Solids, Liquids, and Gases Chapter 14
Matter & Thermal Energy Matter can exist as a solid, a liquid, a gas or a plasma. The Molecular Kinetic Theory of Matter explains their differences and how they can be converted from one form (state) to another. Kinetic energy is the energy of moving objects; the faster they are moving, the more kinetic energy they have.
Kinetic Theory Molecular Kinetic Theory of Matter has four assumptions. All matter is made of tiny particles (atoms, molecules, and ions). These particles are in constant random motion. The particles collide with each other and the sides of the any container in which they are held. The amount of energy that is lost in collisions is negligible.
Gas State Because the particles that make up a gas are moving at high speeds, and in random directions, gases have an indefinite shape and an indefinite volume. The particles spread out to fill whatever container they occupy. Examples: water vapor, air, helium.
Liquid State The particles of a liquid are also moving, but not as fast, so they have less kinetic energy. Because the particles have less kinetic energy, they are less able to overcome the their attractions for each other. The particles are able to slide past each other. Liquids have a definite volume, but an indefinite shape. Examples: water, gasoline, isopropyl alcohol.
Solid State Solids have a definite volume, and a definite shape. The particles are held in position by attractive forces, but they still vibrate in place. Particles in a solid have the least kinetic energy. Many solids are crystalline; their particles form specific geometric arrangements (example: NaCl). Examples: table salt (NaCl), ice, wood, steel.
Thermal Energy Thermal energy is the total energy of a material s particles. The includes both Kinetic energy, and Potential energy (energy from forces that act within or between particles.
Temperature Temperature is a term used to describe how hot or cold an object is. Temperature is the measure of the average kinetic energy of the particles that make up the substance. Three temperature scales are common; Fahrenheit, Celsius, and Kelvin.
Changes of State As you add heat to a substance, the solid typically changes into liquid, and with more heat, the liquid changes into a gas. if you remove heat from a gas, it becomes a liquid, and as more heat is removed it becomes a solid.
Melting and Freezing Melting is the process where a solid becomes a liquid Temperature at which a solid becomes a liquid is its melting point. Heat needed to convert 1 gram of solid into a liquid (or from liquid to solid) is the heat of fusion. Freezing is process where a liquid becomes a solid. It is the reverse of melting. Temperature at which a liquid becomes a solid is its freezing point.
Vaporization & Condensation Vaporization occurs when the liquid particles move fast enough to escape the attractive forces of the other particles. This is the conversion of a liquid into a gas. Condensation is the reverse of vaporization. In condensation the gas becomes a liquid.
Vaporization Vaporization can occur two ways, evaporation and boiling. Evaporation occurs at the surface, and almost at any temperature. The particles at the surface gain enough energy to break the attractive forces of the other particles
Vaporization & Boiling Boiling occurs throughout the liquid at a temperature determined by the pressure on the liquid. Boiling point is the temperature at which the pressure of the vapor in the liquid equals the pressure on the surface. Boiling point at standard pressure (760 mm of Hg) is 100º Celsius. Heat of vaporization is the amount of heat required to convert 1 gram of water at the boiling point into steam.
Sublimation At certain pressures, some substances convert directly from a solid into a gas. Example: Solid carbon dioxide (dry ice) sublimates and becomes gaseous carbon dioxide
Heating Curve A graph of temperature vs. time for a certain substance. It typically shows plateaus where the solid becomes a liquid, and where the liquid becomes a gas.
Heating Curve (Continued)
Plasma State Atoms in the plasma state have so much energy that their electrons are stripped off of them. Plasma is present in stars (including our sun), fluorescent lights, plasma cutters, etc. Most ordinary of matter in the universe is plasma.
Thermal Expansion Thermal expansion is the increase of the volume of a substance when the temperature is increased. Thermal expansion and contraction is why bridges are build with expansion joints. Thermal expansion is the reason that thermometers work. The particles move faster, spread apart, and occupy more volume.
Thermal Expansion & Hot Air Balloons Heating the air in the balloon causes the air to expand, and since the air in the balloon has the a smaller density, it rises in the more dense atmosphere.
Water s Strange Behavior Water has its greatest density at 4º C. It contracts until it hits that temperature. Below 4º C its density increases; it expands. This is caused by the fact that water molecules have areas of strong positive and negative charges, and below 4ºC they line up, and this leaves empty spaces in the structure, so the water (and eventually ice) expands. This explains why ice floats, and without that aquatic life in northern lakes and rivers would be killed off.
Amorphous Solids Most solids melt at a specific temperature (which is related to the pressure on the solid). For example, ice melts at 0 º C at 760 mm of Hg pressure. Some solids, such as butter and glass, melt over a range of temperatures. These solids lack a crystalline structure, and are called amorphous solids.
Liquid Crystals When heated, liquid crystals start to flow, but they don t lose the ordered arrangement. They are highly responsive to temperature changes and electrical charges. They are used to make liquid crystal displays (LCDs) for cell phones, calculators, and notebooks.