Behaviour of Fluids

Pressure

It’s a beautiful summer day when you and your friends go outside to play volleyball, much like the kids in  Figure 1. There’s only one problem—the ball is flat.You pump air into the ball until it is firm. The firmness of the ball is the result of the motion of the air particles in the ball. As the air particles in the ball move, they collide with one another and with the inside walls of the ball. As each particle collides with the inside walls, it exerts a force, pushing the surface of the ball outward. A force is a push or a pull. The forces of all the individual particles add together to make up the pressure of the air. Pressure is equal to the force exerted on a surface divided by the total area over which the force is exerted.

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When force is measured in newtons (N) and area is measured in square meters (m2), pressure is measured in newtons per square meter (N/m2). This unit of pressure is called a pascal (Pa). A more useful unit when discussing atmospheric pressure is the kilopascal (kPa), which is 1,000 pascals.

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Figure 1

Force and Area

You can see from the equation on the opposite page that pressure depends on the quantity of force exerted and the area over which the force is exerted. As the force increases over a given area, pressure increases. If the force decreases, the pressure will decrease. However, if the area changes, the same amount of force can result in different pressure. Figure 2 shows that if the force of the ballerina’s weight is exerted over a smaller area, the pressure increases. If that same force is exerted over a larger area, the pressure will decrease.

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Figure 2

Changes in Gas Pressure

In the same way that atmospheric pressure can vary as conditions change, the pressure of gases in confined containers also can change. The pressure of a gas in a closed container changes with volume and temperature.

Pressure and Volume

If you squeeze a portion of a filled balloon, the remaining portion of the balloon becomes more firm. By squeezing it, you decrease the volume of the balloon, forcing the same number of gas particles into a smaller space. As a result, the particles collide with the walls more often, thereby producing greater pressure. This is true as long as the temperature of the gas remains the same. You can see the change in the motion of the particles in  Figure 3. What will happen if the volume of a gas increases? If you make a container larger without changing its temperature, the gas particles will collide less often and thereby produce a lower pressure.

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Figure 3

Float or Sink

You may have noticed that you feel lighter in water than you do when you climb out of it. While you are under water, you experience water pressure pushing on you in all directions. Just as air pressure increases as you walk down a mountain, water pressure increases as you swim deeper in water. Water pressure increases with depth. As a result, the pressure pushing up on the bottom of an object is greater than the pressure pushing down on it because the bottom of the object is deeper than the top. The difference in pressure results in an upward force on an object immersed in a fluid, as shown in Figure 4. This force is known as the buoyant force. If the buoyant force is equal to the weight of an object, the object will float. If the buoyant force is less than the weight of an object, the object will sink.

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Figure 4

Archimedes’ Principle

What determines the buoyant force? According to Archimedes’ (ar kuh MEE deez)  principle, the buoyant force on an object is equal to the weight of the fluid dis- placed by the object.  In other words, if you place an object in a beaker that already is filled to the brim with water, some water will spill out of the beaker.

Pascal’s Principle

What happens if you squeeze a plastic container filled with water? If the container is closed, the water has nowhere to go. As a result, the pressure in the water increases by the same amount everywhere in the container—not just where you squeeze or near the top of the container. When a force is applied to a confined fluid, an increase in pressure is transmitted equally to all parts of the fluid. This relationship is known as Pascal’s principle.

 Source:  Glencoe Science-The Nature of Matter-SE_0078617650

Changes of State

Thermal Energy and Heat

Shards of ice fly from the sculptor’s chisel. As the crowd looks on, a swan slowly emerges from a massive block of ice. As the day wears on, however, drops of water begin to fall from the sculpture. Drip by drip, the sculpture is transformed into a pud- dle of liquid water.What makes matter change from one state to another? To answer this question, you need to think about the particles that make up matter.

