answer :
decrease
Response:
That's the correct answer
Density
The particle model of matter explains how changes in temperature and pressure affect the volume of a gas. When the gas volume varies due to a change in temperature or pressure, the density of the gas is also affected. As you have learned previously, density is the mass per unit volume of a substance, usually expressed in units of grams per cubic centimeters, g/cm3. Density can be expressed mathematically as follows:
The density of a substance depends on how closely its particles are packed. Under any specific conditions of pressure and temperature, the density of any substance is constant. Different substances have different densities under the same conditions. For example, at normal room temperature and pressure, the density of water is 1.0 g/cm3, the density of iron is 7.87 g/cm3, and the density of cork is 0.2 g/cm3. Density does not depend on the volume of a sample of a substance. A drop of water has the same density as a liter of water.
A substance placed in a liquid floats if its density is lower than the liquid’s density and sinks if its density is greater than the liquid’s density. For example, a piece of iron sinks in water because its density is higher than that of water. A cork floats in water because its density is lower than that of water.
Earlier, you learned that the volume of a gas changes as temperature or pressure changes. Consider examples of how the pressure and temperature changes can change the density of the same gas sample.
Suppose you use a pump to exert pressure on a gas. As the pressure increases, the gas particles are compressed into a smaller volume. The mass of the gas does not change, but the volume it occupies decreases. This causes the density of the gas to increase.
Now suppose you place an inflated balloon in the freezer. After several minutes, you observe that the volume of the balloon has decreased. The mass of the gas particles inside the balloon has not changed, but the volume occupied by the gas particles has decreased. This causes the density of the gas to increase.
Differences in density can explain why a hot air balloon rises into the air. A hot air balloon rises into the air because the density of the air inside the balloon is lower than the density of air surrounding the balloon. The air inside the balloon is heated, which causes the air particles to move faster. As the particles move faster, the spaces between the particles increase, causing an increase in the gas volume. As a result, the density of the air inside the balloon decreases. When the density of the balloon becomes less than the density of the air outside, the balloon lifts off the ground and floats up into the sky.
Figure 6 Hot air inside a balloon is less dense than cool air outside the balloon.
Fill in the blank with “increase”, “decrease”, or “stay the same”.
Heating the air inside a balloon, causes the density of the air inside the balloon to
..............
answer :
decrease
Response:
That's the correct answer
5.0 mL of mercury weighs 34 g.
What is the density of mercury?
0.15 g/cm3
5.0 g/cm3
6.8 g/cm3
170 g/cm3
answer :
6.8 g/cm3
Response:
That's the correct answer
Comparing Gases
Scientists record the temperature and pressure of a gas when they use it in an experiment because gas volume changes when temperature or pressure is changed. This allows other scientists to accurately repeat the same experiment. Scientists are also careful to keep conditions of temperature, pressure, and volume constant when comparing different gases; otherwise, the comparison is not useful.
The table below lists several gases and their properties. You can see from the data that, under the same conditions of volume, temperature, and pressure, different gases have different masses.
The density of each gas under the temperature and pressure conditions shown in the table above can be calculated by dividing the mass of the gas by its volume. However, you can do a quick analysis of the data to compare the densities of these gases without actually doing these calculations. The volumes of the gases are all identical under the same conditions of temperature and pressure. Only the masses of the gases differ. Thus, the densities of these gases vary directly with their masses.
Gas Densities and Flight
You have seen how gases having the same volume but different masses have different densities under the same conditions of temperature and pressure. You can apply this idea to explain how gas balloons lift into the air and enable flight.
Suppose you filled identical balloons with different gases. If you release the balloons into the air, the balloons filled with gases having densities lower than the density of air will rise and gases having densities greater than the density of air will sink to the floor. This difference in density is the principle behind the operation of a gas balloon.
 |
Figure 7 The balloon on the left is filled with a gas that is more dense than air.
The balloon on the right is filled with a gas that is less dense than air. |
Figure 7 The balloon on the left is filled with a gas that is more dense than air.
The balloon on the right is filled with a gas that is less dense than air.
Air contains 78% nitrogen (N2), 21% oxygen (O2), 1% argon (Ar), and other gases. In order to be useful for filling a flying balloon, a gas has to have a density less than the mixture of gases that make up air. Helium has less mass than the elements or compounds found in air. Because of this, helium is less dense than air under the same temperature and pressure. In contrast, krypton, another noble gas, is denser than air because its particles have more mass than any particles found in air.
Which of the following statements is true?
At the same temperature and pressure equal volumes of different gases have the same masses.
At the same temperature and pressure different volumes of the same gases have the same masses.
At the same temperature and pressure equal volumes of the same gases have different masses.
At the same temperature and pressure equal volumes of different gases have different masses.
Which of the following statements is true?
Under the same conditions of temperature and pressure, the densities of different gases vary directly with their volumes.
Under different conditions of temperature and pressure, the densities of the different samples of the same gas are equal.
Under the same conditions of temperature and pressure, the densities of different gases vary directly with their masses.
Under the same conditions of temperature and pressure, the densities of different gases are equal.
Fill in the blank with “a higher density than”, “a lower density than”, or “the same density as”.
