Mixtures vs Compounds – Long Answer Questions (Class 9 Science, Chemistry)
Medium Level (Application & Explanation)
Q1. Explain how you would separate a mixture of sand and salt. Describe the steps and state why each step works.
Answer:
To separate sand and salt, follow these steps:
Add water to the mixture and stir. Salt (NaCl) is soluble in water, while sand is insoluble. This causes salt to dissolve, forming a homogeneous salt solution, while sand remains as solid particles.
Filter the mixture through filter paper. The sand is caught on the filter as residue; the salt solution passes through as filtrate. Filtration works because of the difference in particle size and solubility.
Evaporate the filtrate gently or heat it. As water evaporates, salt crystals (NaCl) reappear because the dissolved salt cannot stay in the vapor. Evaporation separates a dissolved solute from its solvent.
Each step uses a physical method and works because components of a mixture retain individual properties (solubility and particle size) and do not chemically combine.
Q2. Compare and contrast mixtures and compounds with three examples each. Explain one real-life situation where choosing a mixture over a compound is advantageous.
Answer:
Comparison (short points):
Mixtures have variable composition, components retain their properties, and can be separated by physical methods. Examples: air, salt and sand, sugar water.
Compounds have fixed composition, elements lose their original properties, and require chemical reactions to separate. Examples: water (H₂O), table salt (NaCl), carbon dioxide (CO₂).
Real-life advantage: Choose a mixture when you want reversibility or easy separation. For example, making lemonade (water + sugar + lemon juice) is a mixture; if you want to recover sugar or concentrate the drink, you can use simple evaporation or filtration. This flexibility is useful in cooking and industrial processes where ingredients must be adjusted or recovered.
Emphasized idea: Mixtures are best when adjustability and easy separation are needed.
Q3. Describe how the properties of water (H₂O) demonstrate that compounds have different properties from their constituent elements. Include the properties of hydrogen and oxygen for comparison.
Answer:
Water (H₂O) is a compound formed from hydrogen and oxygen. Look at properties:
Hydrogen is a colourless, odourless, highly flammable gas.
Oxygen is a colourless, odourless gas that supports combustion but is not flammable itself.
Water is a liquid at room temperature, non-flammable, has a high boiling point relative to hydrogen and oxygen, and is a good solvent for many substances.
These differences show that when elements chemically combine, they form a compound with new properties. The atoms are held together by chemical bonds, producing characteristics (like liquid state, polarity, and solvent ability) that neither hydrogen nor oxygen alone exhibits.
Emphasize: In a compound, elements lose original properties and gain new, distinct properties.
Q4. Explain why air is considered a mixture. Discuss whether air is homogeneous or heterogeneous and justify your answer with examples.
Answer:
Air is a mixture because it contains different gases—mainly nitrogen (~78%), oxygen (~21%), argon, carbon dioxide (CO₂), and small amounts of other gases—each retaining its own properties and present in variable proportions depending on place and altitude.
Air is generally considered a homogeneous mixture (a solution of gases) because the gases are evenly mixed at the molecular level and composition appears uniform in everyday samples. For example, a breath of air from different parts of a room has virtually the same composition.
However, in some situations air can be heterogeneous, such as when it contains dust, smoke, or water droplets (aerosols), creating visible differences (fog, smoke).
Emphasize: Air is normally homogeneous, but environmental factors can make it heterogeneous.
Q5. A student prepared a sample by dissolving sugar in water and then mixed oil on top. Classify this overall system and explain how the components are arranged and how you might separate them.
Answer:
Classification: This system is a heterogeneous mixture because it shows distinct layers: an aqueous sugar solution and an immiscible oil layer. The sugar fully dissolves in water forming a homogeneous solution, but oil and water do not mix, so the overall system is heterogeneous.
Arrangement and separation:
The bottom layer is the sugar solution (denser than oil), and the top layer is oil (less dense).
