milk sours because of a chemical reaction that takes place between some of the chemicals that make up the milk. Why does a carton of milk left out on a countertop sour sooner than a carton of milk left in the refidgerator

A lower temperature slows down the reaction that causes souring, making the milk last longer in the refrigerator.

Explanation:

Milk is made up of different constituents which includes:

--> water,

--> fat,

--> proteins,

--> lactose (milk sugar) and minerals (salts).

Milk is also considered to be a heterogeneous mixture because of the presence of fat and water which are two immiscible liquid phases that are present.

Milk contains a sugar called lactose. It also contains harmless bacteria called lactobacillus, which uses lactose for energy and creates lactic acid as a by-product. It is the lactic acid which makes milk taste sour. Therefore when milk is kept at a LOWER temperature, these bacteria growths are limited but at a HIGHER temperature such as room temperature, the growth is increased. As they increase in growth, the bacteria makes use of the lactose for energy and creates lactic acid as a by-product as earlier mentioned, leading to the sour taste observed.