Molecular Motion of Matter and the Effect of Lemon in Reducing the Smell of a Fish

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Author: Janice VanCleave

Matter is anything that takes up space and has weight. It appears to be made of minute individual particles, or molecules, which are separated from one another and are in constant motion.

In this project, you will explore the molecular motion of matter as you determine why a fish smells "fishy" and what effect lemon juice has in reducing the smell. You will also examine the effect of temperature on molecular motion and the difference in molecular motion among the different phases of matter, and you will experiment with a "non-Newtonian" fluid.

Getting Started

Purpose:  To determine whether lemon juice reduces the fishy smell of fish.


  • 3 cups
  • water
  • 4 tablespoons (60 ml) of lemon juice
  • spoon
  • marking pen
  • masking tape
  • 3 pieces of uncooked fish about 2 inches (5 cm) square
  • dark scarf (large enough to be a blindfold)
  • helpers (as many as possible)
  • 3 forks


  1. Fill two of the cups half full with cold tap water.
  2. Add 4 tablespoons (60 ml) of lemon juice to the water in only one of the cups and stir.
  3. With the marking pen, write "Air," "Water," and "Lemon" on pieces of masking tape and tape these labels to the appropriate cups.
  4. Place a piece of fish in each of the cups.
  5. Allow the fish to remain in the cups for 15 minutes.
  6. Blindfold one helper with the scarf.
  7. Use one of the forks to remove the fish from the mixture of lemon juice and water.
  8. Hold the lemon-soaked fish near, but not touching, the nose of the blindfolded helper.
  9. Ask your helper to smell the fish and make a mental note of its smell.
  10. Repeat the procedure twice with the same helper, first using the fish soaked in water, and then using the fish sitting in air. Use a separate fork each time.
  11. Ask your helper to compare the smell of the three pieces of fish (see Figure 22.1).
  12. Repeat the entire procedure with each helper.


The fish sitting in air has the strongest fishy smell. The water-soaked fish has less odor, and the lemon-soaked fish has the least odor.


Olfaction (the sense of smell) is the response of chemoreceptors (sensory nerves) to gases that enter the nasal cavity. A material "smells" or has a scent only if it releases gas molecules. The degree or intensity of the scent is due to the concentration of gas that enters the nasal cavity. The specific smell is due to the chemical makeup of the gas molecules. Gas molecules have translational energy (energy associated with gas molecules as they move linearly from one point to another). The self-spreading of the gas due to its translational energy is known as diffusion.

The fishy smell of fish comes from amines (organic chemicals associated with decaying flesh). In the cup filled with air, the amine molecules can escape from the fish and diffuse through the spaces around the air molecules. This increased concentration of amine molecules in the air produces the strong fishy smell.

The water-soaked fish has a reduced odor because the water surrounding the fish restricts the movement of the amine gas into the air and some of the gas molecules also dissolve in the water. Thus, the concentration of fishy-smelling gas in the air is reduced.

Molecular Motion: A Matter of Energy

The lemon-soaked fish smells the least because many of the amine molecules are chemically changed before they have time to vaporize. Amines behave as bases, and lemon juice is an acid. Like all bases, amines combine with an acid to form a salt. In this reaction, the salt produced does not give off aromatic vapor; thus, the fishy-smelling amines are chemically eliminated by the addition of lemon juice.

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