Q1. Define a mixture. Explain with examples how to identify whether a mixture is homogeneous or heterogeneous.
Answer:
A mixture is a physical combination of two or more substances in which each substance retains its own chemical identity and can be separated by physical methods.
To identify the type:
Homogeneous mixture: has a uniform composition throughout. You cannot see separate parts. Examples: saltwater (NaCl dissolved in H₂O), air (a blend of N₂, O₂, CO₂ and other gases). These are also called solutions when one component is dissolved in another.
Heterogeneous mixture: has non-uniform composition and you can see or detect different parts. Examples: salad, rocky soil, suspensions like muddy water where particles settle out.
Quick tests: visual inspection, settling on standing, and passing light (Tyndall effect) help classify mixtures. Remember: if you can pick out components, it’s heterogeneous; if not, it’s likely homogeneous.
Q2. Why do the properties of components in a mixture remain unchanged? Give two examples and explain how physical methods separate them.
Answer:
In a mixture, components are combined physically, not chemically; there is no formation of new substances. Therefore, each component keeps its original properties such as melting point, boiling point, solubility, magnetic behaviour, and particle size.
Examples and separation:
Iron filings + Sand: Iron is magnetic, sand is not. Using a magnet you can extract iron filings easily without changing either substance.
Salt + Water (saltwater): Salt dissolves in water forming a solution. By evaporation or distillation, water can be removed and salt recovered because evaporation is a physical change.
These methods work because they exploit physical differences like solubility, magnetism, boiling point, and particle size. No chemical reactions are involved, so components stay chemically the same.
Q3. Describe what happens when salt dissolves in water. Define solute, solvent, and explain saturated & unsaturated solutions with examples.
Answer:
When salt (NaCl) is added to water (H₂O), the polar water molecules surround the positive (Na⁺) and negative (Cl⁻) ions. The ions get pulled away from the salt crystal and become dispersed evenly, forming a homogeneous solution.
Terminology:
Solute: the substance that dissolves (here NaCl).
Solvent: the substance doing the dissolving (here H₂O).
Saturation:
Unsaturated solution: can dissolve more solute at that temperature (e.g., a small amount of salt in a large volume of water).
Saturated solution: contains the maximum amount of solute that can dissolve at that temperature. If more salt is added, it will not dissolve and will remain as solid crystals.
Dissolution is a physical process; salt can be recovered by evaporating the water.
Q4. How would you separate a mixture of oil and water? Describe at least two methods and explain why they work.
Answer:
Oil and water are immiscible liquids (they do not mix) and form a heterogeneous mixture. Methods to separate them:
Decantation: Allow the mixture to settle. Oil (lower density) floats on top, water stays below. Carefully pour off the oil or the water. This works because of density difference and immiscibility.
Separating funnel (laboratory method): Pour the mixture into a funnel, wait for layers to form, then open the tap to release the denser liquid (water) first, and close it before the oil flows out. This is precise and prevents mixing during transfer.
These methods rely on density difference and immiscibility; no chemical change occurs, so both liquids keep their properties and can be collected separately.
Q5. Explain the main differences between a mixture and a compound with two examples for each. Why is this distinction important?
Answer:
Differences:
Formation: A mixture is formed by physical combining of substances; a compound is formed by chemical combination and involves chemical bonds.
Properties: Components of a mixture retain their original properties; in a compound, new properties appear that are different from the elements (e.g., Na is a metal, Cl₂ is a gas, but NaCl is a crystalline solid).
Separation: Mixtures can be separated by physical methods (filtration, evaporation, magnet), while compounds require chemical reactions or decomposition to separate elements.
Examples:
Mixtures: air (N₂ + O₂ + other gases), salad (vegetables mixed).
Compounds: water (H₂O), carbon dioxide (CO₂).
Importance: Knowing whether a sample is a mixture or compound tells us what separation methods to use and predicts chemical behaviour in reactions and practical applications.
