Quick Tip: Exhaled air has higher CO₂ and lower O₂ than inhaled air.

Step 1: Reaction.

Complete oxidation: C₆H₁₂O₆ + 6O₂ \(\rightarrow\) 6CO₂ + 6H₂O + Energy (ATP).

Step 2: Output.

Products are CO₂, H₂O and energy.

Step 3: Conclusion.

Therefore, “All of these” is correct.
Quick Tip: Energy from glucose is captured mainly as ATP during respiration.

Step 1: Idea.

The plasma membrane allows selective passage of substances—some pass easily, others are restricted.

Step 2: Term.

This property is called selective or semi-permeability.

Step 3: Conclusion.

Hence, the cell membrane is semi-permeable.
Quick Tip: “Selectively permeable” and “semi-permeable” are used interchangeably in Class 10 context.

Step 1: Definition.

ATP is the energy currency of the cell, consisting of adenine, ribose (together adenosine) and three phosphate groups.

Step 2: Clarify options.

It is not “adenine triphosphate”; the correct nucleoside is adenosine.

Step 3: Conclusion.

Thus, ATP stands for adenosine triphosphate.
Quick Tip: Breaking the terminal phosphate bond of ATP releases usable energy.

Step 1: Test.

Iodine solution turns blue-black in presence of starch due to formation of a starch–iodine complex.

Step 2: Others.

Safranin, eosin, and methylene blue are general stains, not specific starch indicators.

Step 3: Conclusion.

Therefore, iodine is used to test starch.
Quick Tip: A blue-black colour confirms starch; no colour change means no starch.

Step 1: Factors.

Photosynthesis rate depends on light intensity (amount) and quality (wavelength/colour), among other factors (CO₂, temperature).

Step 2: Application.

Red and blue light are more effective; higher intensity increases rate up to a limit.

Step 3: Conclusion.

So both properties and amount of light affect the rate.
Quick Tip: Red and blue wavelengths are most efficient for photosynthesis.

Step 1: Identification.

Calcium oxychloride (CaOCl₂) is commonly known as bleaching powder.

Step 2: Use.

It is used for bleaching, disinfection, and water treatment.

Step 3: Conclusion.

Thus, CaOCl₂ is bleaching powder.
Quick Tip: Bleaching powder releases chlorine, which kills germs and bleaches fabrics.

Step 1: Composition.

Iodized salt contains table salt (NaCl) fortified with iodine—commonly as potassium iodate (KIO₃) or potassium iodide (KI).

Step 2: Purpose.

It prevents iodine-deficiency disorders like goitre.

Step 3: Conclusion.

Therefore, both (A) and (B) correctly represent iodized salt.
Quick Tip: Check “iodized” on salt packets—fortified with KI or KIO₃.

Step 1: Understanding alloys.

Bronze is an alloy mainly of copper (Cu) and tin (Sn).

Step 2: Percentage composition.

The given mixture has 90% Cu and 10% Sn, which is the typical composition of bronze.

Step 3: Eliminate others.

- Solder is an alloy of lead and tin.

- Brass is copper + zinc.

- German silver is copper + zinc + nickel.

Step 4: Conclusion.

Hence, the alloy is bronze.
Quick Tip: Bronze is historically important and used in making statues, medals, and tools.

Step 1: Oxygen molecule structure.

Oxygen exists as O₂. Each oxygen atom has 6 valence electrons.

Step 2: Bonding.

They share two pairs of electrons, forming a double bond (O=O).

Step 3: Conclusion.

Therefore, there are two covalent bonds between O atoms.
Quick Tip: O₂ has a double bond, while N₂ has a triple bond.

Step 1: Concept.

Philosopher’s wool is the common name for zinc oxide (ZnO).

Step 2: Reason.

It forms as white fluffy wool-like substance when zinc metal is oxidized.

Step 3: Conclusion.

Hence, Philosopher’s wool = ZnO.
Quick Tip: ZnO is amphoteric – reacts with both acids and bases.

Step 1: Reaction.

Saponification of fats and oils with NaOH gives sodium salts of fatty acids.

Step 2: Identification.

These sodium salts are soaps.

Step 3: Conclusion.

Therefore, the answer is soap.
Quick Tip: Soaps work well in soft water, but not in hard water due to scum formation.

Step 1: Carbonyl definition.

A carbonyl group is C=O, where carbon is double-bonded to oxygen.

Step 2: Elimination.

- –CHO is an aldehyde group.

- –COOH is a carboxyl group.

- –O– is ether linkage.

Step 3: Conclusion.

Thus, the correct symbol is >CO.
Quick Tip: Carbonyl group is common in aldehydes and ketones.

