Top 50 Science Questions Class 10

Chemical name and formula - 

Plaster of Paris

• Chemical name: Calcium sulphate hemihydrate
• Formula: CaSO₄·½H₂O

Gypsum

• Chemical name: Calcium sulphate dihydrate
• Formula: CaSO₄·2H₂O

In the early morning, the Sun is low on the horizon. The sunlight has to travel a longer distance through the Earth’s atmosphere to reach us. The atmosphere contains air molecules and dust particles which scatter light. The shorter wavelengths like blue and violet are scattered away, while the longer wavelengths like red and orange reach our eyes. That is why the Sun appears reddish during sunrise.

Will this phenomenon be observed by an astronaut on the Moon?
No, this will not happen on the Moon. The Moon does not have an atmosphere, so there are no air molecules or dust particles to scatter the sunlight. Since there is no scattering of light, the Sun appears white on the Moon and not reddish.

Stars twinkle because their light passes through the Earth’s atmosphere. The atmosphere has layers of air with different temperatures and densities, which bend the light in different directions. This makes the stars look like they are changing brightness and position, causing them to twinkle.

Decomposition by heat (Thermal decomposition):

CaCO3 → CaO + CO2
(Calcium carbonate decomposes into calcium oxide and carbon dioxide when heated.)

Decomposition by light (Photodecomposition):

2AgCl → 2Ag + Cl2
(Silver chloride decomposes into silver and chlorine gas when exposed to light.)

Decomposition by electricity (Electrolysis):

2H2O → 2H2 + O2
(Water decomposes into hydrogen and oxygen gases when electricity is passed through it.)

Planets do not twinkle because they are closer to the Earth and appear as small discs, not points of light like stars. When their light passes through the atmosphere, the bending of light is averaged over the whole disc, so their brightness and position stay steady. That is why planets shine with a steady light and do not twinkle.

The sky appears dark to an astronaut because there is no atmosphere in space to scatter sunlight. On Earth, the atmosphere scatters blue light in all directions, making the sky look blue. But in space, without air or atmosphere, there is no scattering, so the sky looks black or dark.

The resistivity of alloys is generally higher than that of the pure metals from which they are made.

Why?

• In alloys, different types of atoms are mixed.
• These atoms disturb the regular structure of the metal lattice.
• This scatters electrons more, making it harder for electric current to flow.
• As a result, resistance increases, and hence resistivity is higher.

Example:

• Pure copper has low resistivity (good conductor).
• An alloy like nichrome (nickel + chromium) has much higher resistivity.

The substance ‘X’ is quicklime, also known as calcium oxide (CaO).

When calcium oxide is mixed with water, it reacts vigorously to form slaked lime, also known as calcium hydroxide [Ca(OH)₂]. This solution is used for whitewashing walls.

Chemical Reaction:

CaO (s) + H₂O (l) → Ca(OH)₂ (aq) + Heat

• Calcium oxide (CaO) is quicklime.
• Calcium hydroxide [Ca(OH)₂] is slaked lime (used in whitewashing).
• The reaction is exothermic (produces heat).

Note: When this slaked lime solution is applied on walls, it reacts with carbon dioxide in the air to form a thin layer of calcium carbonate, which gives a shiny white finish.

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

A chemical reaction is a process in which one or more substances (called reactants) are converted into new substances (called products) with different chemical properties.

Key Features of a Chemical Reaction:

• A new substance is formed.
• There is a change in chemical composition.
• Energy may be absorbed or released (such as heat, light, or sound).
• Observable signs may include: 
  • Change in color
  • Formation of a gas
  • Change in temperature
  • Formation of a precipitate (solid)

Example of a Chemical Reaction:

Burning of magnesium in air:

2Mg + O₂ → 2MgO

• Reactants: Magnesium (Mg) and Oxygen (O₂)
• Product: Magnesium oxide (MgO), a white powder

When an iron nail is dipped in copper sulphate (CuSO₄) solution, a chemical reaction takes place. Iron is more reactive than copper, so it displaces copper from the copper sulphate solution. As a result, the blue colour of the solution slowly fades and a reddish-brown layer of copper is deposited on the iron nail.

