By crushing a solid into a powder you increase the reaction rate (the chemical reaction rate). By crushing into a powder, you are increasing the surface area, increasing the exposure to the chemical reaction. The smaller the particles, the faster the chemical reaction that will take place. Think about an ice cube put in a pot of boiling water. The whole ice cube will take longer to melt than if you first crushed that ice cube into small pieces. Small pieces of ice put into boiling water would melt very quickly.
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