📘 Welcome
Hi User, you have selected Read Mode.
This is Time Free Mode for your convenience to understand every question as per your Ease and Time.
Here You get Answer and Details button. After mastering this mode, you can go for a test with Test Mode on the main page designed especially with Exam Features.
This is Time Free Mode for your convenience to understand every question as per your Ease and Time.
Here You get Answer and Details button. After mastering this mode, you can go for a test with Test Mode on the main page designed especially with Exam Features.
Q1. The term “element” was first used by
The term “element” was first used by Robert Boyle, the famous Irish scientist. In his book “The Sceptical Chymist” (1661), Boyle defined elements as simple substances that cannot be broken down into simpler substances by chemical methods. This was a major step in the development of modern chemistry and helped scientists move away from the ancient concept of the four elements (earth, water, air, fire).
Q2. Which method is better than simple evaporation for purification?
Crystallisation is better than simple evaporation for purification. When a solution is evaporated, impurities that are also soluble remain with the solid, contaminating it. In crystallisation, the solid forms as pure crystals while impurities stay dissolved in the solution. This gives much purer products. Examples include obtaining pure copper sulphate or alum crystals from their solutions.
Q3. Crystallisation is mainly used to
Crystallisation is mainly used to purify solids. It is a technique where a solid is dissolved in a solvent to form a solution, and then the solid is recovered as pure crystals. The process works because different substances have different solubilities at different temperatures. Crystallisation is widely used to obtain pure substances like salt, sugar, copper sulphate, and alum from impure samples.
Q4. Ice, water and water vapour are
Ice, water, and water vapour are chemically the same. All three are different physical states of the same substance, H₂O (water). They have the same chemical composition, but their physical states (solid, liquid, gas) differ due to differences in the arrangement and energy of their particles. They are not elements, as water is a compound.
Q5. Bromine at room temperature is
Bromine at room temperature is a liquid. It is one of only two elements that are liquid at room temperature (the other is mercury). Bromine is a reddish-brown liquid with a pungent odour. It is a non-metal and is volatile, meaning it evaporates easily to form a gas. It is not a gas, plasma, or solid at room temperature.
Q6. Metals are generally
Metals are generally lustrous, meaning they have a shiny appearance when freshly cut or polished. This is due to the way they reflect light. Metals are also good conductors of heat and electricity, sonorous (produce sound when struck), and malleable (can be hammered into sheets) and ductile (can be drawn into wires). They are not brittle; brittleness is a property of non-metals.
Q7. The number of gaseous elements at room temperature is
The number of gaseous elements at room temperature is 11. These include the noble gases (helium, neon, argon, krypton, xenon, and radon), as well as gases like hydrogen, nitrogen, oxygen, fluorine, and chlorine. They are in the gaseous state because their particles have weak forces of attraction and high kinetic energy at room temperature.
Q8. Cutting of trees is a
Cutting of trees is a physical change because it only involves a change in size and shape of the wood. The chemical composition of the wood does not change. It is not a chemical change because no new substance is formed. However, it is not reversible (a cut tree cannot be made whole again), but it is still classified as a physical change because the chemical identity remains unchanged.
Q9. Change of state of matter is a
A change of state of matter (melting, freezing, boiling, condensation, sublimation) is a physical change. In a change of state, the chemical composition of the substance remains the same—only the physical form changes. For example, when ice melts to water, the chemical identity (H₂O) remains unchanged. These changes are reversible and do not produce new substances.
Q10. Most elements at room temperature are
Most elements at room temperature are solids. Out of the 118 known elements, about 80 are solids at room temperature. Only a few are gases (about 11) and very few are liquids (only 2: mercury and bromine). The rest are in other states under certain conditions. Solids are the most common state of elements because their particles are closely packed.
Q11. Ability to be drawn into wires is called
The ability to be drawn into wires is called ductility. Ductile metals like copper, silver, and gold can be stretched into thin wires without breaking. This property is important for making electrical wires. Malleability is the ability to be hammered into thin sheets, sonority is the ability to produce sound, and rigidity is the stiffness of a material.
Q12. The only metal liquid at room temperature is
Mercury is the only metal that is liquid at room temperature. It is a heavy, silvery-white metal that remains in the liquid state at normal room temperature. Other metals like aluminium, sodium, and iron are solids at room temperature. Mercury is commonly used in thermometers and barometers. Bromine is a non-metal that is also liquid at room temperature.
