Standard 2 objective 1: I can understand the relationship between properties of matter and Earth s structure Page 1 Note-This information is a copy of the book and on-line books put out by the Utah State Office for the use of 7 th grade science. I purchased a book, as did a few students. The on-line book and the purchased books DO NOT match. In order to teach what the state intended be taught, I have combined the two in hopes we hit all the information that they intended to include. No copyright problems were intended but the students needed a book that was matching and not two different items. Here is the result: Terms to know: density, mixture, particle, sorting. INTRODUCTION: (BTW this is a review for you ) The components of density are: mass and volume. Which one has more mass, a kilogram of feathers or a kilogram of bricks? Though many people will say that a kilogram of bricks is heavier, they actually have the same mass! However, many people confuse mass and density. This confusion causes them to answer the question incorrectly. A kilogram of feathers clearly takes up more space, but this is because it is less dense. What is density, and how can it be calculated, and how can it be used? Density is a physical property. It represents how tightly packed together particles in a substance are. Density is determined by dividing the mass of an object by its volume. Regardless of a sample size, density is always constant. For example, the density of a pure sample of gold is always 19.32 grams per cubic centimeter. This means that whether you have one gram or one kilogram of gold, the density will never vary. Density, a famous concept that was discovered by Archimedes, can be represent by a simply equation: D = M/V or in written terms: Density equals Mass divided by Volume Think of our heart divided in half diagram: That will leave the M on top and the V on the bottom. Keeping this equation in mind, let s move on to a more in-depth discussion of what mass and density mean in the real world.
In the laboratory, density can be used to identify substances. In daily life, density explains everything from why boats float to why air bubbles will try to escape from soda. It even affects your health because the bone density is very important. Based on the equation, it s clear that density can, and does vary from substance to substance due to differences in the ratio of mass to volume. What are mass and volume? Mass The mass of a 1 kg cube is always 1kg whether it is on the top of a mountain, the bottom of the sea, or on the moon. There may be variations in the measurement of mass based on what scientific scale is used (e.g. a digital scale should be more accurate than a triple-beam balance.) Page 2 Triple Beam Balance Scale Digital Scale
Page 3 Volume Volume describes the amount of space an object takes up. The volume of a rectangular shaped object can be found by length x height x width (l x h x w.) For an irregular shaped object, you use water displacement. In this circumstance, you will need a graduated cylinder, water, and paper and pencil to record your numbers and do your calculations. Density: A Further Investigation Units The unit most widely used to express density is g/cm3 or g/ml. Grams per centimeter cubed are equivalent to grams per milliliter (g/cm3 = g/ml.) To solve for density, simply follow the equation D = m/v. For example, if you had a metal cube with the mass of 7.0 g and the volume of 5.0 cm3, the density would be D= 7 g/5 cm3. Notice how the density of various elements on the Periodic Table are represented in Table 1.2: Element Name and Symbol Hydrogen (H).00009 Helium (He).00018 Aluminum (Al) 2.7 Zinc (Zn) 7.13 Tin (Sn) 7.31 Iron (Fe) 7.87 Nickel (Ni) 8.9 Copper (Cu) 8.96 Silver (Ag) 10.5 Lead (Pb) 11.35 Mercury (Hg) 11.55 Gold (Au) 19.32 Platinum (Pt) 21.45 Density (g/cm3) As you can see from this table, the element Platinum is the most dense at 21.45 g/cm3 while the least dense if Hydrogen at.00009 g/cm3.
Page 4 Platinum Copper Both photos above came from the internet google site on element names Now take a look at Table 1.3 which compares some of the Earth s common materials and their densities: Table 1.3 Common Earth Materials Air.0013 Coal 1.4 Basalt 2.8 Granite 2.6 Limestone 2.3 Rock Salt 2.5 Sandstone 2.2 Water 1.0 Density (g/cm3) Archimedes Principle The Greek scientist Archimedes made a significant discovery in 212 B.C. The story goes that Archimedes was asked to find out for the King if his goldsmith was cheating him by replacing his gold for the crown with silver, a cheaper metal. Archimedes did not know how to find the volume of an irregularly shaped object such as the crown even though he knew he could distinguish between substances by their density. While thinking on this puzzle in a bath, Archimedes recognized that when he entered the bath, the water rose. He then realized that he could use a similar process to determine the density of the crown. He then apparently ran through the street naked and shouting Eureka, which means I found it! in Latin. Archimedes then tested the king s crown by taking a genuine gold crown of equal mass and comparing the densities of the two. The king s crown displaced more water than the gold crown
Page 5 of the same mass, meaning that the king s crown had a greater volume and thus had a smaller density than the real gold. Of course, this tale is disputed today because Archimedes was not precise in all his measurements, which would make it hard to determine accurately the differences between the two crowns. Archimedes Principle states that if an object has a greater density than the liquid that it is placed into, it will sink and displace a volume of liquid equal to its own. If it has a smaller density, it will float and displace a mass of liquid equal to its own. If the density is equal, it will not sink or float. Archimedes Principle explains why balloons filled with helium float. Balloons float because they are less dense than the surrounding air. (Photo taken from the internet) Warm air is less dense than cold air so it rises as cold air sinks. It is a density thing!! Helium is less dense than the atmosphere air, so it rises. Archimedes Principle can also be used to explain why boats float. Boats including all the air space within their hull, are far less dense than water.
