Iron is greyish white metal and conducts electricity. It is attracted by a magnet, and reacts with dilute acids to yield hydrogen. Sulphur is a non-metal, and is yellow in colour. It dissolves in carbon disulphide. When powdered iron and sulphur are heated together, a completely new substance, iron sulphide is formed. The properties of iron sulphide are entirely different from those of iron and sulphur. It is black in colour, does not get attracted by a magnet and does not allow electric current to pass through it. It reacts with dilute acids to from hydrogen sulphide gas. In short, the properties of neither iron nor sulphur are exhibited by iron sulphide.
Conditions For Chemical Change
The criterion for a chemical change is the production of one or more new substances. Therefore, certain conditions should be met in order to bring about a chemical change. The necessary conditions are:1) A minimum amount of energy needed to initiate a reaction (activation energy) should be supplied in the form of heat, light or electric current. In a chemical change, the reactants combine to form new products. For this process it is necessary to break the old bonds of the reactants and form fresh bonds, in order to give new products. This requires a certain amount of energy.
2) For the occurrence of any reaction, the molecules or atoms of the reactants must collide with one another, in order to break old bonds and form new bonds.3) The speed with which the chemical reaction takes place is called the rate of the chemical reaction. This should be appreciable, to bring about the change.
4) The rate of a reaction depends on following factors:(a) Temperature (b) Presence of light (c) Presence of catalyst (d) Electricity (e) Pressure.
Temperature
Certain chemical reactions do not take place at room temperature but occur readily at a higher temperature. Thus heat is required to start the reaction e.g. fuels like coal and wood only start burning when heated to a certain temperature (ignition temperature).
Presence of Light
Some reactions take place only in light and do not take place in dark e.g. photosynthesis in green plants or reaction between H2 and Cl2 to form HCl.
Presence of a Catalyst
A catalyst is a substance that increases the rate of chemical reaction without itself undergoing any change.
Examples: Hydrogenation of oils to form fats takes place faster, in presence of nickel.Decomposition of potassium chlorate (KClO3) is greatly accelerated in presence of MnO2 (Manganese dioxide).
Electricity
Certain reactions take place with the help of an electric current.
Example: Decomposition of water (acidulated) to give hydrogen and oxygen gas.
Pressure
Some chemical reactions need very high pressure to proceed. For e.g., in the manufacture of ammonia from nitrogen and hydrogen (Haber's process), a pressure of over 200 atmosphere is required, in presence of a catalyst iron and a temperature of 450oC to 500oC.
Examples of Chemical Changes
Effect of heat on lead nitrate
Lead nitrate is a white, crystalline solid. When heated, it starts decomposing with a crackling sound, producing a reddish brown gas called nitrogen dioxide, and a colourless gas, oxygen. A yellow residue of lead monoxide is left behind in the test tube.
Action of concentrated sulphuric acid on sugar
When concentrated sulphuric acid is added to sugar, it becomes a black mass ofsugar charcoal. The acid removes all the hydrogen and oxygen (as water) from the sugar and absorbs it leaving a residue of spongy carbon.
Action of sodium on cold water
When a small piece (pea size) of sodium is placed in cold water, it darts about on the water with a hissing sound and produces hydrogen. The water left behind, acquires the property of turning red litmus blue. This shows the presence of a basic substance, which is sodium hydroxide.