Step 1: Definition.

Alkynes are hydrocarbons with a triple bond.

Step 2: Options.

- Ethene: alkene (double bond).

- Methane: alkane (single bond).

- Acetylene (ethyne): C₂H₂, with a triple bond.

- Chloroform: not a hydrocarbon.

Step 3: Conclusion.

So acetylene is an alkyne.
Quick Tip: General formula of alkyne: C_nH_2n–2}.

Step 1: Sign convention.

In mirror formula, distances measured against incident light are taken negative.

Step 2: Mirrors.

For concave mirror, focal length is on the left side (against light), hence negative.

For convex mirror, focal length is positive.

Step 3: Conclusion.

So, concave mirror has negative focal length.
Quick Tip: Remember: Concave = negative focal length, Convex = positive focal length.

Step 1: Image formation.

Concave mirrors form different images depending on object position.

Step 2: For magnified virtual image.

When object is between focus (F) and pole (P), the mirror forms a virtual, erect, and magnified image.

Step 3: Conclusion.

Thus, the correct answer is “between focus and pole”.
Quick Tip: Concave mirrors are used in makeup mirrors for magnified virtual images.

Step 1: Definition.

Refraction is bending of light when it passes from one medium to another.

Step 2: Laws.

1. Incident ray, refracted ray and normal lie in the same plane.

2. Snell’s law: \(\dfrac{\sin i}{\sin r} = \mu\), a constant for given media.

Step 3: Conclusion.

Thus, there are 2 laws.
Quick Tip: Snell’s law is key to calculating refractive index.

Step 1: Recall Snell’s law.
\(n = \dfrac{\sin i}{\sin r}\), where \(i =\) angle of incidence, \(r =\) angle of refraction.

Step 2: Concept.

This ratio remains constant for a given pair of media.

Step 3: Conclusion.

Therefore, refractive index is \(\dfrac{\sin i}{\sin r}\).
Quick Tip: Higher refractive index = light bends more towards normal.

Step 1: Understanding power of a lens.

The power of a lens is defined as the reciprocal of its focal length (in meters).
\[ P = \frac{100}{f \, (in cm)} = \frac{1}{f \, (in m)} \]

Step 2: Unit of power.

Since focal length is measured in meters, the unit of power is \( m^{-1} \). This unit is given a special name "Dioptre (D)".

Step 3: Conclusion.

Therefore, the SI unit of power of a lens is Dioptre.
Quick Tip: Power of lens increases when focal length decreases.

Step 1: Definition of magnification.

Magnification produced by a lens is defined as the ratio of the image distance (\(v\)) to the object distance (\(u\)): \[ m = \frac{v}{u} \]

Step 2: Application in concave lens.

In concave lens, the image is always virtual, erect and diminished, hence \( m = \frac{v}{u} \) (always less than 1).

Step 3: Conclusion.

Thus, magnification in a concave lens is \(\frac{v}{u}\).
Quick Tip: For concave lens, the image formed is always virtual, erect and diminished.

Step 1: Refractive index meaning.

Refractive index is the ratio of speed of light in vacuum to the speed of light in medium. Diamond has one of the highest refractive indices among natural substances.

Step 2: Known value.

The refractive index of diamond is 2.42.

Step 3: Conclusion.

Hence, correct answer is 2.42.
Quick Tip: Diamond’s high refractive index makes it sparkle brilliantly by causing total internal reflection.

Step 1: Understanding prism.

A prism is a transparent medium bounded by three rectangular surfaces and two triangular bases.

Step 2: Effective surfaces.

The prism is bounded by three surfaces (two triangular bases + one rectangular refracting surface).

Step 3: Conclusion.

So, a prism is bounded by 3 surfaces.
Quick Tip: Prism is used to study dispersion of light into seven colors (VIBGYOR).

Step 1: Electronic configuration.

Potassium (K) has atomic number 19. Its configuration is: \[ 2, 8, 8, 1 \]

Step 2: Valency concept.

Valency is determined by the number of electrons in the outermost shell. Here, it has 1 electron in outer shell.

Step 3: Conclusion.

So, valency of potassium is 1.
Quick Tip: Alkali metals (like Na, K, Li) all have valency 1.

Step 1: Understanding hydrocarbons.

Alkanes are saturated hydrocarbons with single bonds only.

Step 2: General formula.

For alkanes, the formula is: \[ C_nH_{2n+2} \]

Step 3: Examples.

Methane: \(CH_4\) (n=1), Ethane: \(C_2H_6\) (n=2).

Step 4: Conclusion.

