What change will slow the rate of a chemical reaction?
A decrease in concentration of reactants is an example...
When reactants get used up, their concentration decreases, and so less successful collision between the desired particles occur, therefore slowing down the rate of reaction. A simple lab experiment to do would be reacting hydrochloric acid with a metal, such as magnesium, and then use a gas syringe to calculate how much gas evolved. Time the experiment for around five minutes.
As the reaction progresses, you see less and less bubbles being formed over time until the reaction stops, due to the concentrations of reactants decreasing, and you can see that clearly when the magnesium ribbon slowly gets used up.