The main constituent of water-glass is

Analyze the question: Water-glass is a well-known industrial chemical. The question asks which compound is its main constituent.

Explain the concept: Water-glass is a common name for sodium silicate, Na₂SiO₃. It dissolves in water to give a thick, syrupy, glass-like solution, which is how it got its name.

Learn the naming pattern:

• Silicate = compound containing silicon, SiO₃²⁻ = trioxosilicate(IV)
• Sulphite = compound containing sulphur, SO₃²⁻ = trioxosulphate(IV)

Step-by-step solution:

1. Water-glass contains silicon, so the anion must be trioxosilicate(IV). That rules out B and C, which are sulphur-based (calcium sulphite and sodium sulphite).
2. Decide between calcium and sodium. Water-glass is soluble in water, and it is made by fusing sodium carbonate with sand (silicon dioxide):

Na₂CO₃ + SiO₂ → Na₂SiO₃ + CO₂

Calcium trioxosilicate(IV), CaSiO₃, is the main component of slag and is insoluble, so it does not fit water-glass.

3. The cation is therefore sodium, giving sodium trioxosilicate(IV), Na₂SiO₃.

Answer: D.

Uses of water-glass (often tested):

• Preservative for eggs
• Adhesive for cardboard and paper
• Fireproofing
• Making silica gel and detergents

Exam tip: Remember "water-glass = sodium silicate", where Sodium Silicate are both S's. Also, "silic-" always points to silicon, and "sulph-" to sulphur, so you can quickly eliminate options by looking at the anion's name.