'Nature of Matter: Elements, Compounds, and Mixtures' — Practice

Question 1

Is brass an element, compound, or mixture?

Accepted Answer

✦ Answer: MIXTURE (alloy). Brass is a homogeneous mixture of copper (~65%) and zinc (~35%). The proportions can vary.

Question 2

Which method separates iron filings from sand?

Accepted Answer

✦ Answer: Magnetic separation. Iron is magnetic; a magnet attracts iron while sand stays.

Question 3

Explain the difference between compound and mixture with examples.

Accepted Answer

Step 1 — Compounds.
  Compound = pure substance with FIXED ratio of elements, chemically bonded. Example: water (H₂O) — always 2 hydrogen : 1 oxygen ratio. Properties different from elements (e.g., water is liquid; hydrogen burns, oxygen supports burning).

Step 2 — Mixtures.
  Mixture = combination of substances NOT chemically bonded. Composition is VARIABLE. Components retain their own properties. Example: salt water — can have any amount of salt dissolved.

Step 3 — Comparison.
  COMPOUND: fixed ratio (water always 2:1), chemical bonds, properties different from elements, requires CHEMICAL reactions to separate.
  MIXTURE: variable ratio (salt water can be 5% or 10%), no chemical bonds, properties of components, separated by PHYSICAL methods (evaporation, filtration).

Step 4 — Examples.
  Compounds: H₂O, CO₂, NaCl, glucose, methane.
  Mixtures: air, salt water, brass, fruit salad, oil + water.

✦ Answer: Compound = pure with FIXED ratio, chemically bonded (e.g., H₂O). Mixture = combination with VARIABLE composition, no chemical bonds, components retain properties (e.g., air, salt water). Key difference: chemical bonding vs no bonding; fixed vs variable ratio.

Question 4

Suggest separation techniques for these mixtures: (a) sand and salt in water (b) cream from milk (c) dyes in black ink (d) pebbles from soil (e) sea water to drinking water.

Accepted Answer

Step 1 — (a) Sand, salt, and water mixture.
  Three components — need multi-step:
  • FILTRATION: removes insoluble sand (passes through filter paper)
  • EVAPORATION: of filtrate (sand removed); water evaporates, salt remains.
  Result: sand on filter paper; salt crystals in evaporating dish.

Step 2 — (b) Cream from milk.
  CENTRIFUGATION: spin milk in a centrifuge. Lighter cream rises; denser milk stays. Used in dairies. (At home: just leaving milk; cream rises on top, can be skimmed.)

Step 3 — (c) Dyes in black ink.
  CHROMATOGRAPHY: Put a drop of ink on filter paper. Dip edge in water. Water moves up; dyes move at different rates → separate into bands of different colours. Used in forensics.

Step 4 — (d) Pebbles from soil.
  HAND-PICKING: simply pick out pebbles (large, visible).
  Or SIEVING: pass through coarse sieve; pebbles stay above, soil falls through.

Step 5 — (e) Sea water to drinking water.
  DISTILLATION: Boil sea water in flask. Water vapour rises, passes through condenser, becomes pure water in collection flask. Salt stays behind.
  Used in DESALINATION PLANTS — major water source for some Indian cities (Chennai, Mumbai pilot projects).
  More sustainable: SOLAR DISTILLATION using sunlight.

Step 6 — Indian context.
  India has 7,500 km coastline. Desalination is becoming important for water-scarce coastal cities. Solar distillation is being developed for rural areas.

✦ Answer: (a) Filtration + evaporation. (b) Centrifugation. (c) Chromatography. (d) Hand-picking or sieving. (e) Distillation (desalination). Each method matches the properties of the mixture components.

Practice — 'Nature of Matter: Elements, Compounds, and Mixtures'

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