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NCERT Solutions for Class 9th Science Chapter 9 Chapter opener — Think It Over

Book page 162 Updated on2026-09-08

Q1.
Water can be obtained from various sources. Are all these samples of water chemically identical?
Answer

The water itself is identical; the samples are not. Once each sample is purified, every one of them is the same compound H₂O, with hydrogen and oxygen combined in the fixed mass ratio 1 : 8 — that is exactly what the Law of Constant Proportions says.

What differs is everything the water is carrying with it:

  • River water — suspended silt, dissolved oxygen, traces of organic matter.
  • Borewell water — dissolved calcium and magnesium salts (this is what makes water “hard”), sometimes iron or fluoride.
  • Sea water — about 35 g of dissolved salts per kilogram, mostly sodium chloride.
  • Rain water — dissolved carbon dioxide and other gases from the air.

So these are mixtures, and a mixture may have its components in any proportion. Distil any of them and what comes over is pure water — 9 g of it will always decompose into 1 g of hydrogen and 8 g of oxygen.

Why it happens: the composition of a compound is fixed by how its atoms are bonded — two H atoms to one O atom, always. Nothing about the source can change that. The composition of a mixture is not fixed by any bond, so it can vary freely.
Q2.
Oxygen is sometimes represented as O and sometimes as O2. What is the difference between these symbols?
Answer

O stands for one atom of oxygen; O₂ stands for one molecule of oxygen gas, which contains two oxygen atoms joined by a double bond.

SymbolWhat it namesNumber of atomsMassDoes it exist on its own?
Oone oxygen atom116 uNo — a lone O atom has only 6 valence electrons, so it is unstable
O₂one molecule of oxygen gas232 uYes — both atoms have a complete octet

The symbol O is used when you are counting atoms inside a formula — in H₂O, in CO₂, in CaO. The formula O₂ is used when you mean the gas you breathe.

Why it happens: an oxygen atom is 2, 6 — it is two electrons short of an octet. Two oxygen atoms solve the problem together by sharing two pairs of electrons, so each one now “sees” eight valence electrons. That is the double bond O=O, and it is why free oxygen in air is O₂ and not O.
Did you know? Oxygen atoms can also join in threes to give ozone, O₃. Same element, different molecule, completely different behaviour — O₂ keeps us alive at ground level, O₃ shields us from ultraviolet rays high in the atmosphere.
Q3.
Why does dissolved salt in water conduct electricity, but sugar does not?
Answer

Because salt breaks into free ions in water and sugar does not. An electric current in a solution is carried by charged particles that are free to move; sugar solution has none.

  • Common salt, NaCl, is an ionic compound. The solid is already made of Na⁺ and Cl⁻ ions locked in a lattice. Water pulls that lattice apart, and the ions become free to move. Switch on the battery and Na⁺ drifts to the negative electrode, Cl⁻ to the positive one — that drift is the current, and the bulb glows.
  • Sugar is a covalent compound. It dissolves well, but what floats away into the water are whole, electrically neutral sugar molecules. No charges are released, nothing drifts to either electrode, and the bulb stays dark.
Why it happens: dissolving and ionising are two different things. Both salt and sugar dissolve; only salt ionises. That is the whole difference between an ionic and a covalent compound in water.
Check it yourself: solid salt does not conduct either. In the dry crystal the ions are held in fixed positions, so although the charges are there, they cannot move. Free charge is not enough — it must also be mobile.
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