Chapter 6: Composition of Matter
Solved textbook questions with step-by-step explanations.
Grade 8 Q&A: Chapter 6: Composition of Matter
Welcome to the Questions and Answers section for Grade 8 Science, Chapter 6: "Composition of Matter." This chapter explores the fundamental classification of matter into elements, compounds, and mixtures, their properties, methods of separation, and the distinction between physical and chemical changes, along with the basic laws of chemical combination.
Important Questions and Answers
Q1: Define 'Element' with an example.
Answer: An element is a pure substance that cannot be broken down into simpler substances by ordinary chemical means. It consists of only one type of atom. Example: Oxygen (O), Iron (Fe).
Q2: Define 'Compound' with an example.
Answer: A compound is a pure substance formed when two or more different elements combine chemically in a fixed ratio by mass. The properties of a compound are entirely different from those of its constituent elements. Example: Water (H₂O), Carbon dioxide (CO₂).
Q3: Define 'Mixture' with an example.
Answer: A mixture is a substance formed when two or more substances (elements or compounds) are physically combined in any proportion, without chemical reaction. The components retain their individual properties. Example: Air, Salt solution.
Q4: Differentiate between a Homogeneous and a Heterogeneous mixture.
Answer:
- Homogeneous Mixture: Components are uniformly distributed, and their individual boundaries are not visible (e.g., salt solution, air).
- Heterogeneous Mixture: Components are not uniformly distributed, and their individual boundaries are visible (e.g., sand and water, oil and water).
Q5: List three properties of a compound.
Answer:
- Formed by chemical combination of two or more elements in a fixed proportion.
- Properties of the compound are different from its constituent elements.
- Can only be separated by chemical or electrochemical reactions.
Q6: List three properties of a mixture.
Answer:
- Formed by physical combination of two or more substances in any proportion.
- Components retain their original properties.
- Can be separated by physical methods.
Q7: Explain 'Decantation' as a separation method. Give an example.
Answer: Decantation is a separation method used to separate insoluble solids from liquids or two immiscible liquids by carefully pouring off the top layer without disturbing the settled bottom layer. Example: Separating sand from water after sand settles down.
Q8: When is 'Filtration' used as a separation method?
Answer: Filtration is used to separate insoluble solids from liquids using a filter paper or other porous medium. The liquid passes through, while the solid particles are retained. Example: Separating chalk powder from water.
Q9: Describe 'Evaporation' as a separation technique.
Answer: Evaporation is used to separate a soluble solid from its liquid solvent. The solution is heated, causing the liquid to turn into vapor and escape, leaving the solid component behind. Example: Obtaining salt from salt solution.
Q10: What is 'Distillation' and when is it employed?
Answer: Distillation is a method used to separate components of a liquid mixture based on differences in their boiling points. The liquid with the lower boiling point evaporates first, is condensed, and then collected as a pure liquid. It is employed when both components are liquids or when a pure liquid needs to be obtained from a solution.
Q11: Explain the principle behind 'Chromatography'.
Answer: Chromatography separates components of a mixture that dissolve in the same solvent but are absorbed differently by a stationary phase (e.g., filter paper). Components that are more soluble and less adsorbed travel faster, leading to their separation. Example: Separating different colors in ink.
Q12: How does 'Centrifugation' work? Give an application.
Answer: Centrifugation separates fine suspended particles from a liquid by spinning the mixture at high speed. The denser particles are forced to the bottom of the container due to centrifugal force, leaving the lighter liquid on top. Application: Separating cream from milk, or blood cells from plasma.
Q13: Differentiate between a Physical Change and a Chemical Change.
Answer:
- Physical Change: Temporary, no new substance formed, chemical composition remains same, often reversible (e.g., melting ice).
- Chemical Change: Permanent, new substance(s) formed, chemical composition changes, generally irreversible (e.g., burning wood).
Q14: State the Law of Conservation of Matter (Mass). Who proposed it?
Answer: The Law of Conservation of Matter states that "Matter can neither be created nor destroyed in a chemical reaction." This means the total mass of reactants equals the total mass of products. It was proposed by Antoine Lavoisier.
Q15: State the Law of Constant Proportions. Who proposed it?
Answer: The Law of Constant Proportions states that "In a chemical compound, the elements are always present in definite proportions by mass, irrespective of the source or method of preparation." It was proposed by Joseph Proust.
Q16: Give two examples of a physical change.
Answer: Melting of ice, boiling of water, tearing of paper, dissolving sugar in water.
Q17: Give two examples of a chemical change.
Answer: Burning of wood, rusting of iron, cooking food, digestion of food.
Q18: Is air an element, compound, or mixture? Justify your answer.
Answer: Air is a mixture. It consists of various gases like nitrogen, oxygen, argon, carbon dioxide, etc., which are physically combined in varying proportions and retain their individual properties. No new substance is formed, and its components can be separated by physical means (e.g., fractional distillation of liquid air).
Q19: Can a compound be separated into its constituent elements by physical methods? Why or why not?
Answer: No, a compound cannot be separated into its constituent elements by physical methods. This is because the elements in a compound are chemically bonded together, forming a new substance with different properties. Separation requires chemical or electrochemical reactions.
Q20: What is the key difference in composition between an element and a compound?
Answer: An element is composed of only one type of atom, while a compound is composed of two or more different types of elements chemically combined in a fixed ratio.
Q21: How is the Law of Conservation of Matter demonstrated in a chemical reaction?
Answer: In a chemical reaction, the total mass of the reactants before the reaction is exactly equal to the total mass of the products formed after the reaction. For example, if 12g of carbon reacts with 32g of oxygen, 44g of carbon dioxide will be formed, demonstrating that mass is conserved.
