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Q1. Hydrogen gas is
Hydrogen gas is combustible, meaning it burns in air or oxygen. When hydrogen burns, it produces water (H₂O). This is a chemical reaction that releases energy. Hydrogen is not poisonous, not liquid at room temperature (it is a gas), and it is odourless (no smell). Its flammability makes it useful as a fuel but also requires careful handling.
Q2. Which substance is attracted by a magnet?
Iron is attracted by a magnet because it is a magnetic substance. This property of iron makes it easy to separate from non-magnetic substances like sulphur. Iron sulphide is not attracted by a magnet because the iron and sulphur have chemically combined, and the compound no longer shows magnetic properties. Sulphur and hydrogen sulphide are also not magnetic.
Q3. Compounds show
Compounds show chemical bonding. In a compound, atoms of different elements are held together by chemical bonds (ionic or covalent bonds). This bonding is what gives compounds their fixed composition and unique properties. Compounds do not have variable composition, cannot be easily separated, and involve chemical bonding between the atoms.
Q4. Hydrogen sulphide gas is produced when acid reacts with
Hydrogen sulphide gas is produced when acid reacts with iron sulphide. Iron sulphide (FeS) reacts with dilute hydrochloric acid (HCl) to produce hydrogen sulphide gas (H₂S), which has a characteristic rotten-egg smell. Iron metal produces hydrogen gas with acid, copper does not react easily, and sulphur does not produce hydrogen sulphide with acid.
Q5. Which statement is correct for compounds?
The correct statement for compounds is that a new substance is formed. When elements combine chemically to form a compound, a completely new substance with different properties is created. Compounds have a fixed ratio and cannot be separated easily (only by chemical methods). New substance formation is a key characteristic of compounds.
Q6. A compound involves
A compound involves a chemical reaction. When elements combine to form a compound, a chemical change takes place. This reaction often involves energy changes (heat, light, or electricity). The chemical reaction results in new substances with new properties. There is no easy separation of components, and physical mixing alone does not form compounds.
Q7. Components of a compound can be separated only by
The components of a compound can be separated only by chemical reactions. Because the elements in a compound are chemically bonded, they cannot be separated by physical methods like filtration, magnet, or handpicking. To separate the elements, a chemical reaction must be performed to break the chemical bonds.
Q8. Iron sulphide cannot be separated by
Iron sulphide cannot be separated by physical methods. Since it is a compound formed by a chemical reaction, its components (iron and sulphur) are chemically bonded. Physical methods like using a magnet, filtration, or handpicking cannot separate the iron from the sulphur in iron sulphide. Only chemical methods can decompose it.
Q9. The material formed by heating iron and sulphur has
The material formed by heating iron and sulphur (iron sulphide) has different properties from both iron and sulphur. Iron sulphide is a new compound with its own distinct properties. It is not magnetic (unlike iron), and it is not yellow (unlike sulphur). The formation of new properties is evidence of a chemical change.
Q10. Combustion test of hydrogen in class is
The combustion test of hydrogen in class is not advised. Hydrogen is highly flammable and can form explosive mixtures with air. Burning hydrogen in a classroom setting is dangerous and should be avoided. Demonstrations should only be performed under strict safety conditions, and students should not handle burning hydrogen.
Q11. Which material shows magnetic properties?
An iron and sulphur mixture shows magnetic properties because it still contains uncombined iron. When iron and sulphur are simply mixed (not heated), the iron particles retain their magnetic property and can be attracted by a magnet. After heating, when iron sulphide is formed, the magnetic property disappears because the iron has chemically combined with sulphur.
Q12. Which shows chemical change?
Heating iron and sulphur shows a chemical change. When the mixture of iron and sulphur is heated strongly, a chemical reaction takes place, forming iron sulphide (FeS). This is a chemical change because a new substance with different properties is formed. Crushing, mixing, and separating with a magnet are physical changes.
Q13. Which group produced hydrogen sulphide gas?
The heating group produced hydrogen sulphide gas. After heating iron and sulphur to form iron sulphide, adding acid to the product produces hydrogen sulphide gas (rotten-egg smell). The mixing group only had a mixture of iron and sulphur, which when treated with acid would produce hydrogen gas (not hydrogen sulphide) because the iron is still free.
Q14. In a compound, the elements are combined in a
In a compound, the elements are combined in a fixed proportion. This is a fundamental characteristic of compounds. For example, water (H₂O) always has two hydrogen atoms and one oxygen atom. The fixed proportion is what distinguishes compounds from mixtures, where the components can be in any ratio.
Q15. Heating iron and sulphur results in
Heating iron and sulphur results in a chemical reaction. When the mixture is heated strongly, iron and sulphur react chemically to form iron sulphide (FeS). This is a chemical change because a new substance is formed with properties different from the original elements. The reaction is exothermic (releases heat).
Q16. Iron and sulphur before heating are
Iron and sulphur before heating are a mixture of elements. They are simply mixed together but have not undergone any chemical reaction. The iron and sulphur retain their individual properties (iron is magnetic, sulphur is yellow). This mixture can be separated by physical methods like using a magnet.
