Because the molecules of a liquid are in constant motion and possess a wide range of kinetic energies, at any moment some fraction of them has enough energy to escape from the surface of the liquid to enter the gas or vapor phase. This process, called vaporization or evaporation, generates a vapor pressure above the liquid.
Molecules in the gas phase can collide with the liquid surface and reenter the liquid via condensation. Eventually, a steady state or dynamic equilibrium is reached. The states of matter exhibited by a substance under different temperatures and pressures can be summarized graphically in a phase diagram, which is a plot of pressure versus temperature. Phase diagrams contain discrete regions corresponding to the solid, liquid, and gas phases.
The solid and liquid regions are separated by the melting curve of the substance, and the liquid and gas regions are separated by its vapor pressure curve, which ends at the critical point.
A crystalline solid can be represented by its unit cell, which is the smallest identical unit that when stacked together produces the characteristic three-dimensional structure. Solids are characterized by an extended three-dimensional arrangement of atoms, ions, or molecules in which the components are generally locked into their positions.
The components can be arranged in a regular repeating three-dimensional array. The smallest repeating unit of a crystal lattice is the unit cell.
The major types of solids are ionic, molecular, covalent, and metallic. Students may believe that gas particles are moving slowly in ways similar to what they observe when they see suspended dust particles in a beam of light.
Random particle motion in liquids and gases is a difficult concept for students to appreciate. Students stated that particles were forced apart by heat acting as a substance when gases were heated.
When gases condensed to a liquid, many students attributed this to increased attractive forces between particles. Students frequently find it difficult to appreciate particle movement in solids and this leads to different conceptions about freezing and melting. When they have broken away from each other, they turn from a crystal form to a solution form.
Student 2: "When a block of ice is taken out of a freezer, the sudden change of temperature reacts on the particles making them decrease in size. Atoms are incredibly small and cannot be seen with even the most powerful light microscope. We use multiple models of atoms to help explain chemical processes and describe their behaviour.
In gases the particles move rapidly in all directions, frequently colliding with each other and the side of the container. With an increase in temperature, the particles gain kinetic energy and move faster. The actual average speed of the particles depends on their mass as well as the temperature — heavier particles move more slowly than lighter ones at the same temperature. The oxygen and nitrogen molecules in air at normal room temperature are moving rapidly at between to metres per second.
Unlike collisions between macroscopic objects, collisions between particles are perfectly elastic with no loss of kinetic energy. This is very different to most other collisions where some kinetic energy is transformed into other forms such as heat and sound. It is the perfectly elastic nature of the collisions that enables the gas particles to continue rebounding after each collision with no loss of speed.
Particles are still subject to gravity and hit the bottom of a container with greater force than the top, thus giving gases weight. If the vertical motion of gas molecules did not slow under gravity, the atmosphere would have long since escaped from the Earth. In liquids, particles are quite close together and move with random motion throughout the container. Particles move rapidly in all directions but collide with each other more frequently than in gases due to shorter distances between particles.
The particles in a liquid can:. The attractive forces bonds in a liquid are strong enough to keep the particles close together, but weak enough to let them move around each other.
The table shows some of the properties of liquids and why they are like this:. The particles in a liquid usually are still touching but there are some spaces between them. The gas particles have big distances between them. Solid — In a solid, the attractive forces keep the particles together tightly enough so that the particles do not move past each other. Their vibration is related to their kinetic energy. In the solid the particles vibrate in place.
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