Water becomes smoothie instantly when poured out. Here is a trick to make smoothie drinks at home.

Audit expert: Meng Meng

Associate Researcher, Institute of Physics, Chinese Academy of Sciences

Magic has always brought infinite mystery to people, especially street magic, which can even be performed without any props, such as making a glass of water freeze instantly.

Source: See watermark

Many people have seen the magic of making water freeze instantly, and everyone is very curious about how the magician does it and what the secret is. Let us find out today!

In fact, it is not difficult to make water freeze instantly. All you need is a cup of supercooled water. So what is the so-called supercooled water?

1

What is supercooled water?

In the condensation process with lowered temperature, when there is a lack of solid, liquid and gaseous aerosol particles that act as condensation cores in water, that is, when there is a lack of condensation nuclei, or due to some other reasons, water that remains in liquid form below 0°C is called supercooled water. The temperature of supercooled water is lower than the saturation temperature (the temperature when liquid and vapor are in a dynamic equilibrium state), the temperature of saturated water, wet saturated steam, and dry saturated steam is equal to the saturation temperature, and the temperature of superheated steam is higher than the saturation temperature.

Different from the above concept, in engineering thermodynamics, when the water temperature is lower than the saturation temperature (0°C under normal pressure), it is called supercooled water.

Source: Baidu Encyclopedia

‍2

Supercooled water does not freeze?

The imbalance between the condensation and vaporization of water will last for a long time. The conditions required for water to condense on ice are that there must be a condensate that is pure enough. As long as the conditions are met, water will crystallize and become ice floating in the water, or it can condense on the wall of the container. However, without these conditions, liquid water will not freeze and will stay below the freezing point for a long time. If a condensation nucleus suddenly invades at this time, it will freeze immediately. However, since there are no condensation nuclei in the water, the time and temperature do not meet the requirements, water can still remain in a liquid state below 0°C.

Supercooled water is actually liquid water that is too pure and has no "crystallization nuclei" when it solidifies. If you add a small amount of solid or shake the liquid, it will solidify quickly.

Why does water freeze? Because impurities in water can provide crystals. Supercooled water is very unstable, so if you shake supercooled water, the temperature will rise, and the supercooled water will freeze!

3

Does water that does not freeze below zero mean it is supercooled?

After a brief understanding of supercold water, many people will think that as long as it does not freeze when it is left standing at below zero, it is cooling water. But is this really the case?

This is not the case, at least purified water is a counterexample, and the reasons why the two do not freeze at sub-zero temperatures are completely different!

When the temperature of purified water is below zero, the motion state of water molecules is that water molecules are completely moving around. Two water molecules are close or collide with hydrogen bonds, and can only establish one hydrogen bond. But soon, the momentum of the two water molecules acts on the hydrogen bond. At 0℃ or -40℃, the energy of one hydrogen bond is not equal to the kinetic energy of two water molecules, so water is still liquid.

In the crystallization process of supercooled water, the actual crystallization temperature required is lower than the theoretical crystallization temperature. Under a certain pressure, when the temperature of the liquid is lower than the solidification point of the liquid under pressure, the liquid does not solidify. This liquid is in a thermodynamically unstable state, but under the influence of various external factors, it can solidify quickly and cause the temperature to rise.

In addition, the purity of the liquid is also a very important factor, which determines the degree of supercooling. Studies have shown that high-purity water only starts to freeze at -40°C. This is because the liquid is too pure and does not have enough crystal nuclei to solidify. Therefore, when water must freeze, it needs to overcome the "resistance" of the ice-water interface during the nucleus formation stage, which cannot be done spontaneously. It will only turn into ice when the condensation nuclei reach a certain size.

4

Condensation nuclei of supercooled water

The condensation nuclei of supercooled water have a threshold value during the formation stage. Once it passes through it, the supercooled water will freeze immediately; if it cannot pass through the threshold value, it will not freeze; when the temperature is exactly equal to the freezing point, it will not freeze because it cannot exceed the threshold value.

When the temperature is below freezing, the local temperature changes, resulting in local nuclei. If the crystal nuclei form a certain size, there is nothing to worry about, and they can grow and freeze automatically and quickly. However, if the crystal nuclei generated by the changes are too small and do not exceed the threshold of freezing, the crystal nuclei will automatically melt and no ice will form. In addition to this, it is also important to understand that the higher the temperature, the higher the threshold, and the more difficult it is to generate enough nuclei and grow.

Why does supercooled water appear? The lower the temperature, the lower the threshold, and the easier it is to generate effective atomic nuclei. After the crystal nucleus is formed, the remaining water molecules attach to the nucleus and grow rapidly. This is why supercooled water suddenly freezes. In fact, for supercooled water, the formation of crystal nuclei depends on fluctuations, so uneven water flow is actually conducive to the growth of crystal nuclei.

Why does cold water freeze instantly? This is because the ice-water interface is greatly reduced, and the threshold is also greatly reduced. Supercooled water is a metastable state, so shaking the cup and inserting a glass rod will freeze instantly, and ice will form quickly!

5

Obtaining supercooled water

Put the purified water in the freezer of the refrigerator and freeze it in an environment of -24℃. At this time, the temperature of the water is lower than the freezing point of zero degrees, but it will not freeze. What you get is supercooled water. Next, you must gently take the supercooled water out of the refrigerator. After that, tap the bottle or shake it, and you will see that a bottle of transparent water instantly turns into translucent ice cubes, and the water in the bottle quickly freezes. Because the so-called purified water cannot be completely pure, that is, there are very few condensation nuclei inside it, and if the temperature is further reduced, it will freeze.

Of course, you can also let the outside world give it a disturbance, and shaking it will make it freeze. This is because the crystal nucleus relies on fluctuations, so this is why you have to be very careful when taking the water bottle out of the refrigerator. In addition, you can also fill a clean cup with supercooled water very carefully, then put a small piece of ice in it, and watch the water solidify rapidly around the ice.

Source: Internet

If you want to do this experiment at home, you must know the temperature and freezing time of the refrigerator. Pour boiling water into the bottle and put it in the refrigerator for two to three hours. Remember, you must pay attention to controlling the temperature and time. If it is not easy to control the temperature, you can wrap it with a towel or an insulated box, insert a thermometer in the middle of the bottle, and take it out when the temperature is 0. Be careful not to shake it. When you pour out the water, you will see that it freezes instantly. Go and try it!

Source: Digital Beijing Science Center