Key concepts
Chemistry
Physics
Reaction
Surface area

Introduction
Did you know that flour can explode? Luckily, this does not happen spontaneously on your kitchen counter, but only if the conditions are right. You need a very fine powder of flour to make an explosion happen. In fact, any solid flammable material that is dispersed in the air as a dust cloud will explode if it comes into contact with flame (a reason extreme caution must be used where there is a large amount of grain dust, such as in storage facilities). Why is that? It has to do with the particle size of the solid material, which determines how rapidly a chemical reaction takes place. In this activity, you can try this for yourself—skipping the explosion and creating a big fizz instead!

Background
Some chemical reactions happen very fast (think vinegar and baking soda), whereas others take a very long time (such as rust forming on metal). In chemical reactions that include a solid as one of the reactants, you can actually change the reaction rate by varying the size of the solid that reacts with the liquid or the gas. How does this work? For a chemical reaction to happen, the molecules or atoms of the reactants need to collide with each other. This can only happen at the surface of the solid, as all the molecules trapped within the body of the solid cannot react until they meet the molecules of the other reactant. However, if you take the same material and break it into smaller pieces, there is much more surface area exposed that can interact with the other components—allowing the chemical reaction to occur much more quickly.

Take a flammable material (such as flour) as an example: If you heat up a lump of flour, it will not burn at all or will start burning away slowly in a controlled manner because there is only a limited surface area available that can react with the oxygen in the air. However, the same flammable material dispersed into the air as a fine powder would allow for much greater surface area exposed to the air, allowing for a rapid explosion if ignited. Increasing the surface area of a reactant does not only increase the quantity of material available to react, but will also increase the rate of the reaction. In this activity, you will demonstrate this effect by measuring the rate of a different kind of chemical reaction: the dissolution of sodium bicarbonate from an effervescent antacid tablet in water.  

Materials

  • Effervescent antacid tablets (at least four)
  • Sheet of paper
  • Spoon
  • Four clear 12-ounce (or larger) drinking glasses
  • Measuring cup
  • Teaspoon
  • Tap water
  • Stopwatch
  • Pen

Preparation

  • Take four antacid tablets out of their packages. Take care not to break them, as the tablets are very brittle.
  • Put one whole antacid tablet aside for now.
  • Take the second tablet and break it into half and set both halves aside.
  • Take the third tablet and break it evenly into quarters. First, break it into halves and then break the halves again into two parts. Set all four pieces aside.
  • Take the fourth tablet and ground it into a powder. To do this, put the tablet to be ground inside a clean, folded piece of paper. Place the folded paper on a solid surface and use a spoon to carefully crush the tablet into a powder. Keep the powder folded into the paper and set it aside.
  • Use a measuring cup to add about 250 milliliters (about eight ounces) of tap water to each of the four glasses. The temperature of the water should be the same in each.

Procedure

  • Take one of the glasses with tap water and the whole antacid tablet and put them in front of you.
  • Pick up the whole tablet and hold it above the water surface.
  • Get your stopwatch ready.
  • Drop the whole tablet into the water and at the same time start the stopwatch. What happens once the tablet hits the water? Can you see a chemical reaction happening?
  • Stir the water gently and steadily with the teaspoon. Observe the tablet closely in the water. What do you notice about the tablet? What reaction do you think is taking place?
  • Once all the solid material of the tablet has dissolved in the water and the chemical reaction is completed, stop the stopwatch and write down the reaction time on a sheet of paper. How long did the reaction take? Do you think this reaction is fast or slow?
  • Get a fresh glass of water and this time take the antacid tablet that you broke in half.
  • Take both pieces of the tablet and hold them above the water surface. What do you think will change once you put the two pieces of the tablet into the water compared to the whole tablet?
  • Reset your stopwatch and get it ready.
  • Drop both pieces of the tablet into the water and start the timer again. Compared to the whole tablet, do you see the same reaction happening in the water?
  • Again, stir the water gently and observe how the two tablet pieces dissolve in the water. Do you see more or fewer bubbles forming? Do you think this reaction will be complete faster or more slowly than with the whole tablet?
  • Once all the solid tablet material has completely disappeared and the bubbles have stopped forming, stop the stopwatch and record the reaction time. Did the reaction time change compared to the whole tablet? Was this reaction faster or slower? Why do you think this is the case?
  • With the two remaining glasses, repeat the antacid-adding steps with the antacid tablet that you broke into four pieces and the tablet that you crushed into a powder. Do you observe any changes in the chemical reaction happening in the water? How fast or slow are these tablets dissolving compared to the other tablets? Do you notice any correlation between the reaction time and the size of the tablet pieces?
  • Extra: Can you think of other chemical reactions that you could use to test how the surface area of one of the reactants affects the reaction rate with water? Think of other ingredients in your kitchen that come in various sizes and forms, such as sugar crystals, cubes or powder. Will the same effect be observable for these substances?
  • Extra: What other factors can change the rate of a chemical reaction? Repeat this activity, but only use whole antacid tablets, and this time, vary the temperature of the water in which you dissolve the tablets. How do you think the temperature will influence the reaction rate? Will the tablet dissolve faster or more slowly in hot water compared to cold?

Observations and results
Did you find that the tablet powder dissolved much faster than the whole tablet? What you probably observed in all of your trials was some vigorous bubbling once you dropped the antacid tablet into the water. Effervescent antacid tablets are made from aspirin, citric acid and sodium bicarbonate. When sodium bicarbonate dissolves in water, it reacts with hydrogen ions from the citric acid and forms carbon dioxide. Because carbon dioxide is a gas, it forms bubbles inside the water that you can see as foam on the surface.

The fizzing and bubbling was probably more pronounced the smaller the tablet pieces were that you dropped into the water. At the same time, you probably noticed that the whole tablet took the longest to dissolve, whereas the tablet powder dissolved really quickly. This is because with smaller tablet pieces, there is more surface area of the tablet available that can react with the water, which results in a faster disintegration of the antacid tablet, as you observed.

Cleanup
Pour the water with the dissolved antacid tablets into a sink.

More to explore
Surface Area and Reaction Rate, from ADLC Educational Media
Harmless Flour Is an Incredibly Explosive Substance, from AweSci
Big Pieces or Small Pieces: Which Reacts Faster?, from Science Buddies
Science Activity for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies