Chapter One, Section Three

OBJECTIVES

• Be able to distinguish the different states of matter from each other.
• Understand the concept of density and specific gravity and be able to use them in problems.
• Understand the two different forms of energy and be able to calculate the kinetic energy.
• Understand the concepts of heat and specific heat. Know the units and be able to work problems using the concepts.

I. STATES OF MATTER

We usually speak of matter existing in three states: solid, liquid, and gas. The solid state usually has a definite shape and is not compressible. The liquid has no definite state, is slightly compressible, and will not fill any container into which it is placed. The gas has no definite state, is very compressible, and will fill any type container into which it is place. Changing states of matter is a physical change for the molecules of the matter do not change.

II. DENSITY AND SPECIFIC GRAVITY

Density is determined by dividing the mass of the substance by its volume: D = m/V. Some common liquids and their densities are the following (don't memorize):
 Substance Density (g/cm3) ethanol(l) 0.789 water(l) 0.9982 oxygen(g) 0.00133 salt(s) 2.16

Unless the two substances being mixed are totally miscible (readily mix together), when two substances are put together the substance with the lowest density will be on the top. Density is a function of temperature and usually when the temperature is increased the density will decrease (density is inversely proportional to temperature). However water between 0 oC and 4 oC behaves opposite to what is expected. The maximum density of water occurs at 4 oC. The density of solid water (ice) is less than the density of liquid water, so a pond freezes from the top down. If it were not for this strange behavior, the fish would not survive the freezing of a pond.

Specific gravity numerically is the same as the density at oC if the density is measured in units of g/ml or g/cc.

III. ENERGY AND SPECIFIC HEAT

Energy is defined as the capacity to do work and transfer heat and consists of kinetic energy and potential energy. Kinetic energy is given by KE = 0.5*m*v2 and has a definite zero value: when the velocity of the particle is zero. Potential energy has no general equation form but exists by virtue of position of elements of the system. The Principle of Conservation of Energy is : "Total energy must not change, it just changes form."

Think about a person standing on a diving board. As she stands there very still, she has no kinetic energy. However she does have potential energy because of her position above the surface of the water. As she dives the potential energy is converted into kinetic energy and upon impact the kinetic energy does work to push apart the water molecules.

Let's calculate the kinetic energy of a baseball (1/4 lb = 114 g) traveling at 100 mi/hr (44.7 m/s).

KE = 0.5*m*v2 = 0.5 (114g) (44.7m/s)2 (1kg/1000g) = 114 kgm2/s2 = 114 joule (note that a joule is kgm2/s2).

The SI unit for energy is the joule(j) and 1 cal = 4.184 j.

The specific heat (SH) is the amount of heat (in calories) required to raise the temperature of 1 g of any substance by 1oC. Thus the units of specific heat are cal/(g*deg). Just based upon the units of specific heat, we can see that to obtain a value for the heat added or taken away from a system by a change in temperature we must multiply the SH by the mass of the substance and multiply by the temperature change: cal/(g*deg) * g * deg = cal. So we can write

q = SH*m*(T2 - T1)

This is a very important formula, but it does not need to be memorized since it is so easily obtained from the units of SH. Never memorize something that you can easily derive.

Assignments

1. Go to this site to make calculations using the SH of water to calculate heat and temperature changes for water. You will be using the formula above.
2. Post on the WebCT bulletin board the boiling point and freezing point for some substances other than water. All temperatures must be in Celsius.
3. Calculate the kinetic energy of yourself (use your weight) running at 12 mph. Convert the answer to joules. You will need to express your weight in kgm and your speed in m/s. The answer, if the conversions are done correctly, will be in kgm*m2/s2 which is joules. Post your answer on the WebCT bulletin board.
4. WebCT

Quiz Three

After you have studied this material and practiced some problems, take quiz three. If you score at least 80 on the test then you are ready to take the on-line quiz in WebCT and then to continue to the next chapter.

Web Author: Dr. Leon L. Combs