Worksheet Solutions: Elements, Compound and Mixtures | Chemistry Class 7 ICSE PDF Download

Part A: Multiple Choice Questions 

Q1. Which of the following is a compound?
a) Oxygen
b) Salt (NaCl)
c) Aluminium
d) Gold

Answer: b) Salt (NaCl)
Reasoning: A compound forms when two or more elements chemically combine in a fixed ratio. Salt (NaCl) is made of sodium and chlorine in a 1:1 fixed ratio, unlike oxygen, aluminium, and gold which are pure elements.

Q2. Which of these metals is liquid at room temperature?
a) Aluminium
b) Mercury
c) Iron
d) Copper

Answer: b) Mercury
Reasoning: Most metals are solid at room temperature, but mercury and gallium are exceptions. Mercury is liquid and is often used in thermometers.

Q3. Which of the following is a homogeneous mixture?
a) Oil and water
b) Sand and salt
c) Sugar dissolved in water
d) Soil

Answer: c) Sugar dissolved in water
Reasoning: A homogeneous mixture has the same composition throughout. In sugar solution, the sugar is evenly dissolved in water. Oil-water, soil, and sand-salt are heterogeneous because different parts look and behave differently.

Q4. Which of the following is a noble gas?
a) Chlorine
b) Neon
c) Nitrogen
d) Bromine

Answer: b) Neon
Reasoning: Noble gases (He, Ne, Ar, Kr, Xe, Rn) are chemically inert and present in trace amounts in air. Chlorine, nitrogen, and bromine are reactive non-metals.

Q5. Which separation method is best for separating salt from seawater?
a) Filtration
b) Evaporation
c) Sublimation
d) Chromatography

Answer: b) Evaporation
Reasoning: In seawater, salt is dissolved. Heating evaporates the water, leaving behind solid salt. Filtration won’t work (salt is dissolved), sublimation is for solids like iodine/ammonium chloride, and chromatography is for separating dyes.

Part B: Short Answer Questions

Q6. Define an element and give two examples each of metals and non-metals.
Answer:

• An element is a pure substance made of only one kind of atom and cannot be broken into simpler substances.

• Examples of metals: Iron, Copper (hard, lustrous, conductors).

• Examples of non-metals: Oxygen, Sulfur (dull, brittle, poor conductors).
  This classification is based on distinct physical properties of metals and non-metals.

Q7. Why is water (H₂O) called a compound and not a mixture?
Answer:
Water is formed when hydrogen and oxygen chemically combine in a fixed ratio of 2:1. The properties of water are completely different from hydrogen (flammable gas) and oxygen (supports burning).
In mixtures, substances retain their properties, but in compounds, new properties appear.

Q8. What is atomicity? Classify the following: He, O₂, O₃, P₄.
Answer:

• Atomicity = number of atoms in one molecule of an element.

• He → Monoatomic (1 atom)

• O₂ → Diatomic (2 atoms)

• O₃ → Triatomic (3 atoms)

• P₄ → Polyatomic (4 atoms)
  This classification helps understand molecular structure.

Q9. State two differences between a compound and a mixture.
Answer:

1. Fixed ratio: Compounds have elements in a fixed ratio (H₂O = 2:1), but mixtures can be in any ratio (sand + salt).

2. Properties: Compounds have new properties (water ≠ hydrogen or oxygen), while mixtures retain individual properties (iron + sulfur = both visible).

Q10. Why is paper chromatography useful in separating inks?
Answer:
Different dyes in ink dissolve and travel with solvent at different speeds depending on how strongly they stick to paper. This separates colors on paper. It is widely used for testing pigments, food colors, and medicines.

Part C: Long Answer Questions 

Q11. Explain with steps how you would separate a mixture of salt, sand, and iron filings.
Answer (Stepwise Reasoning):

1. Use a magnet to separate iron filings (magnetic).

2. Mix salt + sand with water. Salt dissolves, sand does not.

3. Use filtration: sand remains on filter paper, salt-water passes through.

4. Heat the salt-water solution. Water evaporates, leaving salt crystals.
   Conclusion: A combination of magnetic separation, filtration, and evaporation is needed.

Q12. Why is distillation better than evaporation for obtaining pure water from salt water?
Answer (Stepwise Reasoning):

1. In evaporation, only salt is obtained, water is lost.

2. In distillation, salt-water is heated → water turns into vapor.

3. Vapor passes through a condenser → cools → becomes pure water.

4. Salt remains behind in flask.
   Conclusion: Distillation is preferred when we need both salt and pure water.

Q13. Compare metals and non-metals under five properties with examples.
Answer (Stepwise Reasoning):

1. State: Metals → mostly solid (Iron, Copper), Non-metals → gases/soft solids (O₂, Sulfur).

2. Lustre: Metals → shiny, Non-metals → dull (except iodine, graphite).

3. Conductivity: Metals → good conductors (Aluminium), Non-metals → poor conductors (Sulfur; graphite exception).

4. Malleability/Ductility: Metals → can be beaten into sheets/drawn into wires; Non-metals → brittle.

5. Sound: Metals → sonorous (produce ringing sound), Non-metals → not sonorous.
   Conclusion: These differences help classify elements based on observable properties.