Why cooling a hot saturated solution gives crystals 15

The general solution has a saturated solubility problem, usually the saturated solubility changes with the temperature, the temperature decreases and the saturated solubility also decreases, then the saturated solution with cooling heat, due to the decrease in temperature, its saturated solubility decreases, and the solution becomes a supersaturated solution, from the thermodynamic point of view it is an unstable state, then the excess solute will be precipitated in the form of crystals to maintain thermodynamic equilibrium, that is, the solution is still maintained in a saturated state (low solubility).

The solubility of most solutes increases with increasing temperature and conversely, as the temperature decreases, the solubility of most solutes decreases. For example, at 60 degrees, 100 grams of a solute can be dissolved; At 40 degrees, only 70 grams of this solute can be dissolved in the same amount of solvent.

Therefore, when the temperature decreases, the solubility of the solute decreases, and so much solute cannot be dissolved in the same amount of water, so the crystals will be precipitated. However, there are some exceptions, such as calcium hydroxide.

Hehe, to put it simply, a saturated solution is a solution that can no longer dissolve solids, when the temperature is reduced, the saturated concentration of the general solution will be reduced, and the excess solute will definitely precipitate in solid form, otherwise there will be no place to go.

The solution is concentrated by heating (reducing the solvent), and then the solution is cooled to crystallize the substances in the solution.

Cooling hot saturated solution method: suitable for solid solutes whose solubility is greatly affected by temperature.

Let's compare it like this:

There are ten bowls (solvent), all filled with rice (solute), now five bowls are added (heating), and then filled with rice (saturated after heating), the bowl is broken (cooled), and the rest of the rice is sprinkled on the table (crystals are released). Hehe.

The solution after the precipitation of the crystals is not necessarily a saturated solution. If there are crystals in the solution and the crystals and the solution coexist in equilibrium, then the solution must be a saturated solution of the substance, and the precipitation crystals are the excess solutes that exceed the current environment, and the solubility of most compounds in the solvent at different temperatures.

There are changes. <>

Generalization of saturated solutionsHomogeneity, the density, composition and properties of the solution are exactly the same, stability, temperature is unchanged, solute, solvent amount is unchanged, solute and solvent will not be separated for a long time, and the mixture, the solution must be a mixture. The rate of dissolution of solutes in a saturated solution is equal to the rate of precipitation of solutes.

A solution that dissolves a certain solute in the cavity at a certain temperature exceeds the saturation limit of this solute is called a supersaturated solution of this solute. The supersaturated solution is unstable, and if the solution is stirred, the solution is shaken, the wall of the container is rubbed, or solid seeds are injected into the solution, the excess solute in the solution will immediately crystallize and precipitate.

Because the solvent is unchanging. However, most of the dissolution mold difficulty s increases with the increase of temperature t. Therefore, the saturated solution with a higher temperature is cooled, according to S 100g = m agent m.

S becomes smaller, M agent does not change, so the most dissolved M substance is reduced. So when it can no longer be dissolved, too many crystals will be precipitated.

The crystals precipitated by cooling of saturated solution must be --- saturated solution, otherwise there will be no crystal precipitation.

--- the saturated solution cools down, crystals will be precipitated from the solution, at this time, the mass of the solute in the solution decreases, the mass of the solution decreases, the mass of the solvent remains unchanged, and the mass fraction of the solute in the solution also decreases, but it is still a saturated solution. (This is for substances whose solubility increases with the increase in temperature, and the solubility increases).

What happens when an unsaturated solution heats up? The unsaturated solution will be more unsaturated when the temperature rises, and there will be no crystal precipitation, so the mass of the solution, the quality of the solute, the quality of the solvent, and the mass fraction of dissolved bright matter will not change, but the solubility increases, so the solution is still an unsaturated solution.

The difference between cooling hot saturated solution and cooling crystallization is that the starting point of cooling is different.

The difference between cooling hot saturated solution and cooling crystallization is that the starting point of cooling is different. "Cooling hot saturated solution crystallization method" is referred to as "cooling hot saturated solution method", also known as "cooling heat saturated solution method".

Cooling hot saturated solution is based on the different solubility of substances at different temperatures (generally high solubility at high temperature, low solubility at low temperature. and a method of separating or purifying solid substances. This method mainly uses the annihilation of substances whose solubility decreases significantly with the decrease of temperature.

First evaporation, concentration, and then cooling to make S change greatly with T (such as potassium nitrate) Evaporation and concentration, cooling crystallization, filtration, washing, and drying. The method is to make the solution saturated at a higher temperature, so that after the temperature is reduced, because the solubility of the substance decreases, the crystals of the substance will be precipitated in the solution.

It is a method of crystallizing saturated solutions. In essence, it is to precipitate the trillions of solutes from the solution in the form of crystals by lowering the temperature (suitable for substances whose solubility increases significantly with the increase of temperature).

Generally, it refers to the process of lowering the temperature of a solution saturated at a relatively high temperature to precipitate the crystal.

The specific measure of recrystallization is to cool the hot-saturated solution, but cooling the hot-saturated solution is not necessarily for the purpose of recrystallization.

Recrystallization is the cooling of a hot-saturated solution multiple times.

The method of recrystallization is to cool the hot saturated solution.

Good morning, the solution that can precipitate crystals is generally saturated solution, and the precipitation crystals are the excess solutes that exceed the current environment, and the solubility of most compounds in the solvent at different temperatures changes, for example, the heated sodium chloride solution will precipitate more than the remaining parts of the current low temperature maximum solubility after cooling, please refer to it.

The temperature is constant, the solution with precipitated crystals is a saturated solution, and the crystals are the crystals of solutes, and the reason why there is crystal precipitation is that the solvent can no longer dissolve the solute, and the solution is saturated.

a The temperature has changed, and the solubility has also changed.

b The crystal does not contain water, of course, the solvent quality does not change, b pair.

c After precipitation of crystals, the solute decreases and c is wrong.

d After the precipitation of the crystal, the solute decreases, while the solvent does not change, and the mass fraction decreases

b Correct.

a Solubility varies with temperature.

c Solute precipitation, of course, the mass in the solution changes.

d As the mass of the solute changes, so does the mass fraction.

The solution should be completely evaporated for evaporation crystallization, and the cooling hot saturated solution is not required; Evaporation crystallization generally requires an evaporation dish, but cooling hot saturated solutions does not need to.

Evaporation crystallization: the solubility remains unchanged, the solvent is reduced, and the solute is precipitated.

Cooling heat saturation: with the decrease of temperature, the solubility decreases, and the solute precipitates.

This depends on whether you want to precipitate higher solubility from low solubility or vice versa