The liquid state of matter is an intermediate phase between solid and gas. Like particles in a solid, particles in a liquid are subject to intermolecular attraction. However, liquid particles have more space between them, so they are not fixed in position.
The attraction between particles in a liquid keeps the volume of the liquid constant.
The movement of particles causes the liquid to be variable in shape. Liquids will flow and fill the lowest part of a container, taking on the shape of the container, but not changing in volume. The limited amount of space between the particles means that liquids have very limited compressibility.
Cohesion and adhesion
Cohesion it is the tendency for the same kind of particles to be attracted to each other. This cohesive “stick” explains the surface tension of a liquid. Surface tension can be considered a very thin “skin” of particles that are more strongly attracted to each other than to the surrounding particles.
As long as these forces of attraction are undisturbed, they can be surprisingly strong. For example, the surface tension of water is large enough to support the weight of an insect. Water is the most cohesive non-metallic liquid.
Cohesive forces are greatest under the surface of the liquid, where particles are attracted to each other from all sides. Particles on the surface are more strongly attracted to identical particles within the liquid than to the surrounding air.
This explains the tendency of liquids to form spheres, the shape with the least amount of surface area. When these liquid spheres are distorted by gravity, they form the classic raindrop shape.
THE accession is when there are forces of attraction between different types of particles. Particles in a liquid will not only be attracted to each other, but are generally attracted to the particles that make up the container that contains the liquid.
Liquid particles are drawn above the level of the liquid surface at the edges where they are in contact with the sides of the container.
The combination of cohesive and adhesive forces means that a slight concave curve, known as a meniscus, exists on the surface of most liquids. The most accurate measurement of the volume of a liquid in a graduated cylinder will be seen by looking at the volume marks closest to the bottom of this meniscus.
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Adhesion is also responsible for capillary action when a liquid is drawn into a very narrow tube. An example of capillary action is when someone takes a blood sample by touching a small glass tube to the drop of blood on the tip of a pricked finger.
Viscosity
Viscosity is a measure of how much a liquid resists free flowing. They say that a liquid that flows very slowly is more viscous than a liquid that flows easily and quickly. A substance with a low viscosity is considered thinner than a substance with a higher viscosity, which is generally considered to be thicker.
For example, honey is more viscous than water. Honey is thicker than water and flows more slowly. Viscosity can usually be reduced by heating the liquid. When heated, the liquid particles move faster, allowing the liquid to flow more easily.
Evaporation
Since particles in a liquid are in constant motion, they will collide with each other and with the sides of the container. Such collisions transfer energy from one particle to another. When enough energy is transferred to a particle on the liquid's surface, it will eventually overcome the surface tension that holds it to the rest of the liquid.
Evaporation occurs when surface particles gain enough kinetic energy to escape the system. As the faster particles escape, the remaining particles have lower average kinetic energy and the temperature of the liquid cools. This phenomenon is known as evaporative cooling.
Volatility
Volatility can be thought of as the probability that a substance is vaporized at normal temperatures. Volatility is a popular property of liquids, but some highly volatile solids can sublime at normal room temperature. Sublimation happens when a substance passes directly from the solid to the gas without going through the liquid state.
When a liquid evaporates inside a closed container, particles cannot escape from the system. Some of the evaporated particles will eventually come into contact with the remaining liquid and lose their energy to condense back into the liquid. When the evaporation rate and condensation rate are the same, there will be no net reduction in the amount of liquid.
The pressure exerted by the vapor/liquid balance in the closed container is called the vapor pressure. Increasing the temperature of the closed system will increase the vapor pressure. Substances with high vapor pressures can form a high concentration of gas particles above the liquid in a closed system.
This can present a fire hazard if the vapor is flammable. Any small spark, even if it occurs from the friction between the gas particles themselves, can be enough to cause a catastrophic fire or even an explosion.
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