Introduction:

A liquid is a sample of matter which matches to the shape of a container in which it is held, and which attains a defined surface in the presence of gravity. The word liquid is as well employed in reference to the state or condition, of matter having this asset.

The liquid state of matter is an intermediate stage between the solid and gas. Similar to the particles of solid, particles in a liquid are subject to the intermolecular attraction; though, liquid particles have more space between them, therefore they are not fixed in position. The attraction between the particles in a liquid maintains the volume of the liquid constant.

The movement of particles causes the liquid to be variable in shape. The liquids will flow and fill the lowest part of a container, taking on the shape of the container however not changing in volume. The limited amount of space among the particles signifies that liquids have just very limited compressibility.

The molecules or atoms of matter in the liquid state are compressed as tightly as those of matter in the solid state; however the atoms or molecules in a liquid can move freely among one other. Illustrations of liquids are water at room temperature (around 20 ºC or 68 ºF), oil at room temperature and alcohol at room temperature.

Whenever a liquid is heated, the atoms or molecules gain kinetic energy. Whenever the temperature becomes adequately high, the liquid becomes a gas, or it might react by chemicals in the atmosphere. Water is an illustration of a liquid that becomes gaseous if it is heated steadily. Alcohol will combust (that is, combined with oxygen in the atmosphere) whenever heated all of a sudden and dramatically.

If a liquid is cooled, the molecules or atoms lose kinetic energy. If the temperature becomes low adequate, the liquid becomes a solid. Water is an excellent illustration. If cooled down, it freezes to the ice.

Comparison of liquids with gases and solids:

Structure of Liquid:

You can consider of a simple liquid like argon or methane as the collection of loosely-packed marbles that can suppose different shapes. However in general, the arrangement of individual molecular units is totally arbitrary, there is some amount of short-range order: the presence of one molecule at a given spot implies that the neighboring molecules should be at least as far away as the sum of the two radii, and this in turn influences the possible locations of more distant concentric shells of the molecules.

An essential result of the disordered arrangement of molecules in a liquid is the presence of void spaces. These, altogether by the increased kinetic energy of colliding molecules which assists push them apart, are responsible for around 15-percent decrease in density that is noticed whenever solids based on simple spherical molecules like Ne and Hg melt into liquids. Such void spaces are assumed to be the key to the flow properties of liquids; the more 'holes' there are in the liquid, the more simply the molecules can slip and slide over one other.

As the temperature rises, thermal motions of the molecules increase and the local structure starts to deteriorate as illustrated in the figure shown below.

Fig: Structure of liquid

There is an extremely little experimental information on the structure of liquids, other than the X-ray diffraction studies that yield plots like in the figure shown above. This is very difficult to design experiments which yield the type of information needed to define the microscopic arrangement of molecules in the liquid state.

The volume of a liquid, dissimilar to that of a gas, is hardly influenced at all via an increase in the pressure; we state that the compressibility (1/V)(dV/dP) is extremely small. The apparent interpretation of this fact is that the molecules of a liquid should be in 'direct contact' by one other, in such a way that any attempt to squeeze them even closer is strongly opposed via the repulsive forces among the electron clouds of the neighboring molecules. This capability of a liquid to transmit a force is the only basis of the hydraulically operated brake system in your car: the pressure of your foot on the brake pedal is immediately transmitted to the cylinders which function mechanical brakes on each of the wheels.

Structure of water:

Most of the liquids are comprised of molecules that apply specific and often highly directional forces on one other. The most notable illustration of such a liquid is water, in which a hydrogen atom of one H₂O molecule is attracted to the oxygen atom of the other molecule. This effect is termed as hydrogen bonding; hydrogen bonds are weaker than the ordinary chemical bonds, therefore in liquid water they are constantly being broken via thermal motions and then reformed in a slightly dissimilar configuration. Hydrogen bonding is mainly responsible for the so-called anomalous properties of water which make it an unusual and exceptional substance.

The most energetically positive configuration of H₂O molecules is one in which each and every molecule is hydrogen-bonded to three neighboring molecules. Owing to the thermal motions illustrated above, this ideal is never attained in the liquid, however whenever water freezes to ice, the molecules settle to exactly this type of an arrangement in the ice crystal. This arrangement need that the molecules be rather farther apart then would or else be the case; as an effect, ice, in which hydrogen bonding is at its maximum, consists of a more open structure, and therefore a lower density than water.

Whenever ice melts, the more energetic thermal motion disturbs much of the hydrogen-bonded structure, permitting the molecules to pack more closely. Water is therefore one of the very few substances whose solid form consists of a lower density than the liquid at the freezing point. Localized clusters of hydrogen bonds still remain, though; these are recurrently breaking and reforming as the thermal motions jiggle and push the individual molecules. As the temperature of the water is increased above freezing, the extent and lifetimes of such clusters reduce, therefore the density of the water rises.

At higher temperatures, the other effect, general to all substances, starts to dominate: as the temperature rises, so does the amplitude of thermal motion. This more energetic jostling causes the average distance among the molecules to increase, decreasing the density of the liquid; this is ordinary thermal expansion.

As the two competing effects (that is, hydrogen bonding at low temperatures and thermal expansion at higher temperatures) both lead to a reduction in density, it follows that there should be some temperature at which the density of water passes via a maximum. This temperature is 4 ?C; this is the temperature of water we will discover at the bottom of an ice-covered lake in which this most dense of all water has moved the colder water and pushed it closer to the surface.