Energy

Simply stated, energy is the ability to do work or cause change. The energy of motion is called kinetic energy. Particles within matter are in constant motion. The amount of motion of these particles depends on the kinetic energy they possess. Particles with more kinetic energy move faster and farther apart. Particles with less energy move more slowly and stay closer together. The total kinetic and potential energy of all the particles in a sample of matter is called thermal energy. Thermal energy, an extensive property, depends on the number of particles in a substance as well as the amount of energy each particle has. If either the number of particles or the amount of energy in each particle changes, the thermal energy of the sample changes. With identically sized samples, the warmer substance has the greater thermal energy. In Figure 2, the particles of hot water from the hot spring have more thermal energy than the particles of snow on the surrounding ground.

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Figure 1

Temperature

Not all of the particles in a sample of matter have the same amount of energy. Some have more energy than others. The average kinetic energy of the individual particles is the temperature, an intensive property, of the substance.You can find an average by adding up a group of numbers and dividing the total by the number of items in the group. For example, the aver- age of the numbers 2, 4, 8, and 10 is (2  4  8  10)  4  6. Temperature is different from thermal energy because thermal energy is a total and temperature is an average. You know that the iced tea is colder than the hot tea, as shown in Figure 2. Stated differently, the temperature of iced tea is lower than the temperature of hot tea. You also could say that 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.

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Figure 2

Heat

When a warm object is brought near a cooler object, thermal energy will be transferred from the warmer object to the cooler one. The movement of thermal energy from a substance at a higher temperature to one at a lower temperature is called heat. When a substance is heated, it gains thermal energy. Therefore, its particles move faster and its temperature rises. When a substance is cooled, it loses thermal energy, which causes its particles to move more slowly and its temperature to drop.

Specific Heat

As you study more science, you will discover that water has many unique properties. One of those is the amount of heat required to increase the temperature of water as compared to most other substances. The specific heat of a substance is the amount of heat required to raise the temperature of 1 g of a substance 1°C. Substances that have a low specific heat, such as most metals and the sand in Figure 3, heat up and cool down quickly because they require only small amounts of heat to cause their temperatures to rise. A substance with a high specific heat, such as the water in  Figure 3, heats up and cools down slowly because a much larger quantity of heat is required to cause its temperature to rise or fall by the same amount.

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Figure 3

VISUALIZING STATES OF MATTER

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Source: Glencoe Science-The Nature of Matter-SE_0078617650

 

 

Matter

What is matter?

Take a look at the beautiful scene in Figure 1.What do you see? Perhaps you notice the water and ice.Maybe you are struck by the Sun in the background. All of these images show examples of matter. Matter is anything that takes up space and has mass. Matter doesn’t have to be visible—even air is matter.

States of Matter All matter is made up of tiny particles, such as atoms, molecules, or ions. Each particle attracts other particles. In other words, each particle pulls other particles toward itself. These particles also are constantly moving. The motion of the particles and the strength of attraction between the particles determine a material’s state of matter.

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Figure 1

There are three familiar states of matter—solid, liquid, and gas. A fourth state of matter known as plasma occurs at extremely high temperatures. Plasma is found in stars, lightning, and neon lights. Although plasma is common in the universe, it is not common on Earth.

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                                                              Gases

Source: Glencoe Science-The Nature of Matter-SE_0078617650

Compounds and Mixtures

Substances

Scientists classify matter in several ways that depend on what it is made of and how it behaves. For example, matter that has the same composition and properties throughout is called a substance. Elements, such as a bar of gold or a sheet of aluminum, are substances. When different elements combine, other substances are formed.

Compounds

The elements hydrogen and oxygen exist as separate, colorless gases.However, these two elements can combine, as shown in Figure 1, to form the compound water, which is different from the elements that make it up. A compound is a substance whose smallest unit is made up of atoms of more than one element bonded together. Compounds often have properties that are different from the elements that make them up. Water is distinctly different from the elements that make it up. It is also different from another compound made from the same elements. Have you ever used hydrogen peroxide (H2O2) to disinfect a cut? This compound is a different combination of hydrogen and oxygen and has different properties from those of water. Water is a nonirritating liquid that is used for bathing, drinking, cooking, and much more. In contrast, hydrogen peroxide carries warnings on its labels such as Keep Hydrogen Peroxide Out of the Eyes. Although it is useful in solutions for cleaning contact lenses, it is not safe for your eyes as it comes from the bottle.