Chemical equation:

Fe (solid) + CuSO₄ (aqueous) → FeSO₄ (aqueous) + Cu (solid)

Explanation:

• Copper sulphate solution is blue because of copper ions (Cu²⁺).
• Iron replaces copper in the solution and forms iron sulphate (FeSO₄), which is greenish in colour.
• The copper that gets displaced is deposited on the nail.

Anhydrous salts are salts that do not contain water molecules in their structure.

Hydrated salts are salts that contain a fixed number of water molecules called water of crystallisation as part of their structure.

Example of a hydrated salt is copper(II) sulphate pentahydrate (CuSO₄·5H₂O), which is blue in colour.

Example of an anhydrous salt is anhydrous copper(II) sulphate (CuSO₄), which is white in colour.

When hydrated copper sulphate is heated, it loses water of crystallisation and becomes anhydrous copper sulphate. This shows the difference between hydrated and anhydrous salts.

Name of the process: Chlor-alkali process

Sodium hydroxide is produced by the electrolysis of brine, which is a concentrated solution of sodium chloride (NaCl).

During the process:

• Hydrogen gas is released at the cathode.
• Chlorine gas is released at the anode.
• Sodium hydroxide is formed in the solution.

Chemical equation:
2NaCl (aq) + 2H₂O (l) → 2NaOH (aq) + Cl₂ (g) + H₂ (g)

This process is called the Chlor-alkali process because it produces two main products: chlorine gas (chlor) and sodium hydroxide (alkali).

Difference in colours of the Sun during sunrise/sunset and noon:

During sunrise and sunset:

• The Sun appears reddish.
• This is because the Sun is near the horizon and its light has to travel a longer distance through the Earth's atmosphere.
• The shorter wavelengths (like blue and violet) get scattered away.
• The longer wavelengths (like red and orange) reach our eyes, so the Sun looks red.

During noon:

• The Sun appears white or yellowish.
• This is because the Sun is overhead, and its light travels a shorter distance through the atmosphere.
• Very little scattering occurs, so all colours of sunlight reach our eyes nearly equally.
• As a result, the Sun looks white or slightly yellow.

Unit of Current:The SI unit of electric current is the ampere (A).

Definition :-  One ampere is the current when one coulomb of charge flows through a conductor in one second.
So,  1 ampere = 1 coulomb / 1 second

Exothermic & Endothermic reactions refer to the flow of energy (usually in the form of heat) during a chemical reaction.

Exothermic Reactions:

An exothermic reaction releases energy, usually in the form of heat, to its surroundings. In these reactions, the energy of the products is lower than that of the reactants. For example:

• Combustion (burning) of fuel like methane:--      CH₄ + 2O₂ → CO₂ + 2H₂O + energy (heat)
• In this reaction, energy is released as heat when methane burns.

Endothermic Reactions:

An endothermic reaction absorbs energy from the surroundings. In these reactions, the energy of the products is higher than that of the reactants. For example:

• Photosynthesis in plants:--        6CO₂ + 6H₂O + energy (sunlight) → C₆H₁₂O₆ + 6O₂
• Plants absorb sunlight to convert carbon dioxide and water into glucose and oxygen.

In summary:

1. Exothermic = Energy released (e.g., burning fuels).
2. Endothermic = Energy absorbed (e.g., photosynthesis).

The correct answer is (c) ciliary muscles.

The ciliary muscles control the change in the focal length of the eye lens.

Damage to the ozone layer is a problem because it lets more harmful UV rays from the sun reach the Earth. This can cause skin cancer, eye problems, and harm animals, plants, and crops.

To stop this damage, people are reducing the use of harmful chemicals like CFCs found in old refrigerators and spray cans. International agreements like the Montreal Protocol help control these chemicals. People are also using safer alternatives and spreading awareness about protecting the ozone layer.