To separate them: use a separating funnel or carefully decant the top oil layer. After removing oil, you can evaporate the water from the sugar solution to obtain crystalline sugar back.
Emphasize: Understanding density, miscibility, and solubility helps choose physical methods for separation.
High Complexity (Analytical & Scenario-Based)
Q6. You are given a sample that could be either a compound or a mixture. Describe an experimental plan with observations that would let you decide which one it is. Explain why each test helps you reach a conclusion.
Answer:
Experimental plan (stepwise):
Physical observation: Note appearance—uniformity, distinct parts, or layers. If distinct parts are visible, likely a heterogeneous mixture. Uniform appearance alone is inconclusive.
Solubility test: Take small portions and try dissolving in water. If components separate (one dissolves, another does not), it indicates a mixture. If entire sample behaves uniformly (dissolves or does not) it could still be compound or homogeneous mixture.
Separation attempt: Use filtration (for insoluble solids) or distillation/evaporation (to recover dissolved substances). If you can recover original substances physically, it’s a mixture because components retain identity.
Chemical test: Perform a chemical reaction expected for known elements (e.g., if suspect carbonate, add acid and look for CO₂). If composition changes chemically and new properties arise, it may be a compound; if separate substances behave independently, likely a mixture.
Why tests help: Physical separation proves components are not chemically bonded. Chemical reactions that alter identity are needed to break compounds, so inability to separate physically but requiring chemical change indicates a compound. Emphasize: Use physical methods first, then chemical tests.
Q7. A chemist mixes hydrogen and oxygen gases and allows them to react to form water (H₂O). Describe how this transformation illustrates fixed composition, change of properties, and energy considerations. Also explain why the reverse process requires a chemical reaction.
Answer:
Fixed composition: Water (H₂O) always contains two hydrogen atoms and one oxygen atom in a fixed ratio by atoms. This demonstrates a definite composition characteristic of compounds.
Change of properties: Hydrogen and oxygen are both gases with very different properties (flammability and combustion support), but water is a liquid at room temperature with properties like high boiling point and solvent ability. These are completely different from the elements, showing the compound has new properties.
Energy considerations: The formation of water from hydrogen and oxygen is exothermic (releases energy). The reaction can be explosive if uncontrolled because forming the H–O bonds releases significant energy.
Reverse process: To decompose water into H₂ and O₂ requires input of energy (endothermic), such as electrolysis, which is a chemical process that breaks the chemical bonds in water. Physical methods cannot separate the elements because hydrogen and oxygen atoms are chemically bonded in fixed proportions.
Emphasize: Compounds have fixed ratios, different properties, and chemical bond energy governs formation and breakdown.
Q8. Consider an alloy like brass (copper + zinc). Explain why brass is treated as a mixture or a homogeneous solution rather than a compound. Discuss its properties, methods of separation (if any), and industrial advantages.
Answer:
Nature of brass: Brass is an alloy, a homogeneous mixture (solid solution) of copper and zinc. The metals are mixed physically in molten form and atoms occupy positions in the solid lattice but do not form new chemical compounds with fixed stoichiometry.
Properties: Brass retains many metallic properties such as malleability, ductility, electrical conductivity, and shiny appearance, with improved strength, corrosion resistance, and workability compared to pure copper. Properties can be varied by changing composition, showing variable composition like mixtures.
Separation: Separating brass into pure copper and zinc is difficult and requires chemical or electrochemical methods like selective oxidation or electrorefining rather than simple physical methods. This difficulty is because the metals are dispersed at atomic level.
Industrial advantages: Brass allows tailoring properties by adjusting the ratio, making it useful for musical instruments, plumbing, and decorative items. Ease of casting and improved mechanical properties are practical benefits.
Emphasize: Alloys are mixtures (often homogeneous) with adjustable properties, favored in industry for flexibility.
Q9. You have two beakers—one containing seawater and the other containing pure water (distilled). Design tests to show t...
