What is meant by a chemical reaction? Explain with the help of an example. (b) Give one example each of a chemical reaction characterised by: (i) Evolution of a gas (ii) Change in colour (iii) Formation of a precipitate (iv) Change in temperature Change in state.
- A chemical reaction results in new substances by undergoing certain reactions and There will be reactants on the left hand side and products on the right hand side.
An example of combustion is methane + oxygen forms carbon dioxide and water. This can be written as a balanced symbol equation:
CH4 + 2O2 →CO2 + 2H2O
- (i) Evolution of gas
Reaction of Zinc and Hydrochloric acid
Zn + 2HCl → ZnCl2 + H2(g)
Hydrogen gas is released in the form of diatomic molecule
- Change in colour
Cu(s) + O2(g) + H2O → Cu(OH)2(s) + O2(g)
Copper is in reddish brown colour and when reacts with oxygen and water it forms hydrated copper carbonate which is green in colour
- Formation of precipitate
AgNO3(aq) + KCl(aq) →AgCl(s) + KNO3
When silver nitrate and potassium chloride in its aqueous form reacts a precipitate of potassium nitrate is obtained
- Change in temperature CaO + H20 →Ca(OH)2
When Quick lime is added to water to form slaked lime a change in temperature is obtained as it reacts vigorously and produce heat which is an exothermic reaction
- Change in state
NH3(g) + HCl(g) → NH4Cl(s)
Ammonia and Hydrogen chloride gas reacts to form Ammonium chloride which is a solid