Is rusting of iron almirah a physical change or chemical change? Explain. (2)
(b) What is meant by an element?

Solution:

When iron rusts it oxidises i.e. iron in the presence of moisture reacts with oxygen to form Fe (III) oxide. The reaction occurring is-
 
                                                                4Fe +3O2+2H2O→2Fe2O3.H2O
                                                           (Iron) (oxygen) (water)  (hydrated iron (III) oxide)
                                                                                                          (Rust)
 

• This is a chemical change.
• A physical change is when a molecule changes state, for example iron going from solid to liquid.
• Generally oxidation cannot occur without reduction.The electrons lost by the substance that is oxidised must be accepted by atoms or ions of another substance.
• However in electrochemical reactions driven by an externally imposed electromotive force and having a partial migration barrier between the two electrodes, it is possible to separate the oxidation and reduction reactions from one another.
• Hence, rusting of iron almirah is an example of a chemical change.

(b) An element is a pure substance which consists of only one type of particles. These particles can be atoms or molecules. For example Carbon is made up of only carbon atoms, gold is made up of only gold atoms, Hydrogen gas consists of Hydrogen molecules (formed from two atoms of Hydrogen) etc. It is the most basic or simplest form of matter.