In the context of the given question, the fastest reaction refers to the rate at which a substance reacts with acid when all other factors, such as the amount and temperature of acid, are kept the same. The options provided (ribbon, powder, crystal, and foil) represent different physical forms of magnesium (Mg) that can react with acid.
The fastest reaction is likely to occur with the magnesium powder (B) rather than the other options. This is because the powder form has a larger surface area compared to the ribbon (A), crystal (C), and foil (D) forms. When a substance has a larger surface area, there are more exposed particles available for the acid to react with. This increases the chances of collision between the acid molecules and the magnesium particles, leading to a faster reaction rate.
To further explain, imagine you have a solid block of magnesium. If you break it down into smaller pieces, such as a ribbon, crystal, or foil, you are increasing the surface area of the magnesium. However, when you crush the solid magnesium into a fine powder, you create the largest surface area possible. This increased surface area allows more acid molecules to come into contact with the magnesium particles simultaneously, resulting in a faster reaction.
In summary, when all other factors are the same, the magnesium powder (B) is likely to react the fastest with acid due to its larger surface area, allowing for more efficient and frequent collisions between the acid molecules and magnesium particles.