Chemical formulas:

• Hydrated copper sulphate: CuSO₄·5H₂O
• Anhydrous copper sulphate: CuSO₄

Activity to show interconversion:

• Take some blue hydrated copper sulphate crystals (CuSO₄·5H₂O) in a watch glass.
• Heat the crystals gently using a burner.
• The blue crystals will lose water and turn into white anhydrous copper sulphate (CuSO₄) powder.
• Now, add a few drops of water to the white powder.
• The white powder will dissolve in water and turn blue again, showing the formation of hydrated copper sulphate.

The rate at which energy is delivered by a current is called power.

Power (P) = Voltage (V) × Current (I)

So, the rate of energy delivery depends on both the voltage and the current.

A balanced chemical equation is a chemical equation where the number of atoms of each element is the same on both sides of the equation — that is, the number of atoms in the reactants equals the number in the products.

Example of balanced chemical equation:-

Balanced equation:

2H₂ + O₂ → 2H₂O

Now:

• On the left: 4 hydrogen atoms, 2 oxygen atoms
• On the right: 4 hydrogen atoms, 2 oxygen atoms

Now the number of atoms on both sides is equal — the equation is balanced.

Why should chemical equations be balanced?

• To follow the law of conservation of mass, which says that matter cannot be created or destroyed in a chemical reaction.
• To show the correct amounts of substances involved in the reaction.
• To make accurate calculations for experiments and industrial processes.

Balancing ensures that the reaction makes sense scientifically and practically.

A precipitation reaction is a type of chemical reaction in which two soluble substances (usually aqueous solutions of salts) react to form an insoluble solid called a precipitate. This solid separates out from the solution as it does not dissolve in water.

General Form of the Reaction:
Soluble salt A (aq) + Soluble salt B (aq) → Insoluble salt (s) + Soluble salt (aq)

Example 1:
Na₂SO₄ (aq) + BaCl₂ (aq) → BaSO₄ (s) + 2NaCl (aq)

• Sodium sulfate reacts with barium chloride.
• Barium sulfate (BaSO₄) is formed as a white precipitate because it is insoluble in water.
• Sodium chloride remains dissolved in the solution.

Example 2:
AgNO₃ (aq) + NaCl (aq) → AgCl (s) + NaNO₃ (aq)

• Silver nitrate reacts with sodium chloride.
• Silver chloride (AgCl) is formed as a white precipitate.
• Sodium nitrate remains in solution.

Key Points:

• Precipitation reactions help in detecting the presence of certain ions in a solution.
• They are commonly used in qualitative chemical analysis and purification processes.

Water of crystallisation is the fixed number of water molecules that are present in the crystal structure of a salt. These water molecules are a part of the salt and give it its shape and colour.

Examples:

• Copper(II) sulphate – CuSO₄5H₂O
• Gypsum – CaSO₄2H₂O

Proof:

1. When blue copper sulphate crystals (CuSO₄·5H₂O) are heated, they lose water of crystallisation and become white anhydrous copper sulphate (CuSO₄).
2. When water is added back to the white powder, it turns blue again, showing that water of crystallisation affects the colour and state of the compound.

Reaction of Zinc with Sodium Hydroxide:

When zinc reacts with sodium hydroxide, it forms a salt called sodium zincate and hydrogen gas. Names of the Products are -

• Sodium zincate (Na₂ZnO₂)
• Hydrogen gas (H₂)

Balanced Chemical Equation:

Zn + 2NaOH → Na₂ZnO₂ + H₂↑

Test for the Gas Evolved:

• Bring a burning matchstick near the mouth of the test tube.
• If hydrogen is present, it will burn with a ‘pop’ sound.

Simple difference between metals and non-metals based on their chemical properties:

• Reaction with oxygen: - Metals usually form basic or amphoteric oxides. Non-metals form acidic oxides.
• Reaction with acids:- Metals react with dilute acids to produce hydrogen gas. Non-metals usually do not react with acids.
• Reaction with water:- Some metals like sodium and potassium react with water to form hydroxides and hydrogen gas. Non-metals generally do not react with water.
• Electropositivity:- Metals tend to lose electrons and form positive ions. Non-metals tend to gain or share electrons.
• Reaction with non-metals:- Metals generally form ionic compounds with non-metals. Non-metals usually form covalent compounds with each other.

