Shulou(Shulou.com)11/24 Report--

Q1, tap water is poured into the mold and put into the refrigerator. How can transparent ice cubes be frozen out? What chemicals need to be added?

By Anonymous

A:

First of all, the transparent ice cubes in fast food restaurant cola are all made by commercial ice makers, so it is difficult to reproduce them at home.

If you really want to make transparent ice at home, there is a way. Find an incubator, fill it with water, don't add the lid, freeze it slowly in the refrigerator, remember, don't freeze through, just freeze the upper part, make sure the bottom is still liquid, so that the upper part of the frozen is transparent ice without bubbles. If you think the ice is too big and not square enough, you can cut it with your own tools.

Water icing is essentially a crystallization phenomenon, which can be explained by the nucleation theory. To put it simply, crystallization first needs a crystal nucleus, and then depends on the continuous growth of the crystal nucleus. Generally speaking, for pure liquid phase, there are three kinds of nucleation power, namely, undercooling at constant pressure, supersaturated vapor pressure at constant temperature and supersaturated concentration at constant temperature and pressure. The power of water freezing comes from the first, that is, when the pressure is constant, when the temperature reaches the freezing point of the liquid phase, the crystal can overcome the potential barrier and nucleate.

However, when there are impurities in the solution, inhomogeneous nucleation will occur. This is because the interface between impurity particles and liquid phase can reduce the potential barrier of nucleation and nucleate more easily. The impurities here are solid particles and bubbles in the water. Therefore, for ordinary water, when it freezes, it often starts from the bubble, and the formed ice freezes the bubble into it, and the ice looks white.

In the scheme given above, the surface of the water is exposed to a cold source, and the water without bubbles on the surface nucleates and crystallizes at low temperatures and continues to grow downward, so that the influence of bubbles can be eliminated and the bubbles can be squeezed into the liquid water below. Of course, if you can completely remove air bubbles and other impurities in the water, you can also freeze transparent ice, but the general boiling can not achieve such purity, you can consider distillation to try.

By Frost White

Q2. Why is the road sloping at the bend? by ordinary high school students

A:

Open the new physics textbook for senior high school, and then find the chapter of centripetal force. The answer is about to come out, that is to provide centripetal force.

When the car is driving at the bend of the road, if it is not tilted, the centripetal force can only be provided by the static friction between the tire and the ground. But the static friction has a maximum value, if the speed is too fast, static friction can not provide the centripetal force needed to turn, then the car will slip out of the corner, which is dangerous.

A schematic diagram of the bend and when the bend is tilted, the centripetal force can be provided by ground support.

Schematic diagram of force analysis finally, there is actually a professional term for bend inclination: Super-elevation. If you are interested, you can take a look at the reference materials. I will not repeat them here.

Reference:

Wang Guishan, Hu Changliang, Bai Haochen, Li Ruijie. Study on the Design of Circular Curve Superelevation and transition Section of Expressway [J]. Highway Traffic Science and Technology, 2021 no. 38 (12): 47-55.

By justiu

Q3. Is there a physical or chemical change in the process of making tofu from soybean? By Anonymous

A:

It should be said that from soybeans to tofu must have experienced chemical changes.

First of all, it should be emphasized that the classification method of dividing material change into physical change and chemical change in middle school physics is very rough, and we should focus on the process and mechanism of material change in the process of learning.

Back to the point, tofu is roughly divided into the following steps: first, sift the beans and soak them in water; then grind the soybeans into soy milk; dilute the ground soymilk with water and filter it; put it in the pan and boil it after filtering! Boil the soybean milk; finally, add coagulant to the cooked soybean milk, the traditional gypsum, brine, and now many tofu use gluconic acid-δ-lactone tofu.

In the above process, it is obvious that soaking, grinding and filtration are all typical physical changes. Heating is needed in the process of cooking soy milk, which will lead to protein denaturation and destroy the spatial structure of proteins. There is no doubt that this step is chemical change.

The key lies in the process of ordering tofu, which results in the accumulation of proteins. The original soybean milk is a kind of colloid, in which protein molecules are uniformly dispersed as colloidal particles, while colloidal particles gather and finally settle down. In this step, the protein is just gathered together, which should be regarded as a physical change.