Thus, correct general formula is \(C_nH_{2n+2}\).
Quick Tip: Remember: Alkanes – single bonds (\(C_nH_{2n+2}\)), Alkenes – double bonds (\(C_nH_{2n}\)), Alkynes – triple bonds (\(C_nH_{2n-2}\)).

Step 1: Recall periodic table.

Fluorine belongs to Group 17 (Halogens).

Step 2: Atomic number.

The atomic number of fluorine is 9. Its electronic configuration is \(2, 7\).

Step 3: Conclusion.

Thus, fluorine has atomic number 9.
Quick Tip: Halogens belong to group 17 and have 7 valence electrons.

Step 1: Litmus test.

Red litmus turning blue indicates that the solution is basic.

Step 2: pH range.

Acidic solutions: \(pH < 7\)

Neutral solutions: \(pH = 7\)

Basic solutions: \(pH > 7\).

Step 3: Conclusion.

Since the solution is basic, its pH is more than 7.
Quick Tip: Always remember: Acids (\(<7\)), Neutral (=7), Bases (>7).

Step 1: Recall oxygen details.

Oxygen has atomic number 8 and its most common isotope is \(O^{16}\).

Step 2: Atomic weight.

The relative atomic mass (atomic weight) of oxygen is 16 u.

Step 3: Conclusion.

Thus, the correct answer is 16.
Quick Tip: Atomic weight of oxygen = 16, hydrogen = 1 → so water \(H_2O\) has molar mass = 18 g/mol.

Step 1: Understanding ores.

Bauxite is the principal ore of aluminium. It mainly contains hydrated aluminium oxides like \(Al_2O_3 \cdot 2H_2O\).

Step 2: Extraction.

Aluminium is extracted from bauxite using electrolytic reduction after refining by the Bayer’s process.

Step 3: Conclusion.

Hence, bauxite is an ore of Aluminium (Al).
Quick Tip: Important ores: Bauxite – Aluminium, Haematite – Iron, Galena – Lead, Cinnabar – Mercury.

Step 1: Activity series.

Standard reactivity series: K \(>\) Na \(>\) Ca \(>\) Mg \(>\) Al \(>\) Zn \(>\) Fe \(>\) Pb \(>\) H \(>\) Cu \(>\) Ag \(>\) Au.

Step 2: Pick the relative order.

Among Na, Mg, Zn, Cu the decreasing reactivity is Na \(>\) Mg \(>\) Zn \(>\) Cu.
Quick Tip: More reactive metals displace less reactive ones from their compounds.

Step 1: Identify the change.

Aluminium replaces iron from iron(III) oxide to form aluminium oxide and iron.

Step 2: Type.

Replacement of one element by another is a single displacement reaction (Thermite reaction).
Quick Tip: In single displacement: \(A + BC \rightarrow AC + B\).

Step 1: Definition.

Compounds containing only carbon and hydrogen are called hydrocarbons (e.g., methane, ethane).
Quick Tip: Hydrocarbons are of three main types: alkanes, alkenes, alkynes.

Step 1: Periodic position.

B and Al are in Group 13 (IIIA), called the boron family.
Quick Tip: Group 13: B, Al, Ga, In, Tl.

Step 1: Fact.

Urochrome (a breakdown product of haemoglobin) imparts the yellow colour to urine.
Quick Tip: Colour can vary with hydration; darker yellow indicates dehydration.

Step 1: Trophic levels.

Producers (1st), herbivores/primary consumers (2nd), carnivores/secondary consumers (3rd).

Step 2: Tiger’s role.

Tiger preys on herbivores (e.g., deer), so it is a secondary consumer at the 3rd trophic level.
Quick Tip: Some chains may add apex predators as 4th level, but in Class 10 context tiger is 3rd.

Step 1: Production vs storage.

Bile is produced by the liver, stored in the gallbladder, and released into the duodenum.
Quick Tip: Bile emulsifies fats—mechanical aid to digestion; it has no enzymes.

Step 1: Concept.

Homologous organs have the same basic structure and origin but different functions.

Step 2: Application.

Human arm, cat’s foreleg and bat’s wing share the same bone arrangement—humerus, radius/ulna, carpals, etc.
Quick Tip: Homology evidences divergent evolution.

Step 1: Definition.

Regeneration is the ability to regrow lost body parts or a new organism from fragments.

Step 2: Example.

Hydra shows remarkable regeneration; frog and cow do not.
Quick Tip: Planaria and Hydra are classic regeneration examples.

Step 1: Terms.

Genotype = genetic makeup; phenotype = observable traits; variation = differences among individuals; heredity = transmission of traits.

Step 2: Conclusion.

Thus, the genic constitution is the genotype.
Quick Tip: Phenotype = Genotype + Environment (simplified view).