Focal length of the concave lens = 2 m
Since it is a concave lens, the focal length is negative.
So, f = –2 m = –200 cm

Using the formula:
Power (P) = 100 / focal length in cm
P = 100 / (–200) = –0.5 dioptres

Hence the power of the concave lens is –0.5 dioptres.

An electric circuit is a closed path through which electric current can flow. It usually includes a power source like a battery, wires to carry the current, and a device like a bulb or fan that uses the electricity.

If the path is complete (closed), electricity flows and the device works. If the path is broken (open), electricity cannot flow and the device does not work.

Plants need four main things for photosynthesis:

1. Sunlight – It provides the energy needed to drive the chemical reactions.
2. Carbon dioxide (CO₂) – Plants take it from the air through tiny openings in their leaves called stomata.
3. Water (H₂O)– Plants absorb it from the soil through their roots.
4. Chlorophyll – This green pigment, found in chloroplasts, captures the sunlight and helps convert carbon dioxide and water into food.

Current will flow more easily through a thick wire than a thin wire of the same material when connected to the same source. 
Because a thick wire has a larger area of cross-section, so it offers less resistance to the flow of current. Lower resistance means more current can flow.

Problems caused by non-biodegradable wastes:

1. Pollution: They cause land, water, and air pollution because they do not break down easily.
2. Harm to animals: Animals may eat plastic or get trapped in it, which can hurt or kill them.
3. Clog drains and cause floods: Plastic and other wastes block drains, leading to waterlogging and floods.
4. Take up space: They pile up in landfills and spoil the beauty of the environment.

Photosynthesis mainly happens in the leaves of the plant, inside parts called chloroplasts.

Inside the leaves, there are tiny structures called chloroplasts, which contain a green pigment called chlorophyll. Chlorophyll captures sunlight and uses its energy to turn carbon dioxide (from the air) and water (from the soil) into glucose (a type of sugar) and oxygen.

Respiration is considered an exothermic reaction because it releases energy in the form of heat and ATP (a type of energy molecule used by cells).

In this process, glucose (a sugar) combines with oxygen to produce carbon dioxide, water, and energy. The energy released is used by the body for various activities, like muscle movement and cell growth.

Here’s how it works:

• The body takes in glucose from food and oxygen from the air.
• These react in the cells of the body, breaking down glucose to release energy.
• The by-products of this reaction are carbon dioxide and water.

Explanation:

• Glucose (C₆H₁₂O₆) reacts with oxygen (O₂).
• This produces carbon dioxide (CO₂) and water (H₂O).
• Energy (ATP and heat) is released during the process.

The reason it’s exothermic is that the energy released (ATP and heat) is greater than the energy required to start the reaction. So, it’s a reaction that releases more energy than it absorbs.

The shiny brown coloured element ‘X’ is copper (Cu).
When copper is heated in air, it reacts with oxygen to form a black coloured compound called copper(II) oxide (CuO).

Reaction:
Cu + O₂ → 2CuO

Explanation:

• Copper (Cu), which is shiny and brown, reacts with oxygen in the air when heated.
• This forms copper(II) oxide (CuO), which is black in colour.
• The surface of the copper turns black due to the formation of this oxide layer.

Answer Summary:

• Element X: Copper (Cu)
• Black compound formed: Copper(II) oxide (CuO)

A convex lens is called a converging lens because it brings parallel rays of light together after they pass through the lens.

When parallel rays of light fall on a convex lens, they are bent inwards and meet at a single point on the other side of the lens. This point is called the focus.

Because the lens converges (joins) the light rays at one point, it is called a converging lens.

Correct answer: (c) Fruit-peels, cake and lime-juice

Reason: All items in option (c) are natural and easily decompose, so they are biodegradable.

Oil and fat containing food items are flushed with nitrogen gas to prevent rancidity.