Look at the following picture and decide which object in the liquid is most dense. Be able to explain why you choose that object: Page 6 Photo taken from the internet Can you explain what is going on with this picture? Is this density or too much mass? Photo taken from the internet Remember that air is a mixture of different substances that exist in a gaseous state. Nitrogen is the most common element found in air. Other substances include Helium, Oxygen, Carbon Dioxide, and water. A mixture is a combination of substances that can be physically removed or easily separated. Like the photo below, a mixture can be easily separated:
Page 7 Having learned the formula for Density (D= m/v), its time to practice. The following are examples of different types of density problems. Write your answers on a clean sheet of lined paper and number properly. Density Problems: 1. If you have a 4 cm3 sample of rock salt with a mass of 10 grams, what is the density? 2. You have a 10 cm3 sample of water with a mass of 10 grams. What is the density? 3. You have an unknown metal with a volume of 4 cm3 and a mass of 42.0 grams. Compare with the charts on page 3 and show the density and the name of the likely unknown metal. 4. Find the density of an unknown liquid in a beaker. The empty beaker s mass is 165 grams. With the unknown liquid inside the beaker, the total mass is 309 grams. The volume of the unknown liquid is 125 ml. Is the liquid pure water? Explain your answer. For a You Tub video on Density, google Density-Mark Drollinger. This one should be easy for you to understand. Another fun You Tube to watch is The Spangler Effect-Density, Science Season 02 Episodes 05 and 06. It is 23:22 long and has an actual 9 layer density tube. It is fun to watch if you need more help on density concepts. Summary of pages 1-7 Density is an important physical property of matter. It reflects how closely packed the particles of matter are. The density of matter can be calculated by dividing the object s mass by its volume.
Vocabulary definition for density: The amount of mass in a given volume of matter; calculated as mass divided by volume. Density and Particle Size Comparing the Density of Various Objects Page 8 We have discussed density. But, how does density affect us every day? We tend to think of density in terms of materials that we are familiar with. If you throw a rock into a lake, the rock will sink to the bottom. Why does the rock sink? The rock sinks to the bottom of the lake because it is denser than the water in the lake. If you throw a block of wood into the lake it will float because wood is less dense than water---unless you have ironwood! Density does not only affect items that are thrown into water. It does not matter what the substances are that are being compared. The denser substance of the two being compared will always sink to the bottom and the less dense substance will always rise to the top. Density does not just determine what sinks and floats when comparing solids and liquids, it applies to any substances that can be mixed with each other. Remember the density demonstration that I showed the class where different densities of saltwater were colored and put into a graduated cylinder? It looked like this: You could get really crazy and do the fancy one on the next page:
Page 9 As you can see, even though all of the substances were liquids with the colored saltwater, the more dense liquids went to the bottom and the less dense liquids rose to the top. From the second picture that was pulled from the Internet Google Images, solids work the same way. If we were to place several different solids in a box and shake the box back and forth, the materials would separate according to their density just like the liquids did in the demonstrations---just like in the pictures of the previous page 8. Below is an experiment that you could try at home: Put various Earth materials (e.g. sand, gravel, twigs, dried leaves, and rocks) into a jar. Gently shake the jar and see what they do. Notice the next picture. What should be on the bottom? What should be on the top? Did you guess correctly? Turn the page and see how it turned out.
Page 10 Picture was taken from the internet The Sorting of Particles Along a Streambed Have you ever walked along a stream and wondered why some areas are sandy and others are strewn with rocks? Different sizes of matter are called particles. Particles are small pieces of matter defined by their size. Clay and sand are examples of smaller particles. Boulders and cobbles are particles that we would normally refer to as rocks. What makes sand deposit in some areas and not in others? Why do the rocks all seem to group together? The answer lies in three factors: 1) the density of the materials involved; 2) the size (its volume) or total mass (the amount of matter in a substance) of the objects; and 3) the rate at which water in the stream is flowing. To understand these questions we need a basic understanding of how water moves in a stream. Heavy, larger objects can only be moved by a stronger current. Smaller and less dense objects can be carried by much weaker currents and are therefore, carried further. When the water in a stream is forced through a narrow canyon or passage, or it drops quickly in elevation, the force of the stream increases. As the water leaves the faster flowing area it spreads out into a larger area. The larger area gives the water
Page 11 more room and the current becomes weaker. Larger rocks and debris are deposited at the end of the faster flow because the current is no longer strong enough to carry them. But, the smaller rocks and debris are carried further downstream until eventually, the current is too weak to carry them and they also stop moving and settle to a stop. The term sorting is used to describe how particