The short answer is because of the difference in PH levels of milk and vinegar. Milk generally has a relatively neutral PH level of around 6.5 - 6.7. While the milk is at this PH level, the molecules of the main protein in milk (casein) have a negative charge. This means the molecules are relatively soluble, because they repel other molecules due to their charge.
When milk is added to an acidic substance such as vinegar, it reaches its isoelectric PH level of 4.6. At this PH level, the number of negatively charged molecules within the milk becomes balanced by the number of positively charged molecules. This causes the molecules to stick together by forming ionic bonds. This causes the milk to become thicker and more gelatinous - in other words, the milk curdles. Other household acidic substances can be used to the same effect, including lemon juice.
If you mix milk and vinger in the right proportions, and using the right technique, you can actually create a type of plastic. Although the plastic is not particularly durable (especially if rolled out into a thin sheet), it's an interesting experiment and certainly something fun for the kids to do, that's not dangerous and doesn't require much cleaning up!
The ratio for mixing vinegar and milk is 16 parts milk to one part ratio, or about one tablespoon of vinegar for every cup of milk. You should heat the required amount of milk on the stove. Make sure you do this slowly, so that the milk does not stick to the pan or heat up too quickly. Once the milk starts to boil (you will notice foam appearing on the surface of the milk once it reaches its boiling point), take the milk off the heat and add the vinegar. Stir for about one minute. Use a sieve or a colander and pass the liquid through it - you may want to pass the liquid through the colander a few times in order to collect the most lumps of casein. Put the solid casein onto some wax paper and press on it using paper towels to get rid of excess liquid. You can then mould your "plastic" into whatever you want.
When milk is added to an acidic substance such as vinegar, it reaches its isoelectric PH level of 4.6. At this PH level, the number of negatively charged molecules within the milk becomes balanced by the number of positively charged molecules. This causes the molecules to stick together by forming ionic bonds. This causes the milk to become thicker and more gelatinous - in other words, the milk curdles. Other household acidic substances can be used to the same effect, including lemon juice.
If you mix milk and vinger in the right proportions, and using the right technique, you can actually create a type of plastic. Although the plastic is not particularly durable (especially if rolled out into a thin sheet), it's an interesting experiment and certainly something fun for the kids to do, that's not dangerous and doesn't require much cleaning up!
The ratio for mixing vinegar and milk is 16 parts milk to one part ratio, or about one tablespoon of vinegar for every cup of milk. You should heat the required amount of milk on the stove. Make sure you do this slowly, so that the milk does not stick to the pan or heat up too quickly. Once the milk starts to boil (you will notice foam appearing on the surface of the milk once it reaches its boiling point), take the milk off the heat and add the vinegar. Stir for about one minute. Use a sieve or a colander and pass the liquid through it - you may want to pass the liquid through the colander a few times in order to collect the most lumps of casein. Put the solid casein onto some wax paper and press on it using paper towels to get rid of excess liquid. You can then mould your "plastic" into whatever you want.
