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NCERT Solutions for Class 9th Science Chapter 1 Real problems need several branches together — Ready to Go Beyond

Book page 7 Updated on2026-09-08

Q1.
How does a mask really work?
Answer

Not by sieving. A mask works because particles are made to touch a fibre and stick to it, and three different mechanisms do that for three different particle sizes.

1. Impaction Heavy droplets (5 – 100 µm) are too massive to follow the bending air, so they run straight into the fibre. 2. Interception A mid-size particle (about 1 µm) does follow the air, but passes so close that it grazes the fibre and is held. 3. Diffusion Very small particles (below 0.1 µm) are knocked about by air molecules and wander onto a fibre within a few layers.
The grey circles are the cross-sections of single mask fibres. A mask is a deep tangle of thousands of such fibres, so a particle that escapes one layer meets many more.

Now the branches, exactly as the box lists them:

  • Physics — particle motion and electrostatic attraction. The melt-blown middle layer carries a permanent electric charge that pulls even neutral particles in, by inducing charge on them.
  • Chemistry — the properties of polymer fibres. Polypropylene is used because it is non-polar, repels water, and holds a static charge for months.
  • Biology — the size and behaviour of viruses. A virus is roughly 0.1 µm across, but it does not travel alone: it rides in respiratory droplets 1 – 100 µm wide, which are far easier to trap.
  • Mathematics — modelling airflow and filtration efficiency. Each layer removes a fixed fraction, so the efficiencies multiply, not add.
if one layer stops 60% of particles, it lets 0.4 through
three such layers let 0.4 × 0.4 × 0.4 = 0.064 through
overall efficiency = 1 − 0.064 = 93.6%
Why it happens: the gaps between mask fibres are tens of micrometres wide — hundreds of times larger than a virus — so if a mask were a sieve it would be useless. It is not a sieve; it is a trap. Impaction catches the big particles and diffusion catches the very small ones, which leaves particles of about 0.3 µm as the hardest to catch: too light to be thrown into a fibre, too heavy to wander into one. That size is called the most penetrating particle size, and it is the size at which mask standards are deliberately tested — a mask rated at its worst size performs better at every other size.
Tip: this also explains why fit matters so much. Air that leaks around the nose or cheeks meets no fibres at all, so the 93.6% calculated above collapses. Filtration efficiency describes the material; a mask worn loosely does not use it.
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