Q13. Burning of paper results in
Burning of paper results in new substances. When paper burns, it reacts with oxygen in the air to form new substances like carbon dioxide, water vapour, and ash. This is a chemical change because the chemical composition of the paper changes completely. The change is irreversible—burnt paper cannot be turned back into paper. New substances are always formed in chemical reactions.
Q14. Kerosene and petrol (miscible, BP difference > 25°C) are separated by
Kerosene and petrol are separated by simple distillation because they are miscible liquids with a boiling point difference greater than 25°C. When the mixture is heated, the component with the lower boiling point (petrol) vaporises first and is condensed and collected. Filtration is for insoluble solids, chromatography is for separating solutes, and fractional distillation is used when boiling point differences are smaller.
Q15. Cesium becomes liquid
Caesium becomes liquid slightly above room temperature. Its melting point is about 28.5°C, which is just above normal room temperature (about 25°C). This means that on a warm day, caesium can be liquid. It is one of the few metals that melts near room temperature. Gallium is another metal with a low melting point (about 30°C).
Q16. An element cannot be broken into simpler substances by
An element cannot be broken into simpler substances by chemical reactions. Elements are the simplest forms of matter and cannot be decomposed into simpler substances by ordinary chemical means. They can only be converted into other elements through nuclear reactions (which are not chemical reactions). Heating and melting may change the state of an element but do not break it down into simpler substances.
Q17. Salt is separated from sea water by
Salt is separated from sea water by evaporation. Sea water contains dissolved salt (sodium chloride). When sea water is allowed to evaporate in large shallow ponds (salt pans), the water evaporates, leaving behind salt crystals. This is a simple and traditional method used to obtain salt from sea water. The salt is then collected and purified.
Q18. Substances with fixed chemical composition are called
Substances with fixed chemical composition are called pure substances. Pure substances have a definite, constant composition and cannot be separated into simpler substances by physical methods. They are either elements (like gold) or compounds (like water). Mixtures, suspensions, and solutions have variable compositions—they are combinations of different substances.
Q19. Rusting of an almirah is a
Rusting of an almirah is a chemical change. When iron rusts, it reacts with oxygen and moisture in the air to form iron oxide (rust), which is a new substance with different properties. Rusting is irreversible and permanent. It is not a physical change because the chemical composition of the iron changes. This is why rusting is considered a chemical change.
Q20. Drinking water in cities is supplied from
Drinking water in cities is supplied from water works. Water is collected from sources like rivers, lakes, or reservoirs, and then it is treated at water works. The treatment involves processes like sedimentation, filtration, and chlorination to make the water safe for drinking. After treatment, the water is supplied to homes through a network of pipes.
Q21. Crystallisation is used for purification of
Crystallisation is used for purification of alum. Alum (potash alum) is a solid that can be purified by crystallisation. An impure sample of alum is dissolved in water, and the solution is filtered and allowed to cool slowly. Pure alum crystals form, leaving impurities in the solution. Butter is separated by centrifugation, ink by chromatography, and kerosene is a liquid.
Q22. Which of the following is NOT a metal property?
Being a poor conductor is NOT a property of metals. Metals are generally good conductors of heat and electricity. They are also ductile (can be drawn into wires), lustrous (shiny), and sonorous (produce sound when struck). Non-metals are poor conductors. For example, copper and aluminium are excellent conductors of electricity, which is why they are used in electrical wiring.
Q23. Camphor is separated from salt by
Camphor is separated from salt by sublimation. Camphor is a sublimable solid—it changes directly from solid to gas when heated. When a mixture of camphor and salt is heated, the camphor sublimes and escapes as vapour, leaving the salt behind. The camphor vapour can be collected and condensed back to solid. This is a simple and effective separation method for sublimable substances.
Q24. Some impurities remain dissolved even after
Some impurities remain dissolved even after filtration. Filtration removes only insoluble solid particles from a liquid. Dissolved impurities, like salt dissolved in water, pass through the filter paper along with the liquid. To remove dissolved impurities, other methods like evaporation, distillation, or crystallisation are needed. Filtration is not effective for dissolved substances.