High Complexity (Analytical & Scenario-Based)
Q6. You are given a mixture of sand, common salt, iron filings, and water. Describe a step-by-step procedure to separate and recover each component, and explain the principle used in each step.
Answer:
Step 1 — Remove iron filings: Use a magnet to attract and lift iron filings out of the mixture. Principle: magnetism — iron is magnetic while other components are not. Recover iron by removing it from the magnet.
Step 2 — Separate sand from saltwater: Pour the remaining mixture through filtration. The sand (insoluble solid) stays on filter paper; salt dissolved in water passes through as filtrate. Principle: filtration based on particle size and insolubility.
Step 3 — Recover salt from salt solution: Heat the filtrate to evaporate water. Continue until water evaporates and salt crystals remain. Principle: evaporation — different boiling points; water vapour leaves while salt, a non-volatile solute, remains.
Step 4 — Collect water (optional): Instead of simple evaporation, use distillation to condense water vapor and collect pure water, leaving salt behind. Principle: boiling and condensation using differing boiling points.
Throughout, keep items dry and labelled. These separations exploit magnetism, solubility, particle size, and boiling point differences.
Q7. Explain why air is considered a homogeneous mixture. What are its major components, and how can oxygen be separated from air for laboratory or industrial use?
Answer:
Air is a homogeneous mixture because its gaseous components are evenly mixed at the molecular level and form a uniform composition in typical conditions. You cannot see or pick out different gases with the naked eye.
Major components:
Nitrogen (N₂) ≈ 78%
Oxygen (O₂) ≈ 21%
Argon (Ar) ≈ 0.93%
Carbon dioxide (CO₂) and other trace gases make up the rest.
Separation of oxygen:
Fractional distillation of liquefied air: Air is cooled and compressed until it liquefies. When slowly warmed, gases boil off at different temperatures; oxygen (bp −183 °C) and nitrogen (bp −196 °C) can be separated because of different boiling points. This is used industrially to produce oxygen and nitrogen.
Laboratory methods: Chemical methods (e.g., decomposition of hydrogen peroxide using a catalyst) produce small amounts of oxygen. For pure oxygen at large scale, liquefaction and fractional distillation is practical.
Understanding air’s homogeneity and components helps in breathing support, industrial gas supply, and environmental monitoring.
Q8. Differentiate between a solution, a suspension, and a colloid. Give two examples for each and explain how the Tyndall effect helps in their identification.
Answer:
Solution:
Homogeneous mixture with solute particles molecularly or ionically dispersed (very small). Examples: salt solution, sugar solution.
Does not scatter light and shows no Tyndall effect.
Suspension:
Heterogeneous mixture where large particles are suspended temporarily and settle on standing. Examples: muddy water, sand in water.
Particles do not pass through filter paper and usually do not show Tyndall effect strongly because particles settle.
Colloid:
Intermediate: particles are small enough to stay dispersed and do not settle, but larger than soluble molecules. Examples: milk (fat in water), fog (liquid droplets in air), starch in water.
Colloids scatter light; this scattering of a light beam is the Tyndall effect. Solutions do not scatter light, while colloids do; suspensions may scatter light briefly but mostly settle.
The Tyndall effect is a quick test: shine a beam of light through the mixture — if you see a visible beam, it's likely a colloid; if invisible, it's a true solution.
Q9. A student needs to separate a mixture of ethanol and water. Suggest a laboratory method and explain why it works. Also mention a limitation of this method.
Answer:
Method: Simple distillation can be used because ethanol (bp ≈ 78.4 °C) and water (bp 100 °C) have different boiling points. The mixture is heated; ethanol, having a lower boiling point, vaporizes first. The vapour is then condensed in a condenser and collected as distillate enriched in ethanol. Principle: separation based on difference in boiling points.
Limitation: Ethanol and water form an azeotrope at about 95% ethanol by volume; thus, simple distillation cannot produce pure (100%) ethanol — it cannot separate beyond the azeotropic composition. For higher purity, fractional distillation with special columns or chemical drying agents (e.g., molecular sieves) are needed.
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