Question

how do decomposition reaction differ from combination reaction

Answer 1

Decomposition reactions involve the breakdown of one substance into 2 or more products and absorbs heat.

Combination reactions involve 2 or more substances to combine to form 1 product .

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Answer 2

Decomposition reaction: A reaction where, one single compound breaks into two or more simpler compounds.
The reaction is mostly endothermic [heat is gained]

Example:
CaCO₃⇒ CaO + CO₂
2HgO⇒ 2Hg+O₂
2AgBr⇒ 2Ag +Br₂
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Combination reaction: A reaction where, two or more compounds react together to form a single complex compound.
The reaction is exothermic in nature [ heat is given out]

Examples:
S+ O₂ ⇒ SO₂
H₂+Cl₂⇒ 2HCl
Ba+F₂⇒ BaF₂
2S+3O₂⇒2SO₃