Grade 7 Q&A: Chapter 14: Elements, Compounds and Mixtures

Concept Questions

Q1: What is an element? Give two examples.

Answer: An element is a pure substance that cannot be broken down into simpler substances by ordinary chemical means. It is made up of only one kind of atom. Examples include Oxygen (O) and Gold (Au).

Q2: Define a compound. How is it different from an element?

Answer: A compound is a pure substance formed when two or more different elements chemically combine in a fixed ratio by mass. Unlike an element, which is made of only one type of atom and cannot be broken down, a compound is made of different types of atoms chemically bonded together and can be broken down into its constituent elements by chemical methods.

Q3: What is a mixture? Provide two examples.

Answer: A mixture is an impure substance formed when two or more substances (elements, compounds, or both) are physically mixed together without chemical combination. Examples include air (mixture of gases) and saltwater (salt dissolved in water).

Q4: Differentiate between a homogeneous mixture and a heterogeneous mixture.

Answer:

• Homogeneous Mixture: Has a uniform composition throughout, meaning the components are evenly distributed and indistinguishable (e.g., sugar solution).
• Heterogeneous Mixture: Has a non-uniform composition, where the components are not evenly distributed and can often be seen separately (e.g., sand and water).

Q5: Explain why elements and compounds are considered 'pure substances', while mixtures are 'impure substances'.

Answer: Elements and compounds are pure substances because they have a definite and uniform composition, and their properties are constant. Elements consist of only one type of atom, and compounds consist of elements chemically combined in a fixed ratio. Mixtures are impure substances because their components are only physically mixed, retain their individual properties, and can be present in variable proportions.

Q6: List three properties of metals.

Answer: Three properties of metals are:

1. They are lustrous (shiny).
2. They are malleable (can be hammered into sheets).
3. They are good conductors of heat and electricity.

Q7: List three properties of non-metals.

Answer: Three properties of non-metals are:

1. They are generally dull (not shiny).
2. They are brittle (break easily).
3. They are poor conductors of heat and electricity (insulators), with exceptions like graphite.

Q8: Can a compound be separated into its constituent elements by physical means? Justify your answer.

Answer: No, a compound cannot be separated into its constituent elements by physical means. This is because the elements in a compound are chemically bonded together, forming a new substance with different properties. Only chemical methods (like electrolysis) can break these bonds and separate the elements.

Q9: Describe the principle of filtration and give an example of its use.

Answer: Filtration is a method used to separate insoluble solids from liquids. The principle is that the mixture is passed through a filter medium (like filter paper) that has pores large enough for the liquid to pass through but small enough to trap the solid particles. An example is separating sand from water.

Q10: When would you use evaporation as a separation technique? Explain with an example.

Answer: Evaporation is used to separate a soluble solid from a liquid in a solution. The liquid is heated, causing it to turn into vapor and leave the solid component behind. For example, evaporation is used to obtain salt from saltwater; the water evaporates, leaving the salt crystals.

Q11: What is magnetic separation? Give an example of a mixture that can be separated using this method.

Answer: Magnetic separation is a method used to separate magnetic substances from non-magnetic substances in a mixture. A magnet is brought close to the mixture, attracting only the magnetic component. An example is separating iron filings from sulfur powder or sand.

Q12: Explain the term 'ductility' in relation to metals.

Answer: Ductility is the property of metals that allows them to be drawn or stretched into thin wires without breaking. For instance, copper is ductile, which is why it is used to make electrical wires.

Q13: What is the significance of a fixed ratio in the formation of a compound?

Answer: The significance of a fixed ratio in compound formation is that it ensures the compound always has a specific chemical formula and consistent properties. For example, water is always H₂O, meaning it always has two hydrogen atoms for every one oxygen atom, giving it its unique characteristics.

Q14: Give an example of an element that is a liquid at room temperature.

Answer: Mercury (Hg) is an example of an element that is a liquid at room temperature.

Q15: What is the difference between an atom and a molecule?

Answer: An atom is the smallest indivisible unit of an element that retains the chemical properties of that element. A molecule is formed when two or more atoms (which can be of the same or different elements) are chemically bonded together.

Q16: How would you separate a mixture of oil and water? Name the method.

Answer: A mixture of oil and water (which are immiscible liquids) can be separated using **decantation**, often with the aid of a separating funnel. Since oil is less dense, it forms a separate layer on top of the water, allowing the water to be drained out from the bottom.

Q17: Describe one application of distillation.

Answer: One application of distillation is to obtain pure water from saltwater. When saltwater is heated, the water evaporates, leaving the salt behind. The water vapor is then cooled and condensed back into liquid pure water, which is collected.

Q18: Why is air considered a mixture and not a compound?

Answer: Air is considered a mixture because its components (like nitrogen, oxygen, argon, carbon dioxide) are physically mixed and not chemically combined. They retain their individual properties, and their proportions can vary (e.g., humidity affects water vapor content). If it were a compound, its components would be in a fixed ratio and lose their original properties.

Q19: What is the chemical symbol for Iron and Gold?

Answer: The chemical symbol for Iron is Fe (from Ferrum), and for Gold is Au (from Aurum).

Q20: Provide an example of a solid non-metal and a gaseous non-metal.

Answer: An example of a solid non-metal is Carbon (C) or Sulfur (S). An example of a gaseous non-metal is Oxygen (O) or Hydrogen (H).

Q21: How can you tell if a substance is a pure substance or a mixture based on its properties?

Answer: Pure substances (elements and compounds) have definite and fixed melting and boiling points, and their composition is uniform throughout. Mixtures, on the other hand, have variable melting and boiling points (they melt or boil over a range of temperatures), and their composition can be non-uniform (especially heterogeneous mixtures).

Q22: What is the main difference in how components are held together in a compound versus a mixture?

Answer: In a compound, components (elements) are held together by strong chemical bonds, forming a new substance. In a mixture, components are only physically intermingled and are not chemically bonded; they retain their individual identities.

Q23: Name a separation technique used to separate components of ink.

Answer: Chromatography (specifically paper chromatography) is a separation technique used to separate the different colored dyes present in ink.

Q24: Why is sieving an effective method for separating some mixtures?

Answer: Sieving is effective because it separates solid particles based on their size. If the components of a solid mixture have significantly different particle sizes, a sieve with appropriate mesh openings can allow smaller particles to pass through while retaining larger ones.

Q25: Give an example of a common compound and its chemical formula.

Answer: A common compound is Water, and its chemical formula is H₂O.

Q26: What is the property of malleability? Which type of element exhibits this property?

Answer: Malleability is the property of a substance that allows it to be hammered or pressed into thin sheets without breaking. Metals exhibit this property.

Q27: Provide an example of a homogeneous mixture that is a solution.

Answer: Sugar dissolved in water is a homogeneous mixture and is an example of a solution.

Q28: If you mix sand and iron filings, which separation method would be most effective?

Answer: Magnetic separation would be most effective because iron filings are magnetic, while sand is not, allowing a magnet to easily separate the iron from the sand.

Q29: How does the formation of a compound differ from the formation of a mixture in terms of energy?

Answer: The formation of a compound typically involves a chemical reaction, which is accompanied by a significant absorption or release of energy (e.g., heat, light). The formation of a mixture, however, is a physical process and usually involves little to no energy change.

Q30: What is the purpose of a chemical symbol for an element?

Answer: The purpose of a chemical symbol is to provide a short, universally recognized abbreviation for an element. This simplifies writing chemical equations and communicating about elements across different languages.