Xenon is not suitable for filling a flying balloon because it has
.......
air.
answer :
a higher density than
Response:
That's the correct answer
Gay-Lussac’s Law of Combining Volumes
So far, we have only considered samples of pure gases. What happens when gases are mixed and react with one another? This question was pursued by several scientists including French scientist Joseph Louis Gay-Lussac.
Figure 8 Joseph Louis Gay-Lussac experimented extensively with gases. He made several ascents
in balloons–considered daring at the time–in order to study gas behavior at different altitudes.
 |
Figure 8 Joseph Louis Gay-Lussac experimented extensively with gases. He made several ascents
in balloons–considered daring at the time–in order to study gas behavior at different altitudes. |
When experimenting with gases in the laboratory, Gay-Lussac was careful to use constant temperature and pressure conditions. As he did, Gay-Lussac found that oxygen gas reacted with hydrogen gas to form water vapor. He also found that no matter what volume of oxygen gas he used, it reacted with two volumes of hydrogen gas to form two volumes of water vapor.
Figure 9 Gay-Lussac found that hydrogen and oxygen always reacted in a 2:1 volume ratio to form water.
 |
Figure 9 Gay-Lussac found that hydrogen and oxygen always reacted in a 2:1 volume ratio to form water.
|
Additional experiments with other gas reactions gave similar results. Gay-Lussac found that every reaction between gases involved volumes that were proportional by some small whole number. He summarized his findings in a principle called Gay-Lussac’s law, which states as follows: Volumes of reacting gases at the same temperature and pressure combine in whole number ratios.
A chemist has 5 liters of oxygen at 4 atm and 250°C. Under the same conditions, how many liters of hydrogen would be needed to completely react with this amount of oxygen, and how many liters of water vapor would be produced?
10 liters of hydrogen are needed and 5 liters of water are produced.
5 liters of hydrogen are needed and 10 liters of water are produced.
10 liters of hydrogen are needed and 10 liters of water are produced.
5 liters of hydrogen are needed and 5 liters of water are produced.
Avogadro’s Hypothesis
Amedeo Avogadro, an Italian scientist working in the same time period as Gay-Lussac, published a hypothesis that could explain Gay-Lussac’s experimental findings. He tested various properties of pure samples of gas and compared them. His results led him to observe that different gases at constant temperature, pressure, and volume had different masses and therefore different densities, as shown in Table 2.
At this point, Avogadro used the idea of particles to explain differing gas densities. Avogadro proposed that the density of a gas is proportional to the mass of one particle of that gas. Avogadro reasoned that the difference in mass between hydrogen gas and oxygen gas, for example, was due solely to differences in the masses of the hydrogen and oxygen gas particles because both gases contained the same number of particles. Based on this idea, he made the following proposal, known as Avogadro’s hypothesis: Equal volumes of gas at the same temperature and pressure contain the same number of particles of gas.
The figure below illustrates how three different gases composed of particles of different mass might be represented. The sizes of the balls represent the relative masses of the particles. The densities of these gas samples would be expected to increase from A to C since the total mass of the sample increases as the volume remains constant.
 |
Figure 11 A comparison of three different gases under the same volume, temperature, and pressure conditions
shows that they all contain the same number of particles even though the particles differ in mass. |
In making his hypothesis, Avogadro had to recognize that some gases exist as individual atoms, while others exist as molecules containing two or more atoms bonded together. For example, a particle of helium gas is simply an atom of helium (He), whereas a particle of hydrogen gas is a molecule of two hydrogen atoms (H2).
This realization was important in explaining Gay-Lussac’s findings. For example, Gay-Lussac’s experiment showed that two volumes of hydrogen gas combine with one volume of oxygen gas to produce only two volumes of water vapor, not three volumes. In other words, the volumes did not add together as you would add whole numbers in an addition equation. Avogadro successfully explained this by proposing that some gases are composed of molecules that contain two or more atoms. These molecules broke apart so that the atoms could rearrange to form new molecules in the product.
We now know that the whole number volume ratios that Gay-Lussac observed are due to the fact that gas molecules react in whole number ratios. In other words, a volume ratio reflects the same ratio of reacting molecules. At the time Gay-Lussac made his observations, scientists had not yet developed the particle model of matter. Because of this, Gay-Lussac may not have realized why he observed this whole number ratio of reacting gas volumes.
Figure 12 Avogadro’s hypothesis explains why gases combine in whole number ratios as Gay-Lussac observed.
Avogadro’s hypothesis states:
Equal volumes of gas at the same temperature and pressure contain the same number of particles of gas.
Equal volumes of gas at different temperature and pressure contain the same number of particles of gas.
Different volumes of gas at the same temperature and pressure contain the same number of particles of gas.
Equal volumes of gas at the same temperature and pressure contain different number of particles of gas.
Avogadro’s hypothesis states:
Equal volumes of gas at the same temperature and pressure contain the same number of particles of gas.
Equal volumes of gas at different temperature and pressure contain the same number of particles of gas.
Different volumes of gas at the same temperature and pressure contain the same number of particles of gas.
Equal volumes of gas at the same temperature and pressure contain different number of particles of gas.
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