We know what we mean by a centimetre (cm) - we mean 1/100 of a metre or 1 x 10-2 m

We know what we mean by a millimetre (mm) - we mean 1/1000 of a metre or 1 x 10-3 m

You may know  a micrometre - we mean 1/1000000 of a metre or 1 x 10-6 m

 A nanometre (nm) - means 1/1000000000 of a metre or 1 x 10-9 m

So nanoparticles are between 1 and 100 nm in diameter.

Nanoparticles are a lot smaller than any particle you can see.

A coarse sand grain is around  1mm in diameter or 0.001 m     (1x10-3m)

A fine sand grain is around 0.1 mm in diameter or  0.0001 m    (1x10-4m)

But when scientists talk about particles they're not thinking of sand.

They refer to coarse particles as dust and include particles between 1 x 10-5 m and 2.5 x 10-6 m, which is smaller than fine sand grains.

To make things easier they refer to this range of size as PM10 - particles less than 10 micrometres.

They think of fine particles as being PM2.5 - particles smaller than 2.5 micrometres.

It seems unlikely that the examiner will expect you to recall these numbers even though they are stated on the syllabus but you would be expected to know that Nanoparticles are smaller even than this.

It's all about surface area.

Strangely, the more we break up a particle the more surface it has.

Breaking up the particle into smaller pieces doesn't increase its size (volume) though.

So, the smaller the particles the higher the surface are to volume ratio.

You are expected to know that decreasing the length of the sides by a factor of ten increases the surface area to volume ratio by a factor of 10.

And for solids reactions happen at the surface.

So the smaller the particle, the larger the surface area and the faster its reactions may happen.

This may mean that we can use less material to do the same job.

For instance, nano sized salt tastes stronger than normal salt due to its large surface.

So we can use less salt to add  the same amount of flavour.

More importantly, we may be able to reduce doses of medicines if they are produced in nanoscale.

Sun-block is often made of Zinc Oxide - but bulk Zinc Oxide is white, while nanoparticle Zinc Oxide is invisible.

And invisible nanoparticles would be equally useful in deodorants and cosmetics.

Sometimes, the increased surface area to volume ratio means that nanoparticles will react completely different;y to normal sized grains - being smaller has changed the chemical properties from that of the bulk material.

For instance, catalysts are substances that speed up reactions without being used up.

Often they do they by having other substances stick to their surfaces - so a substance may be a much more effective catalyst in nano-size particles than as a bulk material

Recall that the diameters are:

·         Nanoscience – the study of particles between 1-100 nm

·         Nanoparticle – a particle containing a maximum of a few hundred atoms

·         PM2.5 - fine particles (PM2.5), which have diameters between 100 and 2500 nm (1 x 10-7 m and 2.5 x 10-6 m). 

·         PM10 - Coarse particles have diameters between 1 x 10-5 m and 2.5 x 10-6 m. Sometimes referred to as dust.