Q17. The mixture of iron and sulphur can be separated by
The mixture of iron and sulphur can be separated by physical methods, such as using a magnet. Because the mixture has not undergone a chemical reaction, the iron retains its magnetic property and can be attracted by a magnet, separating it from the sulphur. Filtration and handpicking can also be used in some cases.
Q18. The process taking place when iron and sulphur are heated strongly is a
The process of heating iron and sulphur strongly is a chemical change. When heated, iron and sulphur react to form iron sulphide, which is a completely new substance with different properties. The change is permanent and irreversible—you cannot get the original iron and sulphur back by physical means. This is a characteristic chemical change.
Q19. Which is TRUE for compounds?
The true statement for compounds is that they have a fixed ratio. Compounds always contain elements in a definite, fixed proportion by mass. For example, in iron sulphide (FeS), the ratio of iron to sulphur is always 1:1. Compounds do not have variable properties, cannot be separated by magnetic methods, and are not easy to separate.
Q20. Iron and sulphur together without heating form a
Iron and sulphur together without heating form a mixture. When they are simply mixed, no chemical reaction occurs, so they remain as a mixture of elements. The individual properties of iron and sulphur are retained, and they can be separated by physical methods. Heating is required to form a compound (iron sulphide).
Q21. Mixing iron and sulphur without heating results in
Mixing iron and sulphur without heating results in no new substance. Only a physical mixture is formed where the original substances retain their properties. The iron is still magnetic and the sulphur is still yellow. No chemical reaction takes place, so no new substance like iron sulphide is formed.
Q22. Which is TRUE for mixtures?
The true statement for mixtures is that they are a physical combination. In a mixture, the components are simply physically mixed together without any chemical reaction. They retain their individual properties and can be separated by physical methods. There is no fixed composition, no chemical bonding, and no new substance is formed.
Q23. Hydrogen sulphide gas smells like
Hydrogen sulphide gas smells like rotten eggs. This is its characteristic odour and is a way to identify it in the laboratory. The smell is very unpleasant and noticeable even in small amounts. Ammonia has a pungent smell, alcohol has a distinct smell, and vinegar smells acidic.
Q24. The colour of iron sulphide is
The colour of iron sulphide is different from both iron and sulphur. Iron is greyish, sulphur is yellow, but iron sulphide is a dark, greyish-black or brownish compound. The fact that the compound has a colour different from its constituent elements is evidence that a chemical change has occurred and a new substance has been formed.
Q25. A mixture can be separated
A mixture can be separated by physical methods. Because the components in a mixture are not chemically bonded, they can be separated using physical techniques like handpicking, filtration, sieving, magnetism, evaporation, and distillation. No chemical reactions are needed to separate mixtures.
Q26. The main difference between mixture and compound is
The main difference between a mixture and a compound is the method of formation. A mixture is formed by simple physical mixing without any chemical reaction. A compound is formed by a chemical reaction between elements, resulting in a new substance with new properties. This difference in formation leads to differences in properties and separation methods.
Q27. Which statement is correct for mixtures?
The correct statement for mixtures is that components retain their properties. When substances are mixed, they do not lose their individual characteristics. Iron in a mixture is still magnetic and sulphur is still yellow. This is different from compounds, where the elements lose their original properties and form a new substance.
Q28. The composition of a compound is
The composition of a compound is the same throughout. A compound is homogeneous, meaning that every part of it has the same composition. For example, every molecule of water is H₂O, and every part of pure copper sulphate has the same formula. This uniformity is a characteristic of compounds, unlike mixtures which can vary.
Q29. Which retains individual properties?
A mixture retains the individual properties of its components. In a mixture, the substances are physically combined but not chemically changed, so they keep their original characteristics. For example, in a mixture of iron and sulphur, the iron is still magnetic and the sulphur is still yellow. Compounds do not retain individual properties because they form new substances.
Q30. Mixtures show
Mixtures show variable composition. The components of a mixture can be present in any proportion. For example, you can mix iron and sulphur in any ratio—50:50, 70:30, etc. This is different from compounds, which have a fixed composition. The variable composition is a key distinguishing property of mixtures.
Q31. Hydrogen gas is
Hydrogen gas is colourless and odourless. It cannot be seen or smelled. This is why it is not easily detectable by human senses, making it dangerous—since it is also flammable, it can ignite without warning. Hydrogen is not coloured, not poisonous (though it can cause suffocation by displacing oxygen), not acidic, and does not smell like rotten eggs (hydrogen sulphide does).
Q32. The colour of a compound is
The colour of a compound is uniform throughout. A pure compound is homogeneous, meaning all parts of it have the same colour. For example, pure iron sulphide is uniformly dark in colour. In contrast, mixtures often have patchy or uneven colour because the components are not evenly distributed.
Q33. The texture of a compound is
The texture of a compound is the same throughout. Since a compound is homogeneous, its physical properties, including texture, are uniform in all its parts. This consistency is due to the fixed composition of compounds. Mixtures can have variable texture because their components are not evenly distributed.
