what is Effervescence

EFFERVESCENCE

Certain chemical reaction involves formation of gaseous products. When the reaction is carried out in a aqueous medium, the gaseous product formed get out in form of bubbles. This phenomenon is known as Effervescence.

For example, when zinc metal is treated with dilute sulphuric acid, hydrogen gas is evolved. The hydrogen gas burns with a pop sound. 

Zn + H2SO4 (dil) ZnSO4 + H2  ↑

Zinc Sulphuric acid Zinc sulphate Hydrogen

Another example for effervescence could be reaction of Magnesium carbonate with hydrochloric acid. When we put some crystals of Magnesium carbonate in hydrochloric acid, Effervescence could be observed. 

MgCO3 + HCl MgCl2 + H2O + CO2  ↑

Change of colour

As we know, the world of chemicals comprises of certain colourful chemicals. When one chemical is reacted with another, in some reaction the reactant and products have different characteristic colors which results in change of color in a chemical reaction.

For example, when lead oxide which is red in color is heated strongly, it forms yellow coloured lead monoxide.

2Pb3O4 6PbO + O2

Red lead oxide (red colour)              Lead monoxide (yellow colour) Oxygen

When copper carbonate (green in colour) is heated strongly, it leaves behind a black residue of copper oxide and gives off carbon dioxide gas. 

CuCO3 CuO + CO2

Copper carbonate (green)       Copper oxide (black)     Carbon dioxide

ENERGY CHANGES

All chemical reactions take place either by the absorption or the release of energy, generally in the form of heat energy.

Exothermic reactions: 

This chemical reactions which proceed with the release of heat energy, are called exothermic reactions.

For example, When magnesium ribbon is heated from its tip in a Bunsen flame, it catches fire and burns with a dazzling white flame with release of heat and light energy. The product formed is magnesium oxide. 

2Mg + O2 2MgO + Energy

Magnesium Oxygen Magnesium oxide

Endothermic reactions :

The chemical reactions which proceed with the absorption of heat energy, are called endothermic reactions. The following is an example of endothermic reaction. To score More in your class 8 refer NCERT solutions for class 8 .

2NaHCO3    + Heat energy Na2CO3 + H2O + CO2

Sodium                                        Sodium Steam     Carbon bicarbonate carbonate dioxide