Long Answer Questions — Purity and Mixtures (CBSE Class 9 Science — Chemistry)
Medium Level (Application & Explanation)
Q1. Explain the difference between the common (everyday) idea of “purity” and the scientific definition of a pure substance. Give one everyday example and one scientific example to illustrate the difference.
Answer:
- In everyday language, pure usually means “free from adulteration” or “not mixed with unwanted substances.” For example, people say pure milk meaning milk not mixed with water or detergent.
- In science, a pure substance means matter made of only one type of particle with uniform chemical nature throughout. It can be an element (like oxygen gas, O₂) or a compound (like water, H₂O).
- Everyday “pure milk” is actually a mixture of water, fats, proteins, vitamins and minerals, so it is not pure scientifically.
- Scientific example: Pure sodium chloride (NaCl) contains only Na⁺ and Cl⁻ ions in a definite ratio and identical chemical properties throughout.
- Key point: Everyday purity focuses on absence of contaminants; scientific purity focuses on uniform chemical composition.
Q2. Describe how you would explain to a shopkeeper why mineral water sold as “pure” is not a pure substance scientifically. Mention at least three components commonly found in such water.
Answer:
- I would explain that mineral water is a mixture, not a pure substance, because it contains several different substances dissolved in water.
- Common components include dissolved salts such as calcium ions (Ca²⁺) and magnesium ions (Mg²⁺), dissolved gases like carbon dioxide (CO₂) (in sparkling water), and trace minerals such as bicarbonates or sulfates.
- Even if it looks clear and clean, the chemical composition varies from bottle to bottle and place to place. That variation shows it is a homogeneous mixture (solution) rather than a single chemical substance.
- Emphasize: “Pure” in shop terms means no added contaminants, but scientifically the water contains different dissolved particles, so it is not a pure substance.
Q3. How can the concept of uniform particles in a pure substance help explain why salt and sugar behave differently when dissolved in water? Use simple chemical reasoning.
Answer:
- A pure substance has uniform particles with definite chemical properties. Table salt (NaCl) is an ionic compound made of Na⁺ and Cl⁻ ions; sugar (sucrose) is a molecular compound made of neutral sugar molecules.
- When NaCl dissolves in water, it dissociates into ions (Na⁺ and Cl⁻), conducting electricity. When sucrose dissolves, it remains as molecules and does not produce ions, so the solution does not conduct electricity.
- Because the particles are uniform, a pure sample of NaCl will always behave like an ionic solute and sucrose like a molecular solute. This difference in particle type explains their distinct behaviors on dissolving and in electrical conductivity.
Q4. A sample of ghee is claimed to be “pure.” From a scientific viewpoint, list and explain three possible tests or observations that can show whether it is truly pure or a mixture/adulterated.
Answer:
- Melting behavior: Pure ghee (mostly fats) melts at a narrow temperature range. If it shows multiple melting points or melts over a wide range, it may contain other fats or oils, indicating a mixture.
- Solubility and separation tests: Ghee mixed with cheaper oils may behave differently on cooling or when treated with solvents; for example, adding a small amount to alcohol may show turbidity if adulterated.
- Sensory and chemical tests: Pure ghee has a distinct aroma and color; chemical tests for unsaponifiable matter or presence of vegetable oils (using lab reagents) can detect adulteration.
- These tests examine uniformity, consistent properties, and composition—all aspects of scientific purity.
Q5. Explain why soil and sea water are always considered mixtures. Discuss their composition and why they cannot be classified as pure substances.
Answer:
- Soil is a mixture because it contains many different components: mineral particles (sand, silt, clay), organic matter (decaying plants and microbes), water, air, and dissolved nutrients. Each component has different chemical nature and the proportions vary by location. Because particles are not uniform and composition changes, soil cannot be a pure substance.
- Sea water is a homogeneous solution of many dissolved salts (mainly NaCl), other ions (Mg²⁺, Ca²⁺, SO₄²⁻), dissolved gases (O₂, CO₂), and organic matter. Its composition varies with place and depth, so it is not a single chemical substance.
- Both examples show that natural materials are made of multiple components and therefore are mixtures, not pure substances.
