In some reactions, new matter is added to the original substance and the substance gains in mass. Such a reaction is called a combination or synthesis reaction. Combination reactions are those in which one element reacts with another to form a compound. Such reactions are also known as synthesis reactions. For example, Hydrogen combines with oxygen to give water.
Take a magnesium ribbon and heat it over a flame. It burns giving out a bright white light. The magnesium ribbon becomes white powder. Add some water to the powder and put a drop of red litmus. The red litmus turns blue. This is due to combination reaction. Oxygen in the air combines with magnesium to give white powder of magnesium oxide (MgO). Magnesium oxide reacts with water to give magnesium hydroxide. Being basic, it turns red litmus blue.
The reactions are:
2Mg(s) + O2(g) 2MgO(s)2MgO(s) + 2H2O(l) 2Mg(OH)2(aq)
Compounds can also undergo combination reactions. For example:
NH3 + HCl NH4Cl
CaO + CO2 CaCO3
2CO + O2 2CO2
2SO2 + O2 2SO3