What is Rancidity?

Rancidity is the condition in which fats and oils in food get oxidized, usually by the oxygen present in air. This leads to:

• Bad smell
• Unpleasant taste
• Change in color or texture

Rancid food is not only unappetizing but can also be harmful to health if consumed.

Explanation:

• Oils and fats can react with oxygen in the air, leading to a process called oxidation.
• Oxidation of fats and oils produces unpleasant smells and tastes, which is known as rancidity.
• To prevent this, food packets are flushed with nitrogen, an inert gas that does not react with the oils and fats.
• Nitrogen displaces oxygen inside the packaging and helps in preserving the freshness and shelf life of the food.
• Nitrogen is an inert gas, meaning it does not react easily with other substances.
• By flushing the food package with nitrogen, the oxygen inside the pack is removed or displaced.
• Without oxygen, the oxidation of oils and fats is minimized or completely prevented.
• This helps to keep the food fresh for a longer time, maintaining its taste, smell, and nutritional quality.

Conclusion:

Flushing with nitrogen prevents rancidity by protecting oils and fats from oxidation.

Water is a compound formed by the chemical combination of hydrogen and oxygen in a fixed ratio (H₂O). When hydrogen and oxygen combine to form water, they lose their individual properties. The properties of a compound are completely different from the properties of the elements that form it.

1. Hydrogen is highly inflammable.
2. Oxygen supports combustion.
3. But water is neither inflammable nor supports combustion.

Water is used to extinguish fire because:

1. It cools the burning material below its ignition temperature.
2. It forms a barrier between the fire and oxygen in the air, cutting off the supply of oxygen.

Thus, despite being made from hydrogen and oxygen, water acts as an effective fire extinguisher due to its completely different chemical properties.

Hypermetropia is a common eye problem where a person can see distant objects clearly but has difficulty seeing nearby objects clearly.

This happens because the eye lens focuses the image behind the retina instead of on it, usually because the eyeball is too short or the lens is not curved enough.

People with hypermetropia need a convex lens to help focus the image correctly on the retina so they can see nearby objects clearly.

The cord of an electric heater does not glow because it is made of a material with low resistance, so it does not heat up much.

The heating element glows because it is made of a material with high resistance, which causes it to get very hot and produce light when current passes through it.

To calculate the resistivity (ρ) of the material, we use the formula:

R = ρ × (L / A)

Where:

• R = 20 ohms (resistance)
• L = 1 meter (length)
• A = πr² (cross-sectional area of the wire)
• r = 0.01 cm = 1 × 10⁻⁴ meters (radius)

Step-by-step calculation:

Calculate the area (A) of the wire's cross section:
A = π × (1 × 10⁻⁴)² = π × 10⁻⁸ m² ≈ 3.1416 × 10⁻⁸ m²

Rearrange the formula to solve for ρ:
ρ = R × (A / L)

Substitute the values:
ρ = 20 × (3.1416 × 10⁻⁸ / 1)

Calculate the resistivity:
ρ ≈ 20 × 3.1416 × 10⁻⁸ = 6.28 × 10⁻⁷ Ω·m

Difference between Displacement and Double Displacement Reactions:

Displacement Reaction:

One element displaces another element from a compound. Usually involves a more reactive element replacing a less reactive one. For example:
            Zn + CuSO4 → ZnSO4 + Cu
            (Zinc displaces copper from copper sulfate to form zinc sulfate and copper.)

Double Displacement Reaction:

Two compounds exchange their ions to form two new compounds. Often occurs in aqueous solutions and may form a  precipitate, gas, or water. For example:
             NaCl + AgNO3 → NaNO3 + AgCl
             (Sodium chloride reacts with silver nitrate to form sodium nitrate and silver chloride, which precipitates.)

Key Difference:

• Displacement reaction involves one element replacing another.
• Double displacement reaction involves exchange of ions between two compounds.

