''Freshly cut apple turning brown, the iron articles shiny when new, but gradually become reddish brown when left for sometime. …..''. How do these changes occur ?

The browning of iron when left for sometime in moist air is commonly known as-

Do copper articles get rust? When rusting becomes faster? What change is this?

Can we say that every variation is a proportion A book consists of 100 pages. How do the number of pages read and the number of pages left over in the book vary? What happened to the number of left over pages, when completed pages are gradually increasing? Are they vary inversely? Explain.

A shiny brown coloured element 'X' on heating in air becomes black in colour. Can you predict the element 'X' and the black coloured substance formed? How do you support your predictions?