4.03 Introduction to Heat

boiling

Boiling is the process of transforming a liquid into a gas by adding heat. As heat is added, the particles in the liquid have a greater kinetic energy and move faster. Particles eventually have enough kinetic energy to escape the liquid phase and become a gas. The temperature at which a liquid boils, changing from a liquid into a gas, is called the *boiling point*.

Cooling Curves

Cooling curves are the opposite of heating curves. A cooling curve shows the temperature change as a substance loses energy and cools down. Cooling curves have the same horizontal sections that you saw in heating curves, where the phase changes from a gas to a liquid or from a liquid to a solid.

how do we measure heat during a change in phase?

Under ordinary conditions, matter can exist in three states, or phases. When a crystalline (solid state that has a definite molecular pattern) material changes phase, its internal energy changes but its temperature does not. The heat absorbed during a phase change produces a change in the molecular potential associated with bonds between the molecules, but the temperature remains the same. Because the temperature remains the same, the average kinetic energy of the molecules remains the same. There are 2 things to consider when measuring heat transfer from one object to another during a phase change: the identity and the amount. Identity: The energy transferred when a substance changes its state/phase is called the latent heat (latent is Latin for hidden). Different substances require different amounts of heat to melt when at their melting point or to vaporize when at their boiling point. The *latent heat of fusion* is defined as the amount of heat energy one unit mass of material, at its melting point, must absorb to change from the solid phase to the liquid phase with no change in temperature. The same amount of heat would be released as the liquid, at its freezing point, changes from the liquid phase into the solid phase with no change in temperature. The *latent heat of vaporization* of a material is defined as the amount of heat energy one unit of mass at its boiling point must absorb to change from the liquid phase into the gaseous phase with no temperature. The same amount of heat would be released as the gas, at its condensation point, changes from the gas phase into the liquid phase with no change in temperature. Amount: The amount of a substance also determines the temperature change associated with a certain quantity of heat transferred to the substance. The quantity of heat needed to melt a cupful of ice is much less than the quantity of heat needed to melt a bucketful of ice. The amount of the substance is the *mass* of the substance. The symbol for mass is m. For our work, the mass will be measured in *grams* (g) or *kilograms* (kg). Remember that 1000 grams is equal to 1 kilogram. For example: 454 grams equals 0.454 kilograms.

Phase Changes

When a substance changes from one state, or phase, of matter to another, we say that it has undergone a change of state or a change of phase. For example, ice melts and becomes liquid, or water vapor (gas) condenses to become liquid. These changes of phase always occur with a change of heat. Heat, which is energy, either comes into or out of the material during the change. During a phase change, the kinetic energy of the molecules in a substance does not change, but the bond energy between the molecules does change. During a phase change, the heat energy is used to change the bonding between the molecules but does not change the kinetic energy of the molecules. In the case of melting, added energy is used to overcome the forces of attraction between the molecules. In the case of freezing, energy is removed and the forces of attraction between the molecules increase. It is important to note that although the heat content of the material changes, the temperature does not change during a phase change because there is no change in the kinetic energy of the molecules.

freezing

Freezing is the process of transforming a liquid into a solid by the removal of heat, the reverse of the melting process. As the sample of liquid loses heat, the particles' movement slows down. The particles continue to move slower and slower until the attractive forces between them are able to hold the particles in a fixed position, transforming the liquid into a solid. The temperature at which a given substance transforms from a liquid into a solid is called its *freezing point*, which is the same temperature as the substance's melting point.

thermal equilibrium

Heat spontaneously flows from a warmer substance to a cooler object when the objects are in thermal contact. After these objects reach the same temperature, we say that the objects are in *thermal equilibrium*. When you have a fever, you might want to check your temperature. You do this by placing a clean thermometer under your tongue. You get your temperature reading when the thermometer reaches thermal equilibrium with your body. If you have a fever, heat flows from your body into the thermometer until they have the same temperature. The temperature of the thermometer is the temperature of your body.

what is heat?

If you touch a hot stove, energy will enter your hand from the stove because the stove is warmer than your hand. However, if you touch ice, energy will pass out of your hand and into the cooler ice. The direction of spontaneous energy transfer is always from a warmer substance to a cooler substance. When you added ice to water in a previous activity, you noticed that the temperature of the water decreased as the heat flowed from the warmer substance (the water) to the colder substance (the ice). The energy that transfers from one object to another because of a temperature difference between them is called *heat*. There is a common mistake that is often made by those of us beginning our physics journey. Sometimes, there is a misconception that matter contains heat. In actuality, matter contains energy in several forms, but it does not contain heat. Now, read that sentence again: matter does not contain heat. You should now ask, "What is the right idea?" Heat is energy in transit (moving) from a body of higher temperature to a body of lower temperature. Once the heat is transferred, the energy is no longer considered to be heat. The symbol for the quantity of heat transferred when a substance undergoes a temperature change is "*Q*." The unit for measuring energy is the *Joule*. The symbol for the Joule is "*J*."

