Changes in State Chapter 2 Section 2

Energy How does the ice go from being a solid back to being a liquid? Energy is the ability to do work Energy of motion is called kinetic energy The molecules in particles of matter are in constant motion Particles with more kinetic energy move faster and further apart.

Potential Energy An object can have potential energy depending on it position. A rock at the top of a cliff has potential energy As it falls its kinetic energy rises and its potential energy lowers Energy cannot be created or destroyed. It can change from one form into another.

Thermodynamics and Conservation of Energy Thermodynamics is a branch of physics that describes temperatures and their relation to energy. It also teaches us about macroscopic variables such as pressure internal energy and entropy. There is an increasing amount of chaotic energy in the universe, this is known as entropy. The law of conservation of energy tells us that energy cannot be created or destroyed but it can change from one form into another.

Thermal Energy The total kinetic and potential energy of all the particles in a sample of matter is called thermal energy. If the number of particles or the energy within those particles changes then the thermal energy of a substance will change. With identical sized samples the warmer sample will have the most energy

Which has more thermal energy?

Temperature Not all particles in a substance have the same energy The average energy of all the particles is the temperature. Temperature is different to thermal energy as thermal energy is a total and temperature is an average.

The average kinetic energy of the particles in the iced tea is less than the average kinetic energy of the particles in the hot tea.

Heat Particles in the ice have the lowest average thermal energy (temperature). The particles move very little and only vibrate in place. The room temperature water has more thermal energy, the particles move around more than particles in the ice, but still fairly slowly. The boiling water has the most thermal energy. The particles move round very fast and some contain so much energy they change into gas and leave the container.

Movement of Heat We often confuse heat and temperature. Heat is a type of energy and temperature is a measurement. When a warm object is brought near a cooler object, thermal energy will be transferred from the warmer object to the cooler one. When ice melts in water it is actually taking on the waters energy and cooling that down rather than being melted by the water. Thermal energy moves from a substance with a higher temperature to one with a lower temperature.

Convection and Conduction When two substances are in direct contact with each other, heat is transferred via conduction. The movement of heat in a fluid or a gas is known as convection. Warm air or warm water rises by convection Radiation does not rely on direct contact between substances. For example infrared radiation in the vacuum of space.

Equilibrium Eventually a substance will come into balance with its surroundings. This is known as equilibrium. Equilibrium is a state of physical balance.

Specific Heat Water has many unique properties One of these is the amount of energy it takes to raise its temperature compared to most other substances. The specific heat of a substance is the amount of energy it takes to raise one gram of a substance by 1 o C Substances with a low specific heat (e.g. most metals and things like sand) heat up and cool down quickly. Substances with a high specific heat (e.g. water) heats up and cools down more slowly as a much larger quantity of energy is required to cause its temperature to rise or fall.

Changes Between Solid and Liquid State Matter can change from one state to another when thermal energy is absorbed or released As ice is heated it absorbs thermal energy and its temperature rises. Above the melting point (0 o C) the ice melts and changes from a solid state to a liquid state. Amorphous solids such as rubber and glass don’t melt in the same way as crystalline solids as they don’t have crystal structures to get broken down. Amorphous solids get softer as they are heated.

Freezing The process of melting a crystalline solid can be reversed if the liquid is cooled. This is called freezing. As liquid cools it loses thermal energy its particles slow down and come close together. Attractive forces trap particles and a solid begins to form. The point where a substance changes from a liquid to a solid is called the freezing point.

Changes Between Liquid and Gas States Matter changes between liquid and gas states through vaporization and condensation. Vaporization – as a water is heated its temperature rises until it reaches 100 o C. During this temperature rise water changes phase into water vapor. This is known as vaporization. Boiling Evaporation

Vaporization, Evaporation and Boiling Evaporation and boiling are NOT the same thing! Evaporation occurs when molecules on the surface of a liquid change phase and become a gas. We can see the resulting water vapor. This is known as evaporation. As this water vapor pushes the air out the way the pressure pushing down on the liquid is reduced. This allows the bubbles that we see to form. These bubbles contain a lot of thermal energy. As the bubbles rise up to the surface thermal energy is released.

B Bubbles cannot form since the vapor pressure is less than the atmospheric pressure. Bubbles can form since the vapor pressure has overcome the atmospheric pressure. EvaporationBoiling

Location of Molecules For evaporation to occur faster molecules must be near the surface, and they must also be heading in the right direction. These molecules must avoid hitting any other molecules as they leave. As the fastest molecules have left the substance it will cool the liquid and anything near it. This is why we sweat in order to cool ourselves off.

Condensation As a gas cools its particles slow down As particles slow down enough for their attractions to bring them together, droplets of liquid form. This is the opposite of vaporization and is called condensation. As a gas condenses to a liquid it releases thermal energy. The decrease in energy changes the arrangement of particles. After the change of state is complete the temperature continues to drop.

Condensation

Changes Between Solid and Gas States Some substances can change from a solid directly into a gas without ever being a liquid. This process is known as sublimation. Dry ice (solid CO 2 ) is an example of this Carbon dioxide does not exist as a liquid so as it absorbs thermal energy from water vapor in the air it changes directly into a gas. As the water vapor cools and condenses into liquid a fog forms.

The law of Conservation of Matter The law of conservation of matter states that matter cannot be created or destroyed. It can however change from one form into another. This is like the law of conservation of energy which states that energy cannot be created or destroyed.

Knowledge and Understanding Recognize that heat is a form of energy and that temperature change results from adding or taking away heat from a system. Explain the effect of heat on particle motion through a description of what happens to particles during a change in phase. Give examples of how heat moves in predictable ways, moving from warmer objects to cooler objects until they reach equilibrium.