Liquid, one of the three physical states in which matter can exist. The other two states are solid and gaseous. The molecules of liquids are arranged less tightly than those of solids but more closely than those of gases. Liquids and gases take the shape of their container, unlike solids, which keep their own shape. Liquids and solids maintain a definite volume, or size, while gases will expand to fill a container. For example, a liter of liquid will not expand to fill a two-liter container, but a liter of gas will.

Most substances can exist in the liquid

Most substances can exist in the liquid state at the right temperature and pressure. Water, for example, exists as a liquid at room temperature (20 C68 F) and normal atmospheric pressure (the pressure of the atmosphere at sea level). Helium, on the other hand, is a gas under these conditions. It must be cooled to a very low temperature or compressed to a very high pressure to become a liquid. Iron is a solid at room temperature and normal pressure and must be heated to 1535 C (2795 F) to become a liquid. Only three chemical elementsbromine, gallium, and mercuryexist as liquids at room temperature and normal pressure. All other elements exist as either solids or gases under these conditions. Many compounds (combinations of elements) exist as liquids. Alcohol, gas, oil, and water are examples of compounds that are liquids at room temperature and normal pressure. Many familiar liquids, such as juice, milk, and soda, are water based, meaning they contain substances mixed with or dissolved in water.


Liquids, particularly water, are essential to life. All plants and animals depend on water to transport nutrients and wastes. Liquids are also important in everyday activities, such as cleaning and painting. Liquids clean by dissolving and carrying away dirt and other solid particles. Paints contain colored particles in a liquid base. The liquid enables people to evenly spread color with a brush or roller on a wall or other surface. After the paint solidifies, it forms a coating on the wall or surface ( "see "Paint and Varnish). Many products, such as metal beams and plastic containers, are made by melting materials into liquid form and then pouring them into molds. When the material cools, it solidifies into the desired shape ( "see "Founding). Various fuels that people burn for heat and energy, such as oil to heat houses and gasoline to power automobile engines, are also liquids.

Structure of Liquids


Liquids, like all substances, are made up of atoms or bonded groups of atoms called molecules. The physical state of any substancewhether the substance is a liquid, solid, or gasdepends on the arrangement of the molecules in the substance. The molecules in a liquid are arranged tighter and more orderly than in a gas, but less orderly than in a solid. In crystalline solids, such as table salt or sugar, the molecules or other particles are stacked in a precise order, forming a three-dimensional pattern that repeats throughout the crystal. Scientists say these substances have long-range order. This means the atoms or molecules in the solid are ordered throughout the solid. In gases, the atoms or molecules have no order at all but move freely, allowing gases to expand and fill a container.

Liquids have no long-range order

Liquids have no long-range order, but their molecules have some order with respect to the nearest neighboring molecules. Every molecule in a liquid has the same number of nearest neighbors, and each of these neighboring molecules are roughly the same distance from one other. But the position of molecules that are further away from a particular molecule in a liquid becomes more and more random with increasing distance. Liquids, therefore, have short-range order, or an orderly pattern only at very small distances (a few molecule lengths) from a given molecule.

Certain liquids

Certain liquids, called liquid crystals, can have some long-range order, though not as much as a solid. These liquids contain long, rodlike molecules that, in certain temperature ranges, line up parallel to each other to create a long-range order. Unlike molecules in a solid, however, molecules in a liquid crystal retain the ability to slide past one another. The optical properties of some liquid crystals change with the arrangement of their molecules. For example, digital watches and calculators use liquid crystals that appear opaque when they are aligned in one direction and transparent when they are aligned in another direction.

Order in the molecules of any substance

Order in the molecules of any substance depends on the forces of attraction and repulsion between the molecules. These forces, called intermolecular forces, arise from electrical charges on molecules that attract or repel the charges on other molecules. Intermolecular forces account for many of the physical properties of a liquid, such as its boiling point, freezing point, and surface tension. They are also the reason a liquid can form from a gas or freeze into a solid.

A liquid forms from a gas when a drop in

A liquid forms from a gas when a drop in temperature or an increase in pressure causes the atoms or molecules of the gas to move closer together and acquire a short-range order. As the temperature of a gas drops, its molecules lose energy. The loss of energy causes the molecules to move more slowly, allowing them to come into closer contact with one another. An increase in the pressure of a gas brings molecules closer together by forcing them into a smaller space. Pressure can be increased by moving a given volume of gas from a large container into a smaller container, or by adding more gas to the original container. When the gas molecules move near enough to one another, intermolecular forces of attraction take over. These forces bring the molecules even closer together and into a short-range order, and a liquid forms. Likewise, when molecules in a liquid lose energy and move closer to one another, these same forces cause the liquid to turn into a solid. The molecules attract one another so much, they lock into place next to their neighbors and lose the ability to slide past each other. The molecules form a definite, unmoving pattern, which makes the solid rigid.

