Increasing the concentration of reactants simply means a greater number of collisions. It is much the same as when you find yourself in a crowded room, the frequency with which you collide with others increases.

Burn a magnesium ribbon in air. The reaction occurs according to the equation below with the dramatic release of energy in the form of heat and white light.

Click to see a 120kb movie of magnesium ribbon burning in air.

Now conduct the following experiment.

Obtain a large 800ml beaker and place 30 grams of powdered calcium carbonate in it. Add 40 ml of vinegar and observe the effervescence as carbon dioxide gas is produced.
Place a burning magnesium ribbon in the beaker with the carbon dioxide gas. Goggles must be worn and tongs used to handle the magnesium ribbon.
You will notice that the magnesium ribbon burns quicker than if it was burnt in air. Click to see a 120kb movie of the reaction.

Explain why?

Hint - Magnesium burns in carbon dioxide according to the equation

A glowing splint ignites when placed in a flask with pure oxygen. Explain why?

Click to see a 120kb movie of the reaction.

Nitric acid reacts with copper to produce a toxic brown gas. Concentrated nitric acid was added to small pieces of copper metal placed in a beaker. The reaction proceeds rapidly. When water is added to the beaker containing the reactants the reactions appears to stop. Explain why?
Click to see a 120kb video.