Q25. Properties like colour and density are called
Properties like colour and density are called physical properties. Physical properties are characteristics that can be observed or measured without changing the chemical composition of the substance. Examples include colour, density, melting point, boiling point, hardness, and state of matter. Chemical properties, such as reactivity, describe how a substance changes into a new substance.
Q26. Gold and silver are examples of
Gold and silver are examples of metals. They are lustrous, malleable, ductile, and good conductors of heat and electricity. Both are valuable precious metals used in jewellery, coins, and electronics. They are elements, not compounds, and they are not non-metals or metalloids. Metalloids have properties of both metals and non-metals (like silicon).
Q27. Making a fruit salad is an example of
Making a fruit salad is an example of a physical change. The fruits are cut into pieces and mixed together, but their chemical composition remains the same. No new substances are formed. The taste and colour remain the same. It is a reversible change in the sense that the fruits could be separated, though not practically. Physical changes involve changes in physical properties without changing chemical composition.
Q28. Which of the following is a physical property of matter?
Density is a physical property of matter. It is the mass per unit volume of a substance and can be measured without changing the substance’s chemical identity. Decomposition, burning, and rusting are chemical changes because they involve the formation of new substances. Physical properties include density, colour, hardness, melting point, and boiling point.
Q29. Crystals are separated from the liquid in a china dish by
Crystals are separated from the liquid in a china dish by evaporation. In the crystallisation process, the solution is heated in a china dish to evaporate the solvent. As the liquid evaporates, the solution becomes saturated, and crystals start forming. The remaining liquid is allowed to evaporate completely, leaving behind pure crystals. This is how crystals like copper sulphate or alum are obtained.
Q30. Elements made artificially are called
Elements made artificially are called synthetic elements. These are elements that do not occur naturally on Earth and are created in laboratories through nuclear reactions. Examples include technetium, plutonium, and einsteinium. They are usually radioactive and unstable. Natural elements are those that occur naturally, while noble elements are a class of gases, and metalloids have properties of both metals and non-metals.
Q31. Passing electric current through water producing hydrogen and oxygen is a
Passing electric current through water producing hydrogen and oxygen is a chemical change. Water (H₂O) decomposes into hydrogen gas (H₂) and oxygen gas (O₂). New substances are formed with different chemical compositions. This process is called electrolysis of water. The change is chemical because the chemical composition of the water changes and new substances are produced.
Q32. Substances that are chemically the same but physically different are
Ice and water are substances that are chemically the same but physically different. Both are H₂O (water). Ice is the solid state, and water is the liquid state. They have the same chemical composition but different physical states. Salt and sugar are chemically different, water and oil are different substances, and ice and oil are also different substances.
Q33. Burning of a candle involves
Burning of a candle involves both physical and chemical changes. The melting of wax is a physical change because wax changes from solid to liquid (wax melting). The burning of the wick and the wax vapour is a chemical change because new substances like carbon dioxide, water vapour, and soot are formed. A candle burning is a good example of a process involving both types of changes.
Q34. A chemical reaction always involves
A chemical reaction always involves the formation of new substances. In a chemical change, the original substances (reactants) are transformed into different substances (products) with different chemical compositions. This is the defining characteristic of a chemical reaction. While colour and state changes may occur, they are not always present. Formation of a new substance is the essential feature.
Q35. Oil burns in air because it is
Oil burns in air because it is inflammable. Inflammable means that a substance catches fire easily and burns rapidly. Oil is a flammable liquid that readily reacts with oxygen in the air when heated to its ignition temperature. Its density, liquid state, or colour are not the reasons it burns. The chemical property of being combustible is what allows oil to burn.
Q36. Dissolving salt in water is a
Dissolving salt in water is a physical change. When salt dissolves in water, no new substance is formed. The salt can be recovered by evaporating the water, proving that the change is reversible. The chemical composition of salt (NaCl) and water (H₂O) remains unchanged. A physical change involves a change in physical properties without altering chemical composition.
Q37. Which change does not produce a new substance?
Melting does not produce a new substance. When a solid melts, it changes from solid to liquid state, but the chemical composition remains the same. For example, melting ice gives water, but both are H₂O. Decomposition, rusting, and burning all produce new substances—they are chemical changes. Melting is a physical change because no new substance is formed.
