What are the differences between a solid and a liquid and a gas?

Answer 1

Lets just use water for an example. At temperatures below 0o Celsius, water is a solid (ice). The molecules of ice are tightly packed together and are not allowed to move much, just vibrating in place. Also ice is less dense than water because the molecular structure of ice is more open than liquid water, that is why ice floats! At temperatures above 0o Celsius and below 100o Celsius, water is a liquid. Which means that the polar water molecules are allowed to move somewhat freely. More free than a solid, but less free as a gas. At temperatures above 100o Celsius, water is a gas, where water molecules are allowed to move independent of one another. Just Google the laws of gases. Phases of matter are ultimately dependent on molecular structure and how fast the molecules are moving. Temperature is the average kinetic energy in a system.

Answer 2

There are a couple of notable differences between the three common states of matter, solid, liquid and gas.

The first is the distances between the atoms or molecules which will be called "particles" here for simplicity. The particles are typically far apart in a gas, many times further apart than the typical size of a particle. In a liquid and solid, particles are close with separations that are an angstrom, smaller distances than the size of a typical particle. One would call these condensed phases crowded.

The second observation one can makes is that the forces between particles dominated the structure and behavior of the solid and liquid phases, but has little or no influence on the gas phase. This is because the forces between particles are typically only a few angstroms or a few tens of angstroms and so the gas particles are just too far apart to experience the forces very often as the move around.

The third observation is the gas particles move around quite freely compared to the others. All particles in all materials have a speed that is determined by temperature. (That is part of the thermal energy in any material.) In a gas, the distances between the particles may be many times the size of the particle and so as the particles go bouncing around, they interact significantly with one another relatively infrequently. In a liquid or solid, they distance between two neighboring particles is usually about the same as the size of the particles. They are never outside the range of force of neighboring particles. In the solid and liquid phase there are quite a few neighboring particles.

In the liquid and solid phases particles remain close together, but in the liquid phase the particles don't remain long near any one set of neighbors and the path that a particle in the liquid phase takes is quite random, controlled by collisions with its neighbor and constantly changing direction. In the solid phase, particles rarely move away from the neighbors. Forces between particles restrict them greatly in a solid which is why a solid will retain its shape when a stress is applied and a liquid will not.

Traditionally, matter is classified as being in a solid, liquid or gas state. We also say that matter exists in three "phases," the solid, liquid and gas phases. (There are other, more exotic phases, such as the plasma phase and liquid crystalline phases and there is also the glass state of matter, but those are either either exotic or more technical refinements of the others.)

The last observation that may be important here is to observe that the three phases occur in a standard order typically, solid being te lowest temperature phase, liquids at a higher temperature and gasses at the highest. If the phase is induced by pressure, gas is the lowest pressure and solid are created at the highest.

There is a great deal more than can be said, especially about exceptions to the rules and exotic forms of matter as well as discussing why the forces between particles determine when they enter which phase.

Answer 3

Solids have a fixed shape and volume, with particles tightly packed together. Liquids have a fixed volume but take the shape of their container, with particles that are close together but can move past each other. Gases have neither a fixed shape nor volume, with particles that are far apart and move freely.