Of course, I would like to briefly introduce the process of ordering tofu, and you can judge for yourself whether this is a physical change or not.

Gypsum and brine are essentially salt coagulants, and there is no unified explanation for their mechanism. We focus on lactone tofu. Gluconic acid-δ-lactone is an acid coagulant. During heating, the protein denatured, the internal hydrophobic region was exposed, and the glucose lactone released hydrogen ions during heating, neutralizing the negative charge of soybean protein to form neutral molecules. Hydrophobic interaction causes neutral protein molecules to gather together and finally settle into tofu.

I don't know if this is a physical change or a chemical change in your eyes.

References:

[1] Qiao Xiaoming. Tofu production technology [J]. Agricultural products processing, 2013 (11): 35.

[2] Li Meng. Study on the preparation of organic tofu with salt and polysaccharide [D]. Henan University of Technology, 2014.

By Frost White

Q4. Are the luminous metal elements added to the fireworks their elemental elements or their corresponding compounds? By Anonymous

A:

The luminous metal elements in fireworks include both elements and compounds, and the specific types are generally determined according to safety factors and price factors.

We know that the main component of fireworks is black gunpowder, which explodes through the reaction between sulfur, charcoal and potassium nitrate. In order to get colorful fireworks, we have to add different metal elements to them and use the flame color reaction to make the fireworks give off colorful colors. The flame color reactions of different metal elements are roughly shown in the following table:

To return to the question, the metal elements added to the fireworks usually include aluminum powder, iron powder, zinc powder, magnesium powder and antimony powder, while metal compounds usually have sodium salt, barium salt, strontium salt and so on, such as barium nitrate. It can be seen that the chemical properties of the metal elements are relatively stable, the reaction with air and water is slow or non-reaction, and the preparation is relatively simple, so these elements are usually directly added to the element. On the other hand, the elemental chemical properties of metal compounds are more active and react with air and water, which is not easy to be preserved and dangerous.

For example, the elemental substance of sodium is very active and reacts violently with water to form sodium hydroxide and hydrogen. The reaction equation is as follows:

Therefore, the corresponding metal element compounds are usually added to the fireworks to ensure safety and easy storage.

By single male youth

Q5. Can the pilot of a supersonic plane hear the sound of the plane's engine? I can't figure it out. The sound was thrown out and could not be heard. 2, the air inside the aircraft remains relatively still with the engine, and the sound can be transmitted. I can hear you. Which one is right? For an answer, thank you. By Anonymous

A:

Supersonic aircraft pilots can hear the sound of aircraft engines, but will not hear the sound of airborne aircraft engines. It is not sufficient to answer this question only by considering the speed of sound. Sound is essentially a kind of mechanical wave, and the aircraft engine is the source of the wave. If we set the speed of the plane to u and the speed of sound to v, then there will be three situations:

Comparison of wave source and wave velocity when the speed of the aircraft (wave source) is close to that of sound (wave), the compression wave in front of the aircraft will show a strong superposition. When the velocity of the wave source is higher than the wave velocity, the envelope of the wave surface is conical, which also has a good name: Mach cone. The sound of the plane can only be heard in the cone, while the pilots in front of the cone cannot receive sound waves, so they cannot hear the sound.

However, air is not the only way to spread sound. Sound travels at different speeds in different media.

Speed comparison consider this situation: the sound from an aircraft engine is transmitted to the pilot's ear through the fuselage of the plane (the material is alloy, as pictured above, and the sound travels at a speed of thousands of meters per second). Therefore, pilots flying at supersonic speeds can hear the sound of aircraft engines.

By justiu

Q6. Why do sometimes things that happen make people feel like they have been experienced, or dreamed of them in their dreams? By ques

A:

This phenomenon of "feeling that something has never happened before" is called Deja vu, which is generally translated as "fixed vision", "hippocampal effect", or simply "deja vu" in Chinese. Scientists have not reached a consensus on the cause of the perception. Brain science has several hypotheses about the causes of perception, including memory errors, dreams, different delays in processing information between the left and right sides of the brain, and so on. Let's briefly introduce the theory of memory errors.

The way the brain stores memory is to extract keywords from experiences and then store only those keywords, such as a sentence or an action. If there is a time when the experience matches these keywords, then these memories will be activated, and we will think, "I've done this before" and "I've seen this scene before." sometimes we really experience things in memory, sometimes it's just a memory mistake. The problem is when the experience matches the memory keyword.

The problem may be that the match is wrong. The area of the brain that stores memories has an effect on many memories, and sometimes it may not work properly, reading non-corresponding memories, such as linking the plot of a movie you have seen before to the current real experience, and using the Shing Mun Building as a hip shaft. It is also possible that the match is not accurate. The brain singled out memories that matched only 60% of the real experience, and the city gate building in Xi'an became the one in Nanjing. The third possibility is the loss of details. When the brain stores memories, it may lose some details over time, recording a continuous event into a number of discrete fragments. For example, you once turned left through the Heping Gate in Xi'an to the Forest of Steles. At first, your brain remembered the whole process, but after a few years, the only memory left was the process of passing through the city gate building, and the left turn and the visit were forgotten. Later, you turn right through the Peace Gate and see Xiamaling. At this time, your brain may remember the memory before, and then begin to doubt life: "I have walked through this city gate building. Have I seen Ma Ling before?"

The above is an explanation of the Deja vu phenomenon. At present, scientists' research on this phenomenon is still very superficial, there are many hypotheses, and the explanation is not perfect. Perhaps some of our readers will be able to give a convincing explanation in the future? )

Of course, this answer is aimed at "deja vu" as a normal physiological phenomenon. Please go to the Chinese Department next door (manual dog head) to deliberately say, "this sister I have seen before."

Reference materials:

[1] Why are there some scenes of deja vu, as if in a dream or in a previous life? What exactly is this feeling?

[1] perceived perception

By is a Tibetan fanatic.

Q7. Why does hot water freeze quickly? is it because it evaporates quickly? By 6

A:

Under certain conditions, hot water freezes faster than cold water, and this effect is called Mpamba effect (Mpemba effect). More strictly, the Mpamba effect means that water with slightly higher temperatures freezes faster than slightly cooler water at the same initial mass and the same cooling conditions. The Mpamba effect is unique in that it shows that the system is historically dependent (memory effect). If the cooling rate of water only depends on the current state, then hot water must reach the initial state of cold water after cooling for a period of time. And then continue to cool in the same form as cold water, so hot water must freeze more slowly than cold water.

There are many explanations for the Mpamba effect. A simple explanation is that evaporation consumes the mass of water, so when hot water reaches the initial temperature of cold water, its mass is smaller than that of cold water, and then it cools to the freezing point at a faster rate. If the container containing water is a container with a large open area, such as a surface dish, it does contribute to the Mpamba effect, but under normal experimental conditions, the evaporation effect is not enough to fully explain the Mpamba phenomenon. However, other explanations draw on similar ideas, such as the accumulation of frost on the surface of containers containing cold water, resulting in a decrease in thermal conductivity, so the cooling rate is significantly lower than that of hot water.

Another kind of explanation involves the undercooling of liquids. Liquid water placed in a smooth container is prone to undercooling. Although the temperature of the water is lower than the freezing point and the thermodynamics tends to solidify, it remains liquid due to the lack of condensation nodules. The condensation nuclei can be either impurities or non-uniform fluctuations (spontaneous nucleation) from the liquid itself. during the cooling process, the internal temperature gradient of hot water is larger, which disturbs the liquid, so it is easier to nucleate and crystallize. on the other hand, the internal convection and fluctuation during cold water cooling are weak and tend to be supercooled.

It can be said that, due to the simultaneous involvement of micro, mesoscopic and macro phenomena, there is no accepted explanation for the Mpamba effect, and even the reproducibility of the experiment is often questioned. However, the condensation shortcut, which is the core of the Mpamba effect, has been confirmed and applied in a wider range of scenarios and fields.

References:

[1] Markov Mpamba effect: Lu, Z., & Raz, O. (2017). Nonequilibrium thermodynamics of the Markovian Mpemba effect and its inverse

[2] quantum Mpamba effect: Carollo F, Lasanta A, Lesanovsky I. Exponentially accelerated approach to stationarity in Markovian open quantum systems through the Mpemba effect

By is happy in his heart.

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: Frions

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