Properties of Liquids:

The properties of liquids can be illustrated in terms of molecular arrangement and intermolecular forces. Let us observe some properties of liquids as illustrations.

1) A liquid consists of a definite volume.

Reason: The intermolecular force of attraction is mere strong adequate to lock up the molecules in a definite space.

2) A liquid consists of no definite shape and attains the shape of the container. This can flow from a higher lever to a lower level.

Reason: The intermolecular force of attraction is weaker in a liquid as compared to solid. Liquid molecules can move, slip and slide over one other as their molecular separation is bigger. The liquid attains the shape of the container.

Reason: Intermolecular force of attraction is weaker however molecular speed is higher in a liquid than in a solid. The effect of attraction limits the molecules while the effect of speed makes the molecules fly separately. The joint effect of these two factors is such that a liquid consists of a surface that based on the shape of the container.

3) Liquid is compressible.

Reason: The distance between the nearest molecules is bigger in a liquid than in solid.

4) A liquid can diffuse to other liquid; however this is much slower as compared to the diffusion of gases.

Reason: The molecules move faster in a liquid than in a solid however slower as compared to the molecules of a gas.

5) Liquid on heating converts into gaseous state.

Reason: Heating raises the intermolecular separation of the liquid molecules however reduces their intermolecular force of attraction. On cooling, vapors lose heat and are transformed or changed to liquid.

Cohesion and adhesion:

Cohesion is the tendency for similar type of particles to be attracted to one other. This cohesive stickiness accounts for the surface tension of the liquid. Surface tension can be assumed as a very thin skin of particles which are more strongly attracted to one other than they are to the particles surrounding them. As long as such forces of attraction are uninterrupted, they can be surprisingly strong. For illustration: the surface tension of water is great adequate to support the weight of the insect like the water skipper. Water is the most cohesive non-metallic liquid.

Cohesive forces are best below the surface of the liquid, where the particles are attracted to one other on all sides. Particles at the surface are more strongly attracted to the similar particles in the liquid than they are to the surrounding air. This accounts for the tendency of liquids to form spheres, the shape having the least amount of surface area. Whenever these liquid spheres are distorted via gravity, they make the classic raindrop shape.

Adhesion is whenever forces of attraction exist among different kinds of particles. Particles of a liquid will not only be attracted to one other, however they are usually attracted to the particles which make up the container holding the liquid. The particles of liquid are drawn up above the surface level of the liquid at the edges where they are in contact by the sides of the container.

The grouping of cohesive and adhesive forces implies that a slight concave curve, termed as the meniscus, exists at the surface of most of the liquids. The most precise measurement of the volume of a liquid in a graduated cylinder will be noticed by looking at the volume marks closest to the bottom of the meniscus.

Adhesion as well accounts for capillary action if a liquid is drawn up to a very narrow tube. One illustration of capillary action is when anyone collects a sample of blood by touching a small glass tube to the blood droplet on the tip of the pricked finger.

Viscosity:

Viscosity is the measure of how much a liquid opposes flowing freely. A liquid which flows very slowly is stated to be more viscous than the liquid which flows simply and quickly. A substance having low viscosity is taken to be thinner than a substance having higher viscosity that is generally thought of as being thicker. For illustration, honey is more viscous than water. Honey is thicker than water and flows more slowly. Viscosity can generally be decreased via heating the liquid. Whenever heated, the particles of the liquid move faster let the liquid to flow more simply.

Evaporation:

As the particles of a liquid are in steady motion, they will collide with one other, and by the sides of the container. These collisions transfer energy from one particle to the other. When adequate energy is transferred to a particle at the surface of the liquid, it will ultimately overcome the surface tension holding it to the rest of the liquid. Evaporation takes place whenever surface particles gain adequate kinetic energy to escape the system. As the faster particles escape, the remaining particles encompass lower average kinetic energy and the temperature of the liquid cools. This phenomenon is termed as the evaporative cooling.

Volatility:

The Volatility can be assumed of as how likely a substance will be to vaporize at normal temperatures. The volatility is more frequently a property of liquids; however some highly volatile solids might sublime at normal room temperature. Sublimation occurs whenever a substance passes directly from solid to gas devoid of passing via the liquid state.

Whenever a liquid evaporates within a closed container, the particles can't escape the system. Several evaporated particles will ultimately come into contact by the remaining liquid and lose adequate of their energy to condense back into the liquid. Whenever the rate of evaporation and the rate of condensation are similar, there will be no total decrease in the amount of liquid.

The pressure applied via the vapor/liquid equilibrium in the closed container is termed as the vapor pressure. Rising the temperature of the closed system will raise the vapor pressure. The substances having high vapor pressures can make a high concentration of gas particles above the liquid in the closed system. This can be a fire risk if the vapor is flammable.

Surface Tension:

There is a force of attraction between the molecules in liquids, and liquids can flow till they take on the shape which maximizes this force of attraction. Beneath the surface of the liquid, the force of cohesion (exactly, 'sticking together') between the molecules is similar in all directions. Molecules on the surface of the liquid, though, feel a total force of attraction that pulls them back to the body of the liquid. As an outcome, the liquid tries to take on the shape that consists of the smallest possible surface area -- the shape of a sphere. The magnitude of the force which controls the shape of the liquid is termed as the surface tension. The stronger the bond between the molecules in the liquid, the bigger is the surface tension.

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