Why does matter exhibit different properties when in nanoscale than when its is macroscale?

[thethinkertank]

I just read this quote on wiki;

"Materials reduced to the nanoscale can show different properties compared to what they exhibit on a macroscale, enabling unique applications. For instance, opaque substances can become transparent (copper); stable materials can turn combustible (aluminium); insoluble materials may become soluble (gold). A material such as gold, which is chemically inert at normal scales, can serve as a potent chemical catalyst at nanoscales. Much of the fascination with nanotechnology stems from these quantum and surface phenomena that matter exhibits at the nanoscale.^[26]

^{https://en.wikipedia.org/wiki/Nanotechnology}

How is that possible?

For example why would Gold be soluble at the nano scale and yet not on the macroscale? 

 

Edited June 19, 2019 by thethinkertank

[swansont]

One effect is the available surface area, in proportion to the number of atoms