This question can be answered in one sentence – because the density of the dumpling changes during the cooking process. But such an answer does not give a true understanding of the process, so let’s dig a little deeper.
Why things float
For any object to move anywhere, a force must start acting on it. There are two opposing forces acting on the dumpling in the water: the Earth’s gravitational force and the force pushing it out of the water (Archimedes’ force). The force of gravity pulls the dumpling downward, while the force of Archimedes pulls it upward. Accordingly, if it is pulled down harder, it sinks. If it is pulled more upward, it floats. And if the forces balance each other, the dumpling does not move anywhere – it rests in the water column.
How big the force of attraction will be (with how much force the dumpling is pulled down) is influenced by 3 parameters – the density of the dumpling (ρ of the dumpling), the acceleration of gravity (g), and the volume of the dumpling (V of the dumpling).
The value of Archimedes’ force (the force pulling the dumpling upwards) also depends on three parameters – the density of the liquid in which the dumpling is immersed (ρ of liquid); the acceleration of gravity (g); and the volume of the dumpling (V of dumpling).
You may notice that the force pulling up and the force pulling down have 2 identical parameters – the acceleration of gravity (g) and the volume of the dumpling (V of the dumpling). This means that these parameters do not affect the ratio of forces. For example, if the volume of the dumpling increases 10 times, it will increase the gravitational force 10 times, but the Archimedean force will also increase 10 times, thereby extinguishing the increased gravitational force.
So only the remaining 2 parameters – the density of water and the density of the dumpling – can change the ratio of forces. If the density of the dumpling increases, then the gravitational force will pull the dumpling down harder, while the Archimedean force pulling upwards will remain the same – the density of the dumpling is not among the parameters that can increase or decrease it.
From all of this we can conclude that two factors can affect the fact that the dumplings pop up:
- an increase in the density of water (in which case Archimedes’ force will pull stronger upward) or
- decreasing the density of the dumpling (in this case the force of attraction will pull down less intensively).
How can you change the density
The density of raw dumplings is approximately 1.25 g/cm³ (that is, if the dumpling were in the form of a cube with a length of 1 cm, width of 1 cm and height of 1 cm, it would weigh 1.25 g).
The density of water at room temperature is 1 g/cm³. As the temperature rises, the density of water gradually falls, and at 100 ° C is 0.96 g/cm³. This means that in such water, Archimedes’ force, pulling the dumpling upwards, is even less than before the water was heated. So the dumpling in this case becomes even more difficult to float up. So, the reason is not an increase in water temperature.
When cooking dumplings, we usually salt them. Could this be the reason? Salt water does have a high density. At room temperature, it is about 1.02 g/cm³. And at
100°C it is 1 g/cm³ (the exact value depends on how much salt you add – the more salt, the higher the density of the water). But even this density is not enough to lift a dumpling with a density of 1.25 g/cm³ to the surface.
It seems that the only thing left is to change the density of the dumpling itself. This is possible if the volume of the dumpling increases, but its mass remains the same. This is possible if the dumpling becomes larger and weighs the same as before. Another option of changing the density is if the volume remains the same and the mass decreases. But guided by life experience, we can say that the first option is more realistic – in the process of cooking dumplings usually increase in size.
How you can increase the volume
Most chemicals in nature change under the influence of high temperatures. The substances that make up dumplings are no exception. All animal and human tissues are composed of proteins – complex molecules with a variety of functions. Proteins help muscles contract (actin and myosin proteins), chemical reactions take place (enzyme proteins), they play an important role in the immune response (protecting the body from harmful microorganisms), and perform thousands of other important functions.
One of the most common proteins in the human body is collagen. It is found in large quantities in muscles (meat), skin, cartilage. All of these components are part of the dumpling filling.
Collagen looks like a rope. It consists of three densely packed “threads” which intertwine to form a thicker “rope”. However, under the influence of high temperature this rope becomes less dense. Its “threads” stop sticking tightly to each other and lose their thread shape. Because of this, it begins to take up more space – to take up more volume. At the same time, the number of components that make up the “rope” has not changed. They have not disappeared, which means that its mass remains the same. Similarly, if we loosen the threads of a real rope and loosen them, the rope will occupy more space, but its weight will not change. This means that the density of such a rope will be less.
As a result, more loose and less dense gelatin appears in place of the dense collagen filaments.
A similar process occurs with the main component of dough – starch (this is not a protein, but a complex carbohydrate – a representative of another important class of substances in our body). When heated, the starch turns into a glister, which also has a lower density.
These transformations do not occur instantly, but over a long period of time as the water heats up. The density of the dumpling gradually decreases from 1.25 g/cm³. Until it reaches a threshold value of 1 g/cm³. After this value, Archimedes’ force becomes stronger than gravity, and the dumpling moves upward.
Если вам есть что дополнить в статье — напишите это в комментариях.
