Q.(i) How many significant figures are present in 0.0025 ?

The Intuition: Why Nature Prefers Simple Ratios

Imagine you have a box of identical Lego bricks — all the same size and weight. You can build different structures by snapping them together. A simple tower might use 2 bricks per floor, while a wider tower uses 3 bricks per floor. The number of bricks in each floor is always a whole number — you can't use half a brick.

Atoms behave the same way. When two elements combine, they do so in fixed, discrete numbers of atoms. You can't have 1.5 atoms of oxygen bonding with 1 atom of carbon. It's either 1:1, 1:2, 2:1, or some other small whole-number ratio. This simple fact — that atoms are indivisible units in chemical combination — is the physical reason behind the Law of Multiple Proportions.

The Precise Statement

Breaking It Down with an Example

Consider carbon and oxygen. They form two common compounds:

• Carbon monoxide (CO): 12 g of carbon combines with 16 g of oxygen
• Carbon dioxide (CO₂): 12 g of carbon combines with 32 g of oxygen

Fix the mass of carbon at 12 g (the same in both). The masses of oxygen that combine with it are 16 g and 32 g. Their ratio is:

That's a small whole-number ratio (1:2). This is not a coincidence — it reflects the fact that CO has one oxygen atom per carbon, while CO₂ has two.

Another Classic: Nitrogen Oxides

Nitrogen and oxygen form several compounds. Fix 14 g of nitrogen (one mole of N atoms):

The oxygen masses (8, 16, 32, 40) are in the ratio 1:2:4:5 — all small whole numbers.

Why This Matters …