Q22: Why is it important that elements in a compound are in a 'definite proportion by mass'?
Answer: It is important because it ensures that a chemical compound has a fixed and unique chemical formula and consistent properties. If the proportions were not definite, it would be a mixture, not a compound, and its properties would vary.
Q23: Give an example where magnetic separation can be used.
Answer: Magnetic separation can be used to separate iron filings from a mixture of iron filings and sand, as iron is a magnetic substance while sand is not.
Q24: What is sublimation? Give an example of a substance that sublimes.
Answer: Sublimation is the process where a solid directly changes into a gas without passing through the liquid state, and vice versa. Camphor and naphthalene are examples of substances that sublime.
Q25: Why does dissolving sugar in water represent a physical change?
Answer: Dissolving sugar in water is a physical change because no new chemical substance is formed. The sugar molecules are simply dispersed in the water, and their chemical identity remains unchanged. The sugar can be recovered by evaporating the water.
Exercise Solutions (From Screenshot Page 48)
Q1: Fill in the blanks.
- The smallest particle of an element is called an atom.
- The smallest particle of a compound is called a molecule.
- A mixture in which components are uniformly distributed is called a homogeneous mixture.
- A mixture in which components are not uniformly distributed is called a heterogeneous mixture.
- The process of separating insoluble solids from liquids is called filtration.
- The process of separating a soluble solid from its liquid solvent is called evaporation.
- The process of separating liquids with different boiling points is called distillation.
- A temporary change in which no new substance is formed is called a physical change.
- A permanent change in which new substances are formed is called a chemical change.
- The law that states matter can neither be created nor destroyed in a chemical reaction is the Law of Conservation of Matter.
Q2: Match the pairs.
(Note: As an AI, I cannot create interactive matching. I will provide the correct pairs.)
- Element - Oxygen
- Compound - Water
- Homogeneous mixture - Salt solution
- Heterogeneous mixture - Sand and water
- Physical change - Melting of ice
- Chemical change - Burning of wood
- Law of Conservation of Matter - Antoine Lavoisier
- Law of Constant Proportions - Joseph Proust
Q3: Give scientific reasons.
- Water is a compound, not a mixture.
Reason: Water (H₂O) is a compound because hydrogen and oxygen combine chemically in a fixed mass ratio of 1:8. Its properties are entirely different from hydrogen (flammable gas) and oxygen (supports combustion). Also, its components can only be separated by chemical or electrochemical means, not by physical methods. - Air is a mixture, not a compound.
Reason: Air is a mixture because its components (nitrogen, oxygen, carbon dioxide, etc.) are physically combined in varying proportions and retain their individual properties. They do not react chemically to form a new substance, and the components can be separated by physical methods like fractional distillation of liquid air. - Melting of ice is a physical change.
Reason: Melting of ice is a physical change because only the state of matter changes from solid to liquid. The chemical composition (H₂O) remains the same. No new substance is formed, and the process is reversible (liquid water can be frozen back into ice). - Burning of wood is a chemical change.
Reason: Burning of wood is a chemical change because wood reacts with oxygen to form new substances like ash, carbon dioxide, and water vapor, all of which have different chemical compositions and properties from the original wood. This change is permanent and generally irreversible.
Q4: Answer the following questions.
- Differentiate between elements, compounds, and mixtures.
Answer:Property Element Compound Mixture Composition One type of atom Two or more elements chemically combined in fixed ratio Two or more substances physically combined in any ratio Separation Cannot be separated By chemical/electrochemical means only By physical methods Properties Unique properties Different from constituents Retain properties of constituents Uniformity Uniform Uniform Homogeneous or Heterogeneous - Explain the different methods of separating mixtures.
Answer:- Decantation: Separating insoluble solids from liquids or immiscible liquids by pouring off the top layer.
- Filtration: Separating insoluble solids from liquids using a filter.
- Evaporation: Separating a soluble solid from its solvent by heating.
- Distillation: Separating liquids with different boiling points by vaporizing and condensing.
- Chromatography: Separating components based on differential adsorption/solubility in a stationary/mobile phase.
- Centrifugation: Separating fine suspended particles from liquid by spinning.
- Magnetic Separation: Separating magnetic substances from non-magnetic ones.
- Sublimation: Separating a solid that sublimes from a non-subliming solid.
- Explain the Law of Conservation of Matter with an example.
Answer: The Law of Conservation of Matter, proposed by Antoine Lavoisier, states that 'Matter can neither be created nor destroyed in a chemical reaction.' This means that the total mass of the reactants before a chemical reaction is always equal to the total mass of the products formed after the reaction. Example: When 12 grams of carbon (C) react completely with 32 grams of oxygen (O₂), they form 44 grams of carbon dioxide (CO₂). Here, the total mass of reactants (12g + 32g = 44g) equals the total mass of the product (44g), demonstrating the conservation of mass. - Explain the Law of Constant Proportions with an example.
Answer: The Law of Constant Proportions, proposed by Joseph Proust, states that 'In a chemical compound, the elements are always present in definite proportions by mass, irrespective of the source or method of preparation.' This means that a given chemical compound always contains the same elements in the same fixed ratio by mass. Example: Water (H₂O) always consists of hydrogen and oxygen combined in a fixed mass ratio of 1:8, regardless of whether the water is from a river, a well, or synthesized in a laboratory. This ratio never changes for pure water.
References
- Maharashtra State Board Science and Technology Standard Eight Textbook (Specific Edition/Year) - Chapter 6: Composition of Matter.
- Maharashtra State Board 8th Standard Science Syllabus.
- Balbharati Science and Technology Textbook.