Refining of Silver – Recovery from Silver Nitrate Solution:

In the refining of silver, silver metal is recovered from its salt solution (silver nitrate – AgNO₃) by a displacement reaction using copper metal. This works because copper is more reactive than silver and can displace silver from its compound.

Type of Reaction:
Displacement Reaction (also called Single Displacement or Single Replacement Reaction)

Chemical Reaction:
Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag

Explanation:

• Copper (Cu) is placed into a solution of silver nitrate (AgNO₃).
• Copper, being more reactive than silver, displaces silver from the silver nitrate solution.
• As a result, copper nitrate (Cu(NO₃)₂) is formed in the solution, and silver (Ag) is deposited as a solid.
• This reaction is used in the purification and recovery of silver in metallurgy.

Result:
Silver is obtained in pure form, and the solution now contains copper nitrate.

Ionic compounds have high melting points because the forces of attraction between the oppositely charged ions are very strong.

• Ionic compounds are made of positive and negative ions.
• These ions are held together by strong forces of attraction.
• A lot of heat is needed to break these bonds.
• That’s why ionic compounds have high melting points.

Examples of ionic compounds with high melting points:

• Sodium chloride (NaCl)
• Magnesium oxide (MgO)

Given:
Resistance (R) = 20 ohms
Current (I) = 5 amperes
Time (t) = 30 seconds

Formula:
Heat developed (H) = I² × R × t
H = 5² × 20 × 30 = 25 × 20 × 30 = 15000 joules

Hence heat developed in 30 seconds = 15000 joules

Ozone (O₃) is a gas made up of three oxygen atoms. It is found in the upper layer of the atmosphere, called the stratosphere, and forms the ozone layer.

How ozone affects the ecosystem:

1. Protects living beings from harmful UV rays:
        The ozone layer absorbs most of the Sun's ultraviolet (UV) rays, which are harmful to humans, animals, and plants. Without           it, UV rays can cause skin cancer, eye damage, and harm crops.

2. Maintains balance in the ecosystem:
         By blocking harmful radiation, ozone helps protect ecosystems like forests, oceans, and farms, allowing plants and animals           to survive and grow properly.

An acid-base indicator is a substance that changes its colour in acidic or basic solutions. It is used to identify whether a solution is acidic or basic.

Example of a synthetic acid-base indicator:
Phenolphthalein – It is colourless in acids and pink in bases.

The white powder is baking powder.

Main ingredients:

• Sodium hydrogen carbonate (baking soda)
• Tartaric acid

Functions:

• Baking soda releases carbon dioxide gas on heating, making the cake soft and spongy.
• Tartaric acid neutralizes the bitter taste of sodium carbonate formed.

Chemical reaction:
2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O

Carbon dioxide makes the cake rise and fluffy.

For a human eye with normal vision:

1. Far point is infinity. This means a normal eye can see distant objects clearly, no matter how far they are.
2. Near point is about 25 centimeters from the eye.This is the minimum distance at which the eye can see objects clearly without strain.

The ciliary muscleshelp in changing the shape of the eye lens to focus on objects at different distances. This process is called accommodation.

• When we look at a nearby object, the ciliary muscles contract, making the lens thicker to increase its converging power.
• When we look at a distant object, the ciliary muscles relax, making the lens thinner to decrease its converging power.

Resistance (R) = Resistivity (ρ) × (Length (L) / Area (A))

Given:-
Resistivity (ρ) = 2.8 × 10⁻⁸ ohm meter
Length (L) = 2 meters
Area (A) = 1.55 × 10⁻⁶ square meters

Substitute the values:
R = (2.8 × 10⁻⁸) × (2 / 1.55 × 10⁻⁶)
R = (5.6 × 10⁻⁸) / (1.55 × 10⁻⁶)
R ≈ 0.0361 ohms

Hence answer is  0.0361 ohms

The resistance of a conductor depends on:

1. Length (L):Directly proportional to resistance.
2. Cross-sectional area (A): Inversely proportional to resistance.
3. Resistivity (ρ):Material property; higher resistivity means higher resistance.
4. Temperature (T): Generally, resistance increases with temperature.