The speed at which reactants change into products is controlled by several conditions. Each is explained below.
Step 1: Amount of reactant present (concentration/pressure). More reactant molecules in a given volume means more frequent collisions, so the reaction goes faster. For gases, raising the pressure has the same effect as raising the concentration.
Step 2: Heating the system (temperature). Higher temperature gives molecules more energy, so more of them cross the activation energy barrier and collide effectively. The rate typically increases 2 to 3 times for each 10-degree rise.
Step 3: Chemical character of the reactants. Reactions between ions in solution occur almost instantly, whereas reactions that require rupture of covalent bonds proceed slowly. The molecular structure therefore fixes the natural speed of a reaction.
Step 4: Degree of subdivision (surface area). In solid-liquid or solid-gas reactions, breaking the solid into a fine powder exposes far more surface, allowing many more contact points and a quicker reaction.
Step 5: Catalysts. Adding a catalyst opens a new reaction route needing less activation energy, so the reaction speeds up without the catalyst being consumed; inhibitors do the reverse.
Step 6: Radiation/light. Photochemical reactions such as photosynthesis or the hydrogen-chlorine reaction move faster when exposed to light of suitable energy.
\[\boxed{\text{Concentration, temperature, nature, surface area, catalyst, light govern reaction velocity.}}\]