The rate of a chemical reaction can be influenced by several factors. Let's analyze each option regarding its effect on reaction rate:
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Changing the temperature from 90ºC to 40ºC: Decreasing the temperature typically decreases the rate of a reaction because it reduces the kinetic energy of the molecules, resulting in fewer effective collisions.
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Adjusting the concentration from 30 ng to 60 ng: Increasing the concentration generally increases the rate of reaction, as there are more reactant molecules available to collide and react.
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Placing the substance in a smaller container: This is a bit ambiguous without further context, but if the volume is decreased while keeping the amount of substance constant, it will actually increase the concentration of the substance, potentially increasing the reaction rate.
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Grinding the substance into a powder: Grinding increases the surface area of the reactants, which typically leads to an increase in the reaction rate because more surface area allows for more collisions between reactant molecules.
Given these analyses, the option that will decrease the rate of the reaction is:
Changing the temperature from 90ºC to 40ºC.