Reactions only happen when atoms and molecules . The [?] of reaction depends on the frequency of the collisions. A collision is one which results in a reaction.

By increasing the [?] or concentration, we increase the chance of a successful collision. This is because there are particles within the space.

By increasing the [?], we increase the speed of the molecules. This increases their [?] energy, so it's more likely that a collision is successful.

If we grind up the reactants to a fine powder, we increase the surface . Since reactions only happen at the surface, the bigger the surface area, the more likely it is that the reaction is successful.

All reactions need an energy to get them going. [?] reduce the activation energy to make it easier for the reaction to occur.