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

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.)

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.

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 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.

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.

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

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.

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.

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.

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 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.

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.

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.

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

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.

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.

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

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.

When magnesium is kept in the open air for some time, it reacts slowly with the oxygen in the air to form a thin, white layer of magnesium oxide (MgO) on its surface. This layer sticks tightly to the metal and prevents the fresh magnesium underneath from coming in direct contact with oxygen.

Before burning magnesium in an experiment, this oxide layer must be removed by rubbing the ribbon with sandpaper or a file. This exposes the shiny surface of pure magnesium, which reacts easily and quickly with oxygen when heated.

When clean magnesium is burned, it reacts vigorously with oxygen to form magnesium oxide, producing a bright white flame and intense heat.

Reasons for cleaning the ribbon:

• To remove the oxide layer (MgO).
• To ensure smooth and complete combustion.
• To allow accurate observation of the reaction.

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)

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

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.

Coils of electric toasters and electric irons are made of an alloy rather than a pure metal due to following reasons:

1. Alloys have higher resistance than pure metals, so they produce more heat when electric current passes through them, which is useful for heating devices.
2. Alloys do not melt easily,  they have higher melting points, so the coils can withstand high temperatures without melting.
3. Alloys are more durable and strong,so the coils last longer under repeated heating and cooling.

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.

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.

Alternating Current (A.C.): -Alternating current is an electric current that changes its direction and strength continuously with time. It flows in one direction for a short time and then reverses to flow in the opposite direction. This happens many times in one second. For example, in most countries, A.C. changes direction 100 times per second (50 cycles per second or 50 Hz).

Direct Current (D.C.): - Direct current is the type of electric current that flows in one constant direction and has a steady value. It does not change with time. Batteries and cells produce direct current.

An aqueous solution of an acid conducts electricity because it contains ions.

When an acid (like HCl) is dissolved in water, it ionises and releases hydrogen ions (H⁺) and other negative ions (like Cl⁻).

Example:
HCl   --->  H⁺ + Cl⁻

These free ions help in the flow of electric current through the solution.

Given - 
Power for distant vision = –5.5 dioptres
Power for near vision = +1.5 dioptres

Using the formula:
Power (P) = 100 / focal length (f in cm)
So, focal length (f) = 100 / Power (P)

(i) For distant vision:
f = 100 / (–5.5) = –18.18 cm

(ii) For near vision:
f = 100 / (+1.5) = 66.67 cm

Hence the focal length of the lens required for correcting:

Distant vision = –18.18 cm  &  Near vision = +66.67 cm

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.

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.

When dilute hydrochloric acid (HCl) is added to a reactive metal, hydrogen gas (H₂) is produced.

Chemical Reaction of Iron with Dilute Sulphuric Acid (H₂SO₄):- 

Iron reacts with dilute sulphuric acid to form iron(II) sulphate (FeSO₄) and hydrogen gas (H₂).

Chemical equation:
Fe + H₂SO₄ → FeSO₄ + H₂↑

A person with a myopic eye cannot see objects beyond 1.2 m clearly. To correct this defect, a concave lens should be used.

A concave lens helps to diverge the light rays so that they appear to come from the person's far point (1.2 m) and can be focused properly on the retina.

The far point of the myopic person is 80 cm.
To correct myopia, a concave lens is used.
Power of the lens = –100 / 80 = –1.25 dioptres.

Hence, the lens needed is a concave lens with power –1.25 dioptres.

Metals are good conductors of electricity because they have a large number of free electrons (also called conduction electrons). These electrons are not tightly bound to atoms and can move freely through the metal. When a potential difference is applied, these free electrons flow easily, allowing electric current to pass.

Glass is a bad conductor (insulator) because its electrons are tightly bound to their atoms. It does not have free electrons available to move and carry electric current. As a result, electricity cannot flow through it easily.

In short:

• Metals have free-moving electrons  --->  Good conductors.
• Glass lacks free-moving electrons  ---->  Bad conductor.

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.

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.

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.

1. Myopia (Nearsightedness)

• Cause: The eyeball is too long or the lens is too curved. Light focuses in front of the retina.
• Correction: Concave lens (diverging lens)

2. Hypermetropia (Farsightedness)

• Cause: The eyeball is too short or the lens is too flat. Light focuses behind the retina.
• Correction: Convex lens (converging lens)

3. Presbyopia

• Cause: Loss of elasticity of the eye lens due to aging. The eye cannot focus on nearby objects.
• Correction: Bifocal lens or progressive lens

Advantages of connecting electrical devices in parallel instead of in series:

1. Each device gets the full voltage of the battery:
   In parallel, every device gets the same voltage as the battery voltage, so all devices work properly.
2. Devices work independently:
   If one device stops working or is switched off, the others continue to work because they have their own separate paths.
3. Current is divided among devices:
   Total current is split according to resistance, preventing any one device from getting too much current.
4. Adding more devices does not reduce voltage:
   You can add more devices without reducing the voltage across each device.

An earth wireis a safety wire that connects the metal body of an electrical appliance to the ground. Its main function is to provide a path for electric current to flow safely into the ground in case there is a fault.

Why is it necessary to earth metallic appliances?

• To prevent electric shock: If the live wire touches the metal body of the appliance, the metal can become live and dangerous. The earth wire carries this current safely into the ground, so anyone touching the appliance does not get an electric shock.
• To protect the appliance: Earthing helps prevent damage to the appliance by carrying away leakage current.
• To help the fuse work properly: When a large current flows through the earth wire during a fault, it helps the fuse to blow quickly and stop the electricity supply.

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.

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.

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.

An electromagnet is a type of temporary magnet made by winding a coil of wire around a soft iron core and passing electric current through the coil. When current flows, the coil produces a magnetic field, magnetizing the iron core. The magnetism disappears when the current is switched off.

Two Uses of an Electromagnet:

1. In Electric Bells and Buzzers: -  Electromagnets are used to attract a metal arm, which helps in producing sound when the circuit is completed.
2. In Cranes for Lifting Heavy Iron Objects: -  Large electromagnets are used in junkyards and scrap industries to lift and move heavy pieces of iron or steel.

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.

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.

Plaster of Paris (POP) is prepared by heating gypsum (calcium sulphate dihydrate) at about 150°C. This removes part of the water of crystallisation.

The reaction is:
CaSO₄·2H₂O → CaSO₄·½H₂O + 1½ H₂O (water vapor)

When POP is mixed with water, it absorbs water and sets into a hard mass by changing back to gypsum.

The setting reaction is:
CaSO₄·½H₂O + 1½ H₂O → CaSO₄·2H₂O

This forms a hard, solid mass because of the formation of interlocking crystals.

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.

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.