melting

Melting is the process of a solid transforming into a liquid. When heat is added to a solid, the particles' kinetic energy increases, and they vibrate more and more violently. If enough heat is added, the attractive forces between the particles are not able to hold them together, and the substance melts. The temperature at which a given substance melts is called its *melting point*. Imagine a group of children are holding hands. If each of the children starts to jump around randomly, it becomes more difficult for them to hold onto each other. If the jumping becomes energized and violent enough, the children will not be able to hold on any longer. This is similar to what happens as the particles of a solid gain energy.

physical properties

Physical properties are easily observable and do not change the chemical property of matter. Some examples of physical properties are physical state, color, smell, density, freezing point, boiling point, melting point, opacity, viscosity, conductivity, and hardness, to name a few. Remember that measuring each of these properties will not alter the basic nature of the substance. An important physical property is physical state of matter. There are four basic states of matter: solid, liquid, gas, and plasma. A change in physical state or phase occurs when energy is added or removed. To go from a phase to phase (i.e. from solid to liquid), we will need to either add or remove heat.

Equation to use when there is a phase change:

The equation used to calculate the amount of heat flow in or out of a substance is this: Q = (m)(Hf) Q = the quantity of heat flowing in or out of the substance. It is measured in Joules (J). m = the mass of the substance. It is measured in kilograms (kg). Hf = the latent heat of fusion Q = (m)(Hv) Q = the quantity of heat flowing in or out of the substance. It is measured in Joules (J). m = the mass of the substance. It is measured in kilograms (kg). Hv = the latent heat of vaporization

Equation to use when there is a temperature change:

The equation used to calculate the amount of heat flow in or out of a substance is this: Q = (m)(c)(ΔT) Q = the quantity of heat flowing in or out of the substance. It is measured in Joules (J). m = the mass of the substance. It is measured in kilograms (kg). c = the specific heat capacity of the substance. It is measured in J/(kg * C degrees) Δ T = the change in temperature of the substance. It is measured in C degrees.

condensation

The process of transforming a gas into a liquid through the removal of heat is called condensing. This process occurs when the temperature of the gas is cooled enough for the particles to slow down and attract each other, forming a liquid. The temperature at which this phase change occurs is called the *condensation point*, the same temperature as the substance's boiling point. If you have ever had a cold glass of lemonade on a warm summer day, you may have noticed water droplets appearing on the outside of the glass. Your glass was not leaking; the water vapor from the surrounding air condensed to liquid when it was cooled on the surface of your glass. Condensation in the form of dew often appears on windows and grass in the morning because water vapor from the warm air of the sunny day was cooled to form a liquid during the night.

how do we measure heart during a change in temperature?

There are 3 things to consider when measuring heat transfer from one object to another: the identity, the amount, and the temperature change of the substances. Identity: Different substances have different capacities for storing internal energy. If you have been to the beach, you know that the water heats up much slower than the sand. Have you ever run quickly across the hot sand to get to the water? Water has a greater heat capacity than sand. Water is much slower to warm in the hot sun, but it is also slower to cool in the cold night. Various substances require different quantities of heat to raise the temperature of equal quantities of the substances by the same amount. The amount of heat required to raise the temperature of a unit mass of a substance by one degree Celsius is called the *specific heat capacity*. The symbol for the specific heat capacity is "*c*," and the unit for measuring this quantity is *Joules/(kg * degrees C)*. Amount: The amount of a substance also determines the temperature change associated with a certain quantity of heat transferred to the substance. The quantity of heat needed to bring a cupful of soup to a boil is much less than the quantity of heat needed to bring the whole pot of soup to a boil. The amount of the substance is the mass of the substance. The symbol for *mass* is "m." For our work, the mass will be measured in *grams* (g) or *kilograms* (kg). Remember that 1000 grams is equal to 1 kilogram. For example: 454 grams equals 0.454 kilograms. Temperature difference: Any of the temperature scales we have previously discussed could be used to measure temperature difference. For our purposes we will use the *Celsius* scale to measure the various temperatures. Just remember that "degrees Celsius" is different from "Celsius degrees." A given point on the Celsius scale is noted by the unit "degrees Celsius" (o^C). For example: water freezes at 0 degrees C and boils at 100 degrees C. However, a difference in temperature is measured in "Celsius degrees" (C^o). For example: if the temperature of a sample of water changes from 20. degrees C to 75 degrees C, then the temperature change is 55 C degrees.

Heating Curves

This heating graph or heating curve represents the change in temperature as a substance under constant heat changes from a solid to a liquid to a gas. Notice the slope of the graph changes at different points, even though the substance is heated constantly. Solid: When the solid ice was heated, the temperature increased until it reached the substance's melting point. For water, the melting point is 0°C, and you may have noticed that the temperature stayed constant at 0°C until the entire sample melted. The heat that was being supplied by the Bunsen burner was used to melt the ice. Solid-Liquid Phase Change: When the energy being supplied in the form of heat is used for a phase change, the temperature of the sample always remains constant. This is represented by the horizontal sections in the heating curve. When a phase change is complete, the temperature begins to rise again. Liquid-Gas Phase Change: What does the second horizontal section of the heating curve represent? This flat section of the graph represents the substance's boiling point. When the liquid begins to boil, the temperature remains constant as the heat energy provided by the flame is used to move the particles farther apart to change the liquid to a gas. Once all of the particles are in the gas phase, the temperature will begin to rise again.