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Figure 1

Mixtures

When two or more substances (elements or compounds) come together but don’t combine to make a new substance, a mixture  results. Unlike compounds, the proportions of the substances in a mixture can be changed without changing the iden-tity of the mixture. For example, if you put some sand into a bucket of water, you have a mixture of sand and water. If you add more sand or more water, it’s still a mixture of sand and water. Its identity has not changed. Air is another mixture. Air is a mixture of nitrogen, oxygen, and other gases, which can vary at different times and places. Whatever the proportion of gases, it is still air. Even your blood is a mixture that can be separated, as shown in Figure 2 by a machine called a centrifuge.

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Figure 2

The Simplest Matter

The Elements

Have you watched television today? TV sets are common, yet each one is a complex system. The outer case is made mostly of plastic, and the screen is made of glass. Many of the parts that conduct electricity are metals or combinations of metals. Other parts in the interior of the set contain materials that barely conduct electricity. All of the different materials have one thing in common. They are made up of even simpler materials. In fact, if you had the proper equipment, you could separate the plastics, glass, and metals into these simpler materials.

Identifying Characteristics

Each element is different and has unique properties. These differences can be described in part by looking at the relationships between the atomic particles in each element. The periodic table contains numbers that describe these relationships.

Number of Protons and Neutrons Look up the element chlorine on the periodic table found on the inside back cover of your book. Cl is the symbol for chlorine but what are the two numbers? The top number is the element’s atomic number. It tells you the number of protons in thenucleus of each atom of that element. Every atom of chlorine, for example, has 17 protons in its nucleus.

Isotopes Although the number of protons changes from element to element, every atom of the same element has the same number of protons. However, the number of neutrons can vary even for one element. For example, some chlorine atoms have 18 neutrons in their nucleus while others have 20. These two types of chlorine atoms are chlorine-35 and chlorine-37. They are called  isotopes (Isuh tohps), which are atoms of the same element that have different numbers of neutrons. You can tell someone exactly which isotope you are referring to by using its mass number. An atom’s mass number is the number of protons plus the number of neutrons it contains. The numbers 35 and 37, which were used to refer to chlorine, are mass numbers. Hydrogen has three isotopes with mass numbers of 1, 2, and 3. They are shown in Figure 1. Each hydrogen atom always has one proton, but in each isotope the number of neutrons is different.

Figure 1

Figure 1

Classification of Elements

Elements fall into three general categories—metals, metalloids (ME tuh loydz), and nonmetals. The elements in each category have similar properties. Metals generally have a shiny or metallic luster and are good conductors of heat and electricity.All metals, except mercury, are solids at room temperature.Metals are malleable (MAL yuh bul), which means they can be bent and pounded into various shapes. The beautiful form of the shell-shaped basin in  Figure 2 is a result of this characteristic. Metals are also ductile, which means they can be drawn into wires without breaking. If you look at the periodic table, you can see that most of the elements are metals.

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Figure 2

Other Elements Nonmetals are elements that are usually dull in appearance. Most are poor conductors of heat and electricity. Many are gases at room temperature, and bromine is a liquid. The solid nonmetals are generally brittle, meaning they cannot change shape easily without breaking. The nonmetals are essential to the chemicals of life.More than 97 percent of your body is made up of various nonmetals, as shown in Figure 3. You can see that, except for hydrogen, the nonmetals are found on the right side of the periodic table. Metalloidsare elements that have characteristics of metals and nonmetals. On the periodic table, metalloids are found between the metals and nonmetals. All metalloids are solids at room temperature. Some metalloids are shiny and many are conductors, but they are not as good at conducting heat and electricity asmetals are. Somemetalloids, such as silicon, are used tomake the electronic circuits in computers, televisions, and other electronic devices.

Figure 3

Figure 3

Source: Glencoe Science-The Nature of Matter-SE_0078617650