What happens when milk and vinegar mix?

In this experiment, we will see how the different densities of water and milk will allow us to separate these two organic compounds.


1. A beaker
2. Milk
3. Water
4. A stopper

How to separate milk from water

1. Make a very small hole with an awl or a knife in the stopper.
2. Pour water into a glass.
3. Next, put a stopper with a small hole in it on top of the water.
4. Now, we pour milk into the stopper and see what happens.
5. The milk is denser than the water, and it flows out of the hole to the bottom of the glass without mixing with the water.
6. The milk falls to the bottom of the glass, but little by little, it mixes with the water.


Density is an important concept to understand why the milk does not mix with the water in our experiment. It is a property of compounds that is determined as a function of the mass-to-volume ratio. In other words, density measures how much a substance weighs as a function of how much it occupies. The higher these values, the higher their density. Density is measured using the units of mass over volume. For example, gram per cubic centimeter (g/cm³), kilogram per cubic meter (kg/m³), gram per cubic meter (g/m³), or milligram per cubic meter (mg/m³).

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The density of water and milk

Before we look at how the density of milk and water behave when put together, let’s look at them separately. In the case of water, the density is 1 gram per cubic centimeter (g/cm³) at a temperature of 15°C. Or, in other words, 1 kilogram per cubic meter (kg/m³). In other words, 1,000 grams of water occupy a volume of 1,000 cubic centimeters (g/cm³). This value changes based on factors such as temperature or pressure. In addition, in the case of seawater, the salinity of the water would also influence its density.

The density of milk is also variable, as it is related to the percentage of fat, non-fat solids, and water it contains. To get a clear idea of the density of milk, it is best to measure it when it is still fresh, as exposure to the environment may cause it to vary. In general, the density of milk is 1.03 grams per cubic centimeter (g/cm³).

In this experiment, we have been able to see that the density of water and milk are different. But not so different that they cannot mix.

Properties of milk

There are a lot of experiments that can be done using milk. For example, can you imagine what can happen when you mix milk and lemon?
In order to understand this experiment, it is essential that you understand the different components that milk has. It will also help you to understand this experiment. This liquid contains several compounds, mainly fats, proteins, and sugar. Milk protein is normally suspended in a colloidal solution. As a consequence, the small protein molecules will float free and independently in a continuous manner. To give you a better understanding, these floating protein molecules are responsible for refracting light and giving milk its white appearance that we can observe. Normally, these protein molecules repel each other, which allows them to float without clumping together. Hence, they are in a liquid state. However, when the pH of their solution changes, they can suddenly attract each other and form clumps. This is exactly what happens when milk curdles. For this, it is sufficient to add a certain amount of lemon juice. With it, we will be altering the state of the milk.

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Why is milk curdled with vinegar?

If lemon juice or vinegar is added to milk, its pH is modified, and it curdles. This happens because the acid present in the lemon (citric acid) or in the vinegar (acetic acid) is capable of producing the denaturation of a protein present in the milk called casein.

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Many people wonder, then, what happens when vinegar is mixed with salt?

The mixture of vinegar (acetic acid) with salt (sodium chloride) is the most effective. By adding salt, the chloride ion in the salt acts as a catalyst, i.e., it promotes the reaction by accelerating and improving the process but without being consumed.

However, what happens if I add vinegar to the milk?

When added to hot milk, vinegar results in the formation of white lumps (curds). Milk is mainly composed of water, fat, lactose, and many proteins called casein. The casein proteins normally form spherical shapes, which float in the liquid milk.

But how does milk react with vinegar?

Casein proteins normally form spherical shapes, which float in liquid milk. When you add vinegar (an acid) to the milk, these spherical structures break up. You can see that this chemical reaction results in a white semi-solid product, also called curd or casein polymer.