Step 1: Conditions in the core.
The \( \sim \)200,000 km-wide solar core has \(T\approx1.5\times10^{7}\,\mathrm{K}\) and density \( \approx 150\,\mathrm{g\,cm^{-3}} \), enabling quantum tunnelling so protons overcome Coulomb repulsion.

Step 2: Fusion pathway.
In the p–p chain: \(p+p\rightarrow\,^2\!H+e^{+}+\nu_e\); \(^{2}\!H+p\rightarrow\,^{3}\!He+\gamma\); \(^{3}\!He+^{3}\!He\rightarrow\,^{4}\!He+2p\).
Net: \(4p\rightarrow\,^{4}\!He+2e^{+}+2\nu_e+\) energy (\(\approx 26.7\) MeV). The mass defect \( \Delta m \) transforms to energy via \(E=\Delta mc^{2}\).

Step 3: Energy transport.
Gamma photons random-walk outward, being repeatedly absorbed and re-emitted, and emerge at the photosphere as the solar spectrum (effective \(T\approx 5778\,\mathrm{K}\)). Neutrinos escape almost instantly, verifying fusion. Quick Tip: Fusion releases \(\sim 10^{7}\) times more energy per nucleon than chemical reactions—hence stars shine for billions of years.

Step 1: Mechanism.
A thermal neutron captured by \(^{235}\mathrm{U}\) produces an excited compound nucleus \(^{236}\mathrm{U}^{*}\) which deforms and splits, e.g. \[ ^{235}\mathrm{U}+n \rightarrow\,^{141}\mathrm{Ba}+^{92}\mathrm{Kr}+3n+Q, \]
with \(Q\approx 200\) MeV.

Step 2: Energy origin.
Products have higher total binding energy per nucleon than the parent; the mass defect converts to energy (\(E=mc^{2}\)), mostly as kinetic energy of fragments plus prompt \(\gamma\)-rays and delayed \(\beta\)-decays.

Step 3: Chain reaction control.
The average neutrons per fission sustain a chain reaction. Reactors keep the multiplication factor \(k_{\mathrm{eff}}=1\) using moderators (to slow neutrons) and control rods (B, Cd) that absorb excess neutrons; coolant removes heat to run turbines. If \(k_{\mathrm{eff}}>1\) and uncontrolled, the neutron population grows explosively. Quick Tip: Remember: fusion joins light nuclei; fission splits heavy nuclei—both release energy due to increased binding energy of the products.

Step 1: Refraction at a thin lens.
Using the small-angle (paraxial) approximation, a lens redirects rays according to Snell’s law at each surface; for a convex lens, rays converge, for a concave lens, rays diverge.

Step 2: Principal rays (construction).
To locate images quickly:
1) Parallel ray \(\to\) through \(F\) (convex) / appears from \(F\) (concave).
2) Central ray through optical center \(O\) \(\to\) undeviated.
3) Focal ray aimed at \(F\) \(\to\) emerges parallel to axis.

Step 3: Nature of images.
Convex lens:
— Object beyond \(2F\): real, inverted, diminished (between \(F\) and \(2F\)).
— At \(2F\): real, inverted, same size (at \(2F\)).
— Between \(F\) and \(2F\): real, inverted, magnified (beyond \(2F\)).
— Inside \(F\): virtual, erect, magnified (same side).
Concave lens: always virtual, erect, diminished.


\begin{tikzpicture[scale=0.9]
\draw[-latex] (-4.5,0)--(4.8,0) node[right]{Principal axis;
% Convex lens
\draw ( -0.2,1.5) to[out=-90,in=90] (-0.2,-1.5);
\draw ( 0.2,1.5) to[out=-90,in=90] ( 0.2,-1.5);
\node at (0,1.8){Convex lens;
\foreach \x/\lab in {-2/\(F_1\),2/\(F_2\), -4/\(2F_1\), 4/\(2F_2\){
\draw (\x,0) -- ++(0,0.12) node[below=2pt]{\lab;

\draw[very thick] (-4,0)--(-4,1.0) node[above]{Object;
\draw[->,thick] (-4,1.0)--(0,1.0);
\draw[thick] (0,1.0)--(2,0);
\draw[->,thick] (-4,1.0)--(0,0.3);
\draw[thick] (0,0.3)--(1.6,-0.05);
\draw[very thick] (1.6,0)--(1.6,-0.35) node[below]{Image;
\end{tikzpicture Quick Tip: Sign convention (real is positive to the right of lens): \( \frac{1}{f}=\frac{1}{v}-\frac{1}{u} \), \( m=\frac{v}{u}=\frac{h_i}{h_o} \).