What happens to the boiling and melting points of water when salt is added?

The melting point (which should be the freezing point to be precise) decreases and the boiling point increases. The principle is the dependence of the dilute solution: the vapor pressure of the solvent in the dilute solution decreases, the freezing point decreases, the boiling point increases, and the value of osmotic pressure is only related to the number of particles of the solute in a certain amount of solution and has nothing to do with the properties of the solute itself.

It is only suitable for ideal dilute solutions, and is approximately applicable to dilute solutions. Adding salt to water can be approximated as an ideal dilute solution. Here's how it works:

An increase in the boiling point.

The temperature at which the vapor pressure of a liquid is equal to the outside pressure is called the boiling point of that liquid. The boiling point when the external pressure is is called the normal boiling pointFigure 2-11 shows the curve aa'and bb'The vapor pressure of the pure solvent and the solution as a function of temperature are described, respectively.

At the same temperature, the vapor pressure of the solution is lower than that of the pure solvent, so the line bb'**aa'Under. When the external pressure is p, the boiling point of the solution is tb, while the boiling point of the pure solvent is tb*, obviously, tb>tb*, that is, the boiling point elevation value δtb=(tb-tb*)Raoult obtained the following relation based on the experimental results:

tb=kbbb

where kb is the boiling point rise constant of the solvent and bb is the mass molar concentration of the solute.

Freezing point is reduced.

One. Definition of the freezing point of a solution.

The freezing point (also known as freezing point) of a solution is the temperature at which a solid pure solvent and a liquid solution are equilibrate. At this time, the vapor pressure of the solid pure solvent is equal to the vapor pressure of the solvent in the solution.

Two. The reason for the decrease in the freezing point of the solution.

Because the freezing point is the temperature at which the solid pure solvent and the liquid solution are in equilibrium, and according to Raoult's law, the vapor pressure of the solvent in the solution is less than the vapor pressure of the pure solvent. Therefore, the temperature of the solution and the solid pure solvent in equilibrium will be lower than the freezing point of the pure solvent, which is why the freezing point of the solution is reduced.

Three. The formula for calculating the freezing point reduction.

According to Raoult's law, the freezing point of a non-electrolyte solution with a refractory non-electrolyte decreases tf and is proportional to the molar concentration bb of the mass of the solute. Namely.

tf=(tf*-tf)=kfbb

where kf is the freezing point reduction constant of the solvent.

The boiling point rises and the melting point decreases when salt is added to water.

Water molecules in a liquid state tend to attach to each other to form water droplets, ponds, or oceans. However, water molecules with high energy can overcome this force that binds them together with other water molecules and break away from the surface of the liquid to become water vapor. At any given time, some water molecules detach from the surface of the water, a phenomenon known as "evaporation".

As the water temperature increases, so does the number of water molecules that have enough energy to leave the water's surface. Water boils when the pressure created by the water molecules detached from the surface of the liquid exceeds the pressure of the surrounding air. The water usually begins to boil after heating the water to increase its energy and bring it to a temperature of 100 degrees Celsius.

However, when you add salt to the water, the problem becomes more complicated. Sodium chloride (refined table salt) dissolves easily in water, decomposes into sodium and chloride ions, and diffuses inside the liquid. Now, if a water molecule wants to become water vapor, it needs to get rid of the shackles of sodium and chloride ions while being attracted by other water molecules.

Chemical equilibrium refers to the state in which the positive and negative reaction rates of chemical reactions are equal in reversible reactions under certain macroscopic conditions, and the concentrations of reactants and products do not change. It can be judged by δrgm = 0, A is the chemical formula of substance A in the reaction. According to Le Chatre's principle, if a system that has reached equilibrium is changed, the system will change accordingly to counter the change.

The rate of the positive reaction is equal to the rate of the reverse reaction, and the concentration of the reactants and the concentration of the products do not change anymore, reaching a state of surface quiescence, which is called a state of chemical equilibrium.

The four chemical equilibrium are redox equilibrium, precipitation-dissolution equilibrium, coordination equilibrium, and acid-base equilibrium. Chemical equilibrium has an extremely important application in analytical chemistry.

Extended information: Characteristics of analytical chemistry.

1.The concept of "quantity" is highlighted in analytical chemistry.

For example, the measured data should not be arbitrarily selected; The accuracy of the data and the size of the deviation are related to the analysis method used.

2.Analyzing a specimen is a process of obtaining information and reducing the uncertainty of the system.

3.Experimental.

Emphasis on hands-on ability, cultivate the skills of experimental manipulation, and improve the ability to analyze and solve practical problems.

4.Comprehensive.

It involves chemistry, biology, electricity, optics, computers, etc., reflecting the ability and quality of Lahu.

Analytical chemists should have a strong sense of responsibility.

When a small amount of salt is added to the water, the freezing point decreases and the boiling point increases.

After adding a small amount of salt to the water, the freezing point is reduced, which is explained by chemical knowledge because the solidification of water is the combination of hydrogen bonds, and the mixing of other substances will generally destroy the bond between hydrogen bonds and fibers, making it more difficult for water to solidify, so the freezing point is generally reduced. Salt is also an ionic compound, and its destructive power to hydrogen bonds is relatively strong.

The physical explanation is that there are salt ions in the water, so a part of the water on the surface is taken up by salt. This results in a drop in the vapor pressure of the water. Whereas, the vapor pressure of ice is constant.

The vapor pressure of the ice at the freezing point is less than or equal to the vapor pressure of the water, so after adding salt, the vapor pressure of the water becomes smaller, and the vapor pressure that is equal to the original becomes the ice larger. At this time, because the vapor pressure of the ice drops rapidly, it is necessary to lower the temperature so that the vapor pressure of the ice is equal to the vapor pressure of the ice. This results in a lower freezing point.

The boiling point increases after adding a small amount of salt to the water, because the salt added to the water will ionize into anions and cations, and the anions will attract to the positively charged part of the water molecule, and the cation and the negatively charged part will be attracted, thus increasing the intermolecular force in the liquid water, resulting in an increase in the energy required for gasification.

When a small amount of salt is added to the water, the freezing point decreases and the boiling point rises. This is the dependence of the dilute solution.

Dilute solutions have common property laws, such as the vapor pressure of the solution drops, the boiling point increases, the freezing point decreases and the osmotic pressure increases, etc., they have nothing to do with the nature of the early macromass of the dissolved light, and are determined by the number of solute particles, so it is called the dependence of the dilute solution.

The change in the properties of the dilute solution is directly proportional to the number of solute particles.

The boiling point of brine varies with the concentration of salt, when the salinity increases by 1%, the boiling point temperature of brine increases, and the boiling point of brine with 10% concentration is about. At standard atmospheric pressure, the boiling point of brine is always higher than that of water, and sodium chloride is easily soluble in water and decomposes into sodium ions and chloride ions at the same time, and diffuses inside the liquid. If a water molecule is to become water vapor, it needs to be free from the shackles of sodium and chloride ions while being attracted by other water molecules.

The boiling point of brine varies with the concentration of salt, when the salinity increases by 1%, the boiling point temperature of brine increases, and the boiling point of brine with 10% concentration is about. At standard atmospheric pressure, the boiling point of brine is always higher than that of water, and sodium chloride is easily soluble in water and decomposes into sodium ions and chloride ions at the same time, and diffuses inside the liquid. If a water molecule is to become water vapor, it needs to be free from the shackles of sodium and chloride ions while being attracted by other water molecules.