Evolution of Gas:
Evolution of gas in a chemical equation is indicated by the appearance of a gas as one of the products of a reaction. This can be represented by the symbol (g) next to the formula of the gaseous product. For example, in the reaction 2HCl(aq) → H2(g) + Cl2(g), the evolution of gas is indicated by the formation of hydrogen gas (H2) and chlorine gas (Cl2) as products.
When a gas is evolved in a chemical reaction, it is usually a sign that a new substance is being formed. This can happen when two reactants combine to produce a gas, or when a single reactant breaks down to form a gas. The evolution of gas can also indicate a change in temperature or pressure within the reaction vessel.

Formation of Precipitate:
The formation of a precipitate in a chemical equation is indicated by the appearance of a solid substance that is insoluble in the reaction mixture. This can be represented by the symbol (s) next to the formula of the solid precipitate. For example, in the reaction AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq), the formation of precipitate is indicated by the silver chloride (AgCl) that is formed as a solid product.
When a precipitate is formed in a chemical reaction, it usually means that a reaction has occurred between two soluble substances to produce an insoluble product. This can happen when two ionic compounds react to form a solid precipitate through a double displacement reaction.

Evolution or Absorption of Heat:
The evolution or absorption of heat in a chemical equation is indicated by the inclusion of the terms "heat" or "ΔH" (change in enthalpy) in the reaction. When heat is evolved in a reaction, it is usually written as a product on the right side of the equation. Conversely, when heat is absorbed in a reaction, it is written as a reactant on the left side of the equation.
The evolution or absorption of heat in a chemical reaction can indicate whether the reaction is exothermic (heat is released) or endothermic (heat is absorbed). Exothermic reactions tend to feel warm to the touch, while endothermic reactions tend to feel cool. The amount of heat evolved or absorbed can also be quantified using calorimetry techniques.