Q38. Silicon is classified as a
Silicon is classified as a metalloid. Metalloids are elements that have properties of both metals and non-metals. Silicon is a good example—it is a semiconductor (like metals), but it is brittle and has a metallic lustre. It is used in the electronics industry. Silicon is not a metal, non-metal, or compound; it is an element with intermediate properties.
Q39. Salt obtained from sea water may contain
Salt obtained from sea water may contain many impurities. Sea water contains not only sodium chloride (salt) but also other dissolved salts like magnesium chloride, calcium sulphate, and various minerals. When sea water evaporates, these impurities remain with the salt. The salt is then purified to remove these impurities before it is used.
Q40. Butter is separated from curd by
Butter is separated from curd by centrifugation. Curd is churned, and the mixture is spun rapidly. The lighter fat particles (butter) move to the top, and the heavier buttermilk settles at the bottom. This separation is based on the difference in density between butter and buttermilk. Filtration, sublimation, and distillation are not suitable for separating butter from curd.
Q41. Non-metals are usually
Non-metals are usually poor conductors of heat and electricity. They do not conduct heat and electricity well because they do not have free electrons. They are generally not malleable (cannot be hammered into sheets), not ductile, and are usually not lustrous (except some like iodine). Metals, on the other hand, are good conductors, malleable, and lustrous.
Q42. Which of the following is a chemical change?
Burning of wood is a chemical change. When wood burns, it reacts with oxygen to form new substances like carbon dioxide, water vapour, and ash. The chemical composition of the wood changes completely, and the change is irreversible. Boiling water, dissolving salt, and melting ice are physical changes because no new substances are formed.
Q43. Number of naturally occurring elements is
The number of naturally occurring elements is about 92 (from hydrogen to uranium). These elements occur naturally on Earth. The remaining elements (about 26) are synthetic—they are created in laboratories. The total number of known elements is 118. Natural elements include carbon, oxygen, gold, iron, and many others.
Q44. Burning of paper is a
Burning of paper is a chemical change. When paper burns, it undergoes combustion, reacting with oxygen to form new substances like ash, carbon dioxide, and water vapour. The change is irreversible and permanent—burnt paper cannot be changed back into paper. This is a characteristic of chemical changes, where new substances are formed.
Q45. Sugar gets charred on heating to dryness because it
Sugar gets charred on heating to dryness because it decomposes. When sugar is heated strongly, it undergoes thermal decomposition, breaking down into simpler substances like carbon (char) and water vapour. This is a chemical change. Sugar does not evaporate (it decomposes first), does not freeze (freezing is for liquids), and does not sublime like sublimable solids.
Q46. Which scientist gave the modern definition of an element?
Antoine Lavoisier gave the modern definition of an element. He defined an element as a substance that cannot be broken down into simpler substances by chemical methods. He also produced the first modern list of elements. Robert Boyle first used the term “element,” but Lavoisier provided the modern scientific definition based on experimental observation.
Q47. Elements having properties of both metals and non-metals are called
Elements having properties of both metals and non-metals are called metalloids. Metalloids are elements that show intermediate properties. They may be shiny like metals but brittle like non-metals. They are semiconductors of electricity. Examples include silicon, germanium, arsenic, and antimony. Compounds are combinations of elements, mixtures are physical combinations, and alloys are mixtures of metals.
Q48. Which non-metal is liquid at room temperature?
Bromine is a non-metal that is liquid at room temperature. It is a reddish-brown volatile liquid with a pungent odour. It is the only non-metal that is liquid at room temperature. Iodine is a solid, chlorine is a gas, and carbon is a solid. Mercury is a metal that is liquid at room temperature.
Q49. Crystallisation separates
Crystallisation separates pure solids from solutions. In this method, a solid is dissolved in a solvent, and the solution is allowed to cool or evaporate slowly. The solid comes out as pure crystals, leaving impurities behind in the solution. This is used to purify solids like alum, copper sulphate, and sugar. Filtration separates insoluble solids, and distillation separates liquids.
Q50. Gallium becomes liquid at
Gallium becomes liquid at about 303 K (which is 30°C). Gallium is a metal with a low melting point—it melts in the hand (at body temperature, which is about 310 K). 300 K is about 27°C, which is slightly below the melting point of gallium. 303 K is approximately 30°C, which is the correct melting point. Gallium is used in semiconductors and thermometers.