Homework #4 Name _______________________
(Total 20 pts)
For this homework, the concept of “Density”
and “Heat” will be explored.
Density is the “compactness” of a material, or stated in a
formula it is how much it weighs (mass) divided by how much space it takes up
(volume). For pure water the density is
1 gram for every milliliter of water (1 g/ml). From the table below, determine what elements
are described in Questions 1-3.
Name of element | Density (g/ml) |
Magnesium | 1.74 |
Silicon | 2.33 |
Bismuth | 9.80 |
Zinc | 7.14 |
Sodium | 0.97 |
Boron | 2.34 |
- Item weighs 25 grams and takes up 3.5 ml of volume
_______________ (1 pt)
- Item weighs 117 grams and takes up 50 ml of volume
_______________ (1 pt)
- This item floats on water ___________________ (1 pt)
- If all these items were mixed together, which one is expected
to sink to the bottom? ________ (1 pt)
“Heat” is the next topic.
It is a somewhat difficult concept to grasp but it is very important. I
encourage you first to read Chapter 5. Then carefully study the next figure. This
figure is a graph that plots temperature over thermal energy. To answer
questions 9-12 you need to read, use this graph. No formula is needed!
There are two general types of heat; “Sensible” and
“Latent.” Sensible Heat describes a process where the temperature changes as
heat is added or removed. Latent Heat Thermal Energy is absorbed but the
temperature does not change, for example between points D and C shown in the
graph.
The definition of “Heat Capacity” plays a significant role
in sensible heat; it is the ability of a substance to store heat. For pure
water, the heat capacity is 1 cal / g °C (1 calorie per every gram of water per
every degree change in Celsius), or in words; if I have 1 gram of water and I
want to raise it 1°C, I must add 1 Calorie of heat. Determine the following:
- If I want to raise 2 grams of water 1°C, how much heat should I
add? _____________ (1 pt)
- If I want to raise 1 gram of water 2°C, how much heat should I
add? _____________ (1 pt)
The higher the ‘heat capacity’ of an object, the more heat
it takes to increase its temperature.
Determine the following:
- Wood has 4 times the heat capacity as steel. If I add the same
amount of heat to the same amount of steel and wood, I find the
temperature of the steel increases to 40°C. What is the temperature of the
wood? ___________ (1 pt)
- Water has 5 times the heat capacity as soil. If I add the same
amount of heat to the same amount of water and soil, I find the
temperature of the soil increases to 50°C. What is the temperature of the
water? _________ (1 pt).
The next concept to be considered when dealing with heat, is
“Latent Heat”. The word latent refers to the temperature of the system
remaining the same even as heat is being added or subtracted from it. This will
occur when the system is changing states. Consider a piece of ice.
- The latent heat of fusion (or the
amount of heat required to turn the ice into water) is 80 Calories/gram.
How much heat must be added to melt 2 grams of ice at 0°C and what is the
final temperature after this heat is added? _______________ _______________ (3 pts)
- The latent heat of vaporization
(or the amount of heat required to turn the water into a vapor) is
540 Calories/gram. How much heat must be added to turn 3 grams of
water at 100°C into a vapor and what will be the final temperature after
this much heat is added? _____________
_____________ (3 pts)
Putting sensible and latent heat together, it is now
possible to figure out the temperature of water going through various stages of
heat input/withdrawal. (Heat capacity of water is 1 cal / g °C)
- 60 calories isadded to 1 gram of water at an
initial temperature of 80°C. What is the water’s final temperature and
state (solid, liquid or vapor)?
Temperature = __________________ (2 pts)
State = ________________________ (1 pt)
- 120 calories isremoved from 1 gram of water at an
initial temperature of 40°C. What is the water’s final temperature and
state (solid, liquid or vapor)?
Temperature = __________________ (2 pts)
State = ________________________ (1 pt)