Q34. Iron sulphide does NOT show
Iron sulphide does NOT show magnetic property. Although iron is magnetic, when it chemically combines with sulphur to form iron sulphide, the compound loses the magnetic property. This is a key difference between a mixture (where iron retains magnetism) and a compound (where it does not). This loss of magnetic property is evidence of a chemical change.
Q35. The gas with the smell of rotten eggs is
The gas with the smell of rotten eggs is hydrogen sulphide (H₂S). This characteristic smell is its most distinctive property and makes it easily identifiable even in tiny amounts. Hydrogen, oxygen, and sulphur dioxide do not have this smell. The smell is due to the presence of sulphur in the compound.
Q36. A compound can be separated
A compound can be separated by chemical methods. Since the elements in a compound are chemically bonded, physical methods like filtration, magnet, or evaporation cannot separate them. Chemical reactions are needed to break the bonds and separate the elements. For example, iron sulphide can be decomposed by heating or by reacting with acid.
Q37. Heating iron and sulphur strongly produces a
Heating iron and sulphur strongly produces a compound (iron sulphide). When the mixture is heated to a high temperature, a chemical reaction takes place, and the two elements combine to form a new compound with distinct properties. Iron sulphide is a uniform, non-magnetic, greyish-black substance.
Q38. When iron and sulphur are simply mixed, the result is a
When iron and sulphur are simply mixed, the result is a mixture. No chemical reaction occurs, so the components remain separate. The iron and sulphur can be distinguished visually (yellow and grey particles), and their individual properties are retained. This is a simple physical combination, not a compound.
Q39. Iron sulphide is an example of a
Iron sulphide is an example of a compound. It is formed by the chemical combination of iron and sulphur in a fixed proportion (FeS). It has properties different from both iron and sulphur and cannot be separated by physical methods. This makes it a typical example of a compound.
Q40. In a mixture, the properties are
In a mixture, the properties are the same as the constituents. Since no chemical reaction occurs, each component retains its own physical and chemical properties. For example, in a mixture of iron and sulphur, the iron is still magnetic and the sulphur is still yellow. The properties of the mixture are simply the combined properties of its components.
Q41. Which does NOT retain individual properties?
A compound does NOT retain the individual properties of its constituent elements. When elements combine chemically, they lose their original properties and form a new substance with new properties. For example, iron is magnetic and sulphur is yellow, but iron sulphide is not magnetic and is greyish-black. Elements, mixtures, and solutions retain the properties of their components.
Q42. A compound is formed when two or more elements are
A compound is formed when two or more elements are chemically combined. Chemical combination involves a chemical reaction where atoms of different elements bond together, resulting in a new substance with distinct properties. Physical mixing, light heating, or dissolving do not create compounds—they create mixtures or solutions.
Q43. Iron sulphide is formed by
Iron sulphide is formed by heating iron and sulphur. When the mixture is heated strongly, a chemical reaction occurs, and iron sulphide is formed. Crushing iron, mixing iron and sulphur, or dissolving sulphur does not create a compound—a chemical reaction (heating) is necessary to form iron sulphide.
Q44. The gas produced when dilute acid reacts with iron is
The gas produced when dilute acid reacts with iron is hydrogen (H₂). Iron metal reacts with acids (like hydrochloric acid) to produce hydrogen gas. This is a standard reaction: Fe + 2HCl → FeCl₂ + H₂. Hydrogen sulphide is produced when acid reacts with iron sulphide, not with iron metal.
Q45. A mixture has
A mixture has variable composition. The components of a mixture can be present in any proportion. For example, you can mix different amounts of iron and sulphur to make different mixtures. There is no fixed ratio. This is a distinguishing characteristic of mixtures, unlike compounds which have fixed composition.
Q46. A compound has
A compound has fixed composition. The elements in a compound are always combined in a definite, fixed proportion by mass. For example, iron sulphide always has a 1:1 ratio of iron to sulphur (FeS). This fixed composition is a fundamental characteristic of compounds and distinguishes them from mixtures.
Q47. The process taking place in simple mixing of iron and sulphur is a
Simple mixing of iron and sulphur is a physical change. No chemical reaction occurs, and no new substance is formed. The components retain their individual properties and can be separated by physical methods. The change is merely physical—a change in the arrangement of the components.
Q48. A mixture does NOT involve
A mixture does NOT involve the formation of a new substance. In a mixture, the components are simply physically combined, and no chemical reaction takes place. The original substances retain their properties, and there is no new substance created. This is a key difference between mixtures and compounds, which involve the formation of new substances.
Q49. Chemical combination results in
Chemical combination results in the formation of a new substance. When elements or compounds react chemically, bonds are broken and new bonds are formed, creating substances with different properties from the original reactants. This is the defining feature of chemical reactions and compounds. Easy separation and physical mixtures are not results of chemical combination.
Q50. In a compound, the properties are
In a compound, the properties are totally different from those of the constituent elements. When elements combine chemically to form a compound, they lose their individual characteristics and give rise to a new substance with entirely new properties. For example, sodium (a reactive metal) and chlorine (a poisonous gas) combine to form common salt (NaCl), which is safe to eat.