High Complexity (Analytical & Scenario-Based)
Q6. A family believes bottled juice is “pure” because the label says “100% natural.” As a science student, prepare an explanation for them on why this claim does not mean the juice is a pure substance. Include what “100% natural” might actually imply.
Answer:
- Explain that “100% natural” is a marketing term suggesting no artificial additives, but it does not mean the juice is a pure substance chemically. Natural juice is a mixture of water, sugars (glucose, fructose), organic acids (citric acid), vitamins, pigments, and sometimes pulp.
- The amounts of these components vary between fruits, batches, and processing methods. Such variability proves the juice is composed of different substances—a mixture, even if all are natural.
- “100% natural” may imply no synthetic additives, but not uniform chemical composition. It does not guarantee absence of contaminants or uniformity in every bottle.
- Emphasize: purity in science requires only one type of particle; natural juice contains many different molecules, so it is not pure chemically.
Q7. Design a simple school-lab experiment (practical outline) to show that milk is a mixture and not a pure substance. Describe expected observations and their scientific interpretation.
Answer:
- Procedure outline: Take a small amount of milk and allow it to stand or gently centrifuge to separate layers. Heat a portion gently and observe. Add a few drops of a fat solvent (like ether) in a separated test showing fat dissolving. Another test: evaporate some milk on a watch glass to dryness.
- Expected observations: Milk separates into cream (fat) and watery layer on standing or after centrifugation. Heating may cause denaturation of proteins and skin formation. Evaporation leaves residue of salts, proteins and sugars, not a single substance. Fat dissolves in ether but not in water.
- Interpretation: Different components (fat, proteins, lactose, minerals, water) show different physical behaviors—separation, solubility differences, and residue—confirming milk is a mixture with multiple constituents.
Q8. Consider two samples: (A) a crystallized substance labeled “pure NaCl” and (B) a clear bottled solution labeled “mineral solution.” Propose analytical reasoning and at least three practical tests to distinguish which is a pure substance and which is a mixture.
Answer:
- Analytical reasoning: Pure NaCl is an ionic compound with fixed composition and characteristic properties (melting point, conductivity of its aqueous solution). A mineral solution is a mixture of various dissolved salts and possibly gases.
- Tests:
- Dissolve equal masses in distilled water: both dissolve, but test conductivity—solution of NaCl shows conductivity proportional to expected ionic concentration; mineral solution may show different conductivity reflecting varied ions.
- Evaporation/Crystallization: Evaporate water from each; pure NaCl should crystallize as uniform cubic crystals; mineral solution yields mixed crystals or multiple types of residues.
- Melting point test (solid sample): Pure NaCl melts at a known high temperature (~801°C); an impure sample or mixture will show a lower or broadened melting range.
- Conclusion: Consistent, reproducible properties indicate a pure substance, while variable residues and differing properties show a mixture.
Q9. A student argues that since sea water has a nearly uniform composition over large areas, it should be considered a pure substance. Critically evaluate this argument and provide scientific reasons to accept or reject it.
Answer:
- Reject the argument: Uniformity over large areas means sea water is a homogeneous mixture (solution) on a macroscopic scale, but not a pure substance. A pure substance is composed of only one chemical species (element or compound) with fixed chemical composition; sea water contains many different dissolved ions (Na⁺, Cl⁻, Mg²⁺, SO₄²⁻, etc.) in varying ratios.
- Even if composition is approximately similar across oceans, there are regional and temporal variations in concentration and dissolved gases. Sea water’s properties arise from multiple components, and it exhibits behavior (like selective evaporation or precipitation) that separates components, unlike a pure substance.
- Therefore, scientific definition requires single chemical identity—sea water fails this test and must be classified as a mixture.
Q10. You are given an unknown clear liquid. Describe an analytical approach combining observations and simple laboratory techniques to decide whether the liquid is a pure substance (like distilled water) or a mixture (like a saline solution). Explain how each step helps in conclusion.
Answer:
- Step 1 — Visual and odor check: Look for color or smell. Pure distilled water is colorless and odorless; any color or smell suggests dissolved substances.
- Step 2 — Conductivity test: Measure electrical conductivity. Pure water has very low conductivity, while a saline o...