Combination and decomposition reactions · Chemistry Class 9–10 | StemPy Lab
Course 5.2·Lesson 1 of 3
Combination and decomposition reactions
In a combination reaction, two or more substances join to make one. In a decomposition reaction, one substance breaks into two or more. Decomposition can be caused by heat, light or electricity.
about 25 min1 simulation2 practice questions
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Combination: two become one
In a combination reaction, two or more reactants join to make one product.
A+B→AB
Examples:
Burning of carbon: C(s)+O2(g)→CO2(g).
Formation of water: 2H2(g)+O2(g)→2H2O(l).
Burning of magnesium: 2Mg(s)+O2(g)→2MgO(s).
Slaking of lime. Quicklime joins with water, and a lot of heat is given out. The product is slaked lime, which is used for whitewashing walls.
CaO(s)+H2O(l)→Ca(OH)2(aq)+heat
Decomposition: one becomes two or more
In a decomposition reaction, one reactant breaks into two or more products.
AB→A+B
Decomposition needs energy. The energy can be heat, light or electricity.
By heat (thermal decomposition).
Calcium carbonate (limestone): CaCO3heatCaO+CO2. The quicklime that we get is used to make cement.
Green iron sulphate crystals lose water and then break up when heated. The colour changes from green to brown, and gases with a smell of burning sulphur come out.
2FeSO4heatFe2O3+SO2+SO3
By electricity (electrolysis). Water breaks up into hydrogen and oxygen when an electric current passes through it.
2H2O(l)electricity2H2(g)+O2(g)
By light (photolysis). Silver chloride is white, and it turns grey in sunlight, because silver is made. This is used in black and white photography.
2AgCl(s)sunlight2Ag(s)+Cl2(g)
You should find that the solid always ends at 56% of its starting mass; a hotter flame only gets it there sooner.
Opposite reactions
Combination and decomposition are opposites. Hydrogen and oxygen combine to make water. Water decomposes into hydrogen and oxygen.
Four reaction types shown with coloured blocks. Combination: A plus B gives AB. Decomposition: AB gives A plus B. Displacement: A plus BC gives AC plus B. Double displacement: AB plus CD gives AD plus CB.
Practise with new numbers
Try it in Python
This program names the type of a reaction from what is on each side.
Remember these
Combination: A+B→AB. Decomposition: AB→A+B.
Decomposition needs heat, light or electricity.
Slaking of lime is a combination reaction that gives out a lot of heat.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
What type of reaction is 2H2+O2→2H2O?
What energy breaks up water in electrolysis?
Why is slaked lime used to whitewash walls?
Why are combination and decomposition called opposite reactions?
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