Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What is the chemical formula of pure milk?

  1. May 11, 2009 #1
    What is the chemical formula of pure milk? What is the chemical reaction taking place when milk combines with salt?

    thanks in advance.:smile:
  2. jcsd
  3. May 11, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    Re: milk!

    There isn't a chemical formula for milk as such - it's a mixture of (mostly) water, fats, proteins and 1000s of minor chemicals. It's rather like asking the chemical formula for a cow!

  4. May 11, 2009 #3


    User Avatar
    Science Advisor
    2017 Award

    Re: milk!

    This is not a chemical reaction, but a physical one. Milk is what is known as a colloid: a physical mixture of two substances where particles of one substance is evenly dispersed in the second substance. In the case of milk, small globules of fat are dispersed throughout a solution of (mostly) water. These fat globules are so small that the gravitational force on them is not enough to counter their tendency to diffuse through the solution. Hence, the fat globules stay suspended in solution indefinitely (for larger particles, such as pulp particles in orange juice, gravity is stronger than diffusion so the pulp particles settle at the bottom of the container if you let it sit in the refrigerator).

    Now, particles in a colloid experience a fine balance of intermolecular forces. On one hand, the fat globules would like to stick to each other and clump together. However, these fat globules are charged (I believe they have a slight negative charge), so the particles repel each other electrostatically (remember, like charges repel). In milk, the electrostatic repulsion is what allows the fat particles to stay suspended in solution.

    Adding salt to the mixture alters the balance of forces. The salt dissociates into ions in the milk, and these ions help to screen out and weaken the electrostatic repulsion between fat globules. At a certain concentration of salt, the attractive forces between the fat particles become stronger than the electrostatic repulsion between particles. When this happens, the fat globules aggregate together and form larger particles. These larger aggregates now experience larger gravitational forces that are stronger than their tendency to diffuse, so they fall out of solution and the milk separates. These aggregates of fat particles, if created properly, are more commonly known as cheese.

    The same concept occurs in many other systems. For example, river deltas form because silt particles that are suspended in fresh water aggregate together when they come into contact with saltwater at the mouth of a river. This phenomenon causes a large buildup of silt and mud at the mouths of rivers, creating the characteristic river delta.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook