Types of chemical reactions — combination and decomposition

A combination (synthesis) reaction is one in which two or more simple substances combine together to produce a single, more complex product.

The general pattern is: two or more reactants → one product, written as $\text{A}+\text{B}\to \text{AB}$. Multiple substances are built up into one new compound.

The answer is $\text{A}+\text{B}\to \text{AB}$ — two reactants combine to form one product.

We need the general equation pattern that correctly represents a combination reaction, where substances are built up into a single product.

AB → A + B shows one reactant breaking into two products — that is the pattern for a decomposition reaction, not combination. Eliminate this option.

A → A + B shows one reactant becoming two products, which doesn't make chemical sense for a combination reaction and looks more like a decomposition. Eliminate this option.

AB + C → A + BC shows two reactants and two products — this represents a different type of reaction (a displacement reaction), not combination. Eliminate this option.

A + B → AB is the only option showing two reactants forming one product, which is the hallmark of a combination reaction.

The clues are: green powder, turns black, gas produced that turns limewater milky (indicating carbon dioxide), and the energy source is heat.

Copper carbonate is a green powder that, when heated, breaks down into copper oxide (black solid) and carbon dioxide gas. Carbon dioxide turns limewater milky, matching all observations perfectly.

The word equation is: $\text{copper carbonate}\stackrel{\text{heat}}{\to }\text{copper oxide}+\text{carbon dioxide}$. One reactant breaks into two products with heat — this is thermal decomposition.

All observations match thermal decomposition of copper carbonate.

We need to match the observations — green powder, turns black, gas turns limewater milky, heated — to the correct reaction.

Electrolysis of water uses electricity (not heat) and produces hydrogen and oxygen gases — not carbon dioxide. Water is also not a green powder. Eliminate this option.

Oxidation of magnesium produces a white powder (magnesium oxide) and a bright white flame — it starts with a silver metal ribbon, not a green powder, and produces no carbon dioxide. Eliminate this option.

Dissolving copper sulfate is a physical change — no gas is produced, no colour change to black occurs, and limewater would not turn milky. Eliminate this option.

Thermal decomposition of copper carbonate matches all observations: green → black colour change, carbon dioxide gas produced (limewater turns milky), and heat is the energy source.