One of the simple ways to prevent rusting is to apply a coating of paint or grease on the surface of iron. In fact, these coats should be applied regularly to prevent rusting. Another way to prevent rusting is to deposit a layer of a metal like chromium or zinc on iron.This process of depositing a layer of zinc on iron is known as galvanization.

The process of obtaining large crystals of pure salt from sea water is known as crystallisation.
The process of crystallisation is used to purify a large number of water soluble solids. Common salt obtained from sea water is an impure substance having many impurities in it.

Formation of manure from leaves is a chemical change as a new substance (manure) is formed. The product (manure) has different properties than the original substance (leaves) and this process cannot be reversed.

NaHCO₃ is the chemical formula of Baking soda.

A change can be physical or chemical, depending on whether or not it involves the formation of a new substance. In a physical change, no new substance is formed whereas, in a chemical change, a new substance is formed. In a physical change, the product formed has the same properties as the original substance whereas, in a chemical change, the product formed has different properties than the original substance. The physical change is temporary and can be reversed while the chemical change is permanent and cannot be reversed.
Some of the examples of physical changes are melting of ice, cutting a fruit, etc. Some of the examples of chemical changes are burning of wood, rusting of iron, etc.               

$\bullet$ When candle wax melts, only physical state will change from solid to liquid. When the wax cools, it returns to its solid state.
$\bullet$ Breaking chalk into pieces does not involve any chemical reaction only the size changes. Hence, it is a physical change.
$\bullet$ The digestion of food involves a long series of chemical changes. The complex substances we eat are changed into new simpler substances.
$\bullet$ Burning is also a chemical change in which the substance that burns combine with oxygen to form a new
substance 'ash' along with heat and light.

Turning milk into curd is a chemical change. A chemical change is defined as a change in which composition of substance changes. The compositions of curd and milk are different. After converting milk into curd, we cannot get back the milk again. So, it is an irreversible change.

Changes in which only physical properties of a substance change are called physical changes. In a physical change, no new substance is formed.
For example cutting of wood is a physical change or conversion of water into vapours.
 

Naming: 1 Mark
Equation: 1 Mark
The two conditions essential for rusting are presence of
(i) Oxygen
(ii) Water or water vapour
The chemical equation for rusting of Iron is:
Iron(Fe) + Oxygen$(O_2$) + Water$(H_{2}O$) $\to$ rust(Iron Oxide$F{e}_2{O}_3$)