Molecules that are close together

Molecules that are close together, as in a liquid or solid, also exert a force of repulsion on each other. The outer electrons of each molecule repel the outer electrons of their neighboring molecules. This repulsion makes it difficult to force the molecules any closer to one another, which in turn makes it difficult to reduce the volume of a liquid or solid. This property makes brakes work in automobiles and trucks. The brake pedal is connected to a piston in a cylinder filled with brake fluid. The driver activates the brakes by pushing on the brake pedal, which in turn pushes the piston in the cylinder. The brake fluid in the cylinder cannot be compressed, so the piston moves the fluid from the cylinder through hollow steel tubes filled with fluid and into pistons connected to brake pads. The liquid pushes these pistons into the brake pads, making the pads rub against the rim of the wheels. This rubbing causes friction, slowing down the wheels.

Physical Properties

The physical properties of a liquid describe

The physical properties of a liquid describe how the liquid behaves under different temperatures and pressures and how it behaves when it comes into contact with other substances. Some liquids boil and evaporate at extremely low temperatures, while others boil at extremely high temperatures. For example, liquid helium boils at -269 C (-452 F), while liquid platinum boils at 3825 C (6917 F). Liquids also vary widely in freezing point and viscosity (a property that measures how a liquid flows). In addition, liquids behave differently when they come into contact with solids and when they mix with other liquids.

Boiling Point

The boiling point of a liquid is the temperature

The boiling point of a liquid is the temperature at which molecules escape from the liquid and enter the gaseous state. Heat causes a liquid to boil by adding energy to the liquid`s molecules. As the molecules gain energy, they move about more quickly and range farther from each other. When the molecules are far enough apart, intermolecular forces are too weak to pull them back together, so the molecules form a vapor. Boiling starts when bubbles of vapor form within the liquid. These bubbles rise to the top of the liquid and release the gaseous molecules to the atmosphere above the liquid`s surface. It takes 2,260 Joules (540 calories) of heat energy to evaporate 1 gram of water at 100 C (212 F) at sea level.

At the boiling point

At the boiling point, the vapor pressure of a liquid must equal the pressure of the atmosphere above the liquid. For a liquid boiling in an open container, the atmosphere above the liquid is simply Earth`s atmosphere. The pressure in the bubbles of vapor must equal the pressure of Earth`s atmosphere pressing down on the liquid. If this were not true, the air pressing down would squeeze and collapse the bubbles before they could form and rise to the surface. The boiling point of a liquid is lower at higher elevations because atmospheric pressure decreases as altitude increases. For example, the boiling point of water is 100 C (212 F) at sea level, where the air pressure measures one atmosphere (atm). On top of Mount Everest, which is 8,850 m (29,035 ft) above sea level, water boils at only 70 C (158 F) because the air pressure at this height is only 13 atm.

Different materials have different boiling

Different materials have different boiling points because the forces of attraction between their molecules differ. For example, water molecules strongly attract each other because of their structure. A water molecule consists of one oxygen atom and two hydrogen atoms. The oxygen atom attracts the electrons it shares with the hydrogen atoms more strongly than the two hydrogen atoms do. Electrons have a negative electric charge and thus make the oxygen end of the water molecule more negatively charged, while the hydrogen end of the molecule has a positive charge. This separation of charge makes the water molecule strongly polar. The negative charge on the oxygen atom attracts positive hydrogen atoms from other water molecules, causing the water molecules to bond tightly to each other. Breaking this bond requires considerable heat, which is why the boiling point for water, 100 C (212 F), is relatively high . Without this bonding, water would boil near -80 C (-112 F). Ethyl alcohol is also a polar liquid, and its boiling point is 78.5 C (173.3 F).

Nonpolar liquids have lower boiling points

Nonpolar liquids have lower boiling points than polar liquids because electric charge is evenly distributed around their molecules. This even distribution makes the molecule-to-molecule attractions in nonpolar liquids relatively weak. Examples of nonpolar liquids are the "hydrocarbons", substances that consist entirely of hydrogen and carbon molecules. Many common fuels, such as gasoline and methane, are hydrocarbons. In the molecules of these substances, the carbon and hydrogen atoms share their electrons more equally than do the hydrogen and oxygen atoms of water. As a result, the bonds between the molecules are relatively weak, and the liquids boil at lower temperatures. The hydrocarbon propane boils at 42.1 C (-43.8 F), and butane boils at 0.5 C (31.1 F). These substances exist as gases at room temperature.

Sometimes a liquid can be "superheated"that

Sometimes a liquid can be "superheated"that is, heated above its usual boiling point without changing into vapor. Superheating occurs when vapor bubbles inside a liquid don`t have an appropriate surface on which to form. For example, when water in a smooth-walled container is heated in a microwave oven, it can reach a higher temperature than its boiling point and remain a liquid. If a rough surface enters the liquid, such as a teabag, vapor bubbles can form and the liquid will begin to boil rapidly.

Liquid 1 | Liquid 2 | Liquid 3 |

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