Why cannot we separate hydrogen and oxygen present in water by physical means?
Answer
Because in water the particles of hydrogen and oxygen are so tightly attached to each other that no physical method can pull them apart. Water is a compound, not a mixture.
Why it happens: A physical method — filtering, sieving, evaporating, using a magnet, letting things settle — only sorts particles that are already separate and lying side by side. In a compound the elements have combined chemically in a fixed ratio and are no longer separate particles of hydrogen and oxygen at all; the molecule of water is a new particle. To take them apart you have to force a chemical change, which is exactly what the electric current in Activity 8.3 did.
Tip: This is the sharpest line in the chapter. Salt dissolved in water is a mixture — evaporate the water and the salt is back. Hydrogen and oxygen in water are a compound — no amount of evaporating will ever give them back.
Q2.
Are common salt and sugar elements or compounds? Let us find out.
Answer
Both are compounds.
Common salt is sodium chloride. Sodium is a soft metal and chlorine is a hazardous gas, yet together they make a harmless, taste-enhancing substance that is essential for our lives — made of particles of sodium and chlorine in a 1 : 1 ratio.
Sugar is a chemical compound of the elements carbon, hydrogen and oxygen. Activity 8.4 proves it: heated sugar leaves behind charcoal (carbon) and gives off water, and water itself is hydrogen and oxygen.
Why it happens: The clinching evidence in both cases is the change of properties. Neither salt nor sugar behaves anything like the elements inside it — a compound is a genuinely new substance, not the two elements sitting together.
Q3.
Is it possible to separate sodium chloride into its elements by physical processes?
Answer
No. Sodium chloride is a compound, so no physical process can split it into sodium and chlorine.
Why it happens: Be careful not to confuse two different separations. Dissolved sodium chloride can be recovered from seawater by evaporation — but that only separates salt from water, a mixture. It gives you back sodium chloride, not sodium and chlorine. To break the salt itself into its two elements you would have to break the chemical combination that holds them in the 1 : 1 ratio, and that needs a chemical change, not a physical one.
Check it yourself: Ask of every separation — am I separating a mixture into its components, or a compound into its elements? Only the first can be done physically.