Reactions happen at different rates (speeds) dependant on different factors.  In this section we will take a quick look into what these factors are. Later you will learn why they do what they do.           

                                                                                                                             The four factors that affect the speed of a reaction are: temperature, concentration, particle size and catalysts.        

As you increase temperature the reaction rate increases and as you increase the concentration of a solution you speed up the reaction rate. So, increasing temperature or concentration speeds up the reaction.                                                                                        However increasing particle size has the opposite effect. When you have a huge lump you have a large particle size. The reaction happens on the surface of the substance. So, the big lump has a small surface area. But if you made the lump into powder you would get a faster reaction because you have a larger surface area. The diagram illustrates the point with simple 2D shapes: 

The surface area idea can be hard to get your head around. Hopefully the diagram helps clear this up: