What Are the Properties of Liquids?

Liquid is one of the three major material forms. It has no defined shape and is often affected by the container. But its volume is constant under the environment of constant pressure and temperature. The distance between liquid molecules is relatively long, the molecular movement is also violent, and the attraction between molecules is small. Temperature increase or pressure reduction can generally vaporize a liquid into a gas.

Liquid is one of the three major material forms. It has an indefinite shape and is often affected by the container. What shape the container is, the shape of the liquid when it is filled with liquid. But its volume is constant under the environment of constant pressure and temperature. In addition, the liquid exerts pressure on the sides of the container like other physical states. This pressure is transmitted in all directions, not only does not decrease and increases with the depth (this is why the deeper the water, the greater the water pressure).

Increasing the temperature or reducing the pressure can generally vaporize the liquid into a gas, such as heating water into water vapor. Pressurization or cooling can generally solidify a liquid into a solid, such as cooling water to ice. However, pressurization alone cannot liquefy all gases, such as oxygen, hydrogen, helium, and the like. ^[1]

All substances have three forms: solid, liquid and gaseous. When a substance is in a solid state, it has a certain volume and a certain shape. When a substance is in a liquid state, it has a certain volume without a certain shape. When a substance is in a gaseous state, it has neither a certain volume nor a certain shape. Different from the solid state, the distance between liquid molecules is far, the molecular motion is more violent, and the attractive force between the molecules is smaller. In fact, it has almost no resistance to shear and tensile forces, but can only resist its resistance to compression. power. This means that under pressure, the liquid can reach an out-of-balance state. Under the action of tensile force or shear force, the liquid is easily deformed, which makes the liquid show "flowability" similar to that of gas, which is not found in solids. Therefore, gases and liquids are collectively referred to as fluids. From a mechanical point of view, flowability is that no matter how small a tangential force (or tensile force) acts on a stationary liquid like a water stack, the original equilibrium state of the liquid is immediately destroyed, and it appears as a deformation movement. That is flowing. Therefore, the fluidity of a liquid is often specified as a property that the liquid cannot resist shear force (or tensile force) when it is in equilibrium. ^[2]

The volume of a liquid is constant under the environment of constant pressure and temperature. In addition, liquid pairs

At a certain temperature, the speed of the molecules in the solution and the energy they have are different. Those molecules with higher energy on the liquid surface can overcome the attraction between liquid molecules and escape from the liquid surface to become vapor molecules. This process is called evaporation. On the other hand, some of these gas molecules hit the surface of the liquid and are attracted back to the liquid. This process, which is opposite to the phenomenon of liquid evaporation, is called condensation.

At first, there are no gas molecules above the liquid, and the speed of condensation is zero; as more and more gas molecules are formed, the speed of condensation is getting faster and faster. When the condensation speed is equal to the liquid evaporation speed, that is, the liquid surface overflows in a unit time The number of molecules equals the number of molecules returned from the liquid to the liquid, and the dynamic equilibrium of evaporation and condensation is reached:

The vapor pressure of a liquid increases with increasing temperature. When the vapor pressure of a liquid is equal to the outside atmospheric pressure, a large number of air bubbles are generated inside the liquid and continue to escape, and the phenomenon of violent vaporization occurs simultaneously inside and on the surface of the liquid. This phenomenon is called boiling, and the temperature at this time is called the boiling point of the liquid. Take water as an example. If the water temperature reaches 100 ° C under one atmospheric pressure, the vapor pressure of water is exactly 1 atmosphere, and water starts to boil. 100 ° C is the boiling point of water at 1 atmosphere.

The boiling point of a liquid is related to the external pressure. When the pressure on a liquid increases, its boiling point increases; when the pressure decreases, the boiling point decreases. For example, the steam pressure in a steam boiler is about tens of atmospheres, and the boiling point of water in the boiler can be above 200 ° C. For another example, the atmospheric pressure on Mount Everest is 32kPa, and the water boils when heated to 71 ° C, but the rice is not easy to cook. This is because the atmospheric pressure decreases as the terrain rises, and the boiling point of water gradually decreases as the altitude rises. Therefore, when referring to the boiling point of a liquid, the external pressure conditions must also be indicated. It is customary to use the liquid boiling point at a pressure of 101.325 kPa as the normal boiling point.

In chemical production, the relationship between boiling point and external pressure is often used to deal with problems encountered in production. Vacuum distillation is often used to isolate and purify high-boiling compounds or compounds that are easily decomposed under normal pressure. For example, the pressure cooker for daily life uses the pressure of the liquid surface to raise the boiling point of the liquid, thereby raising the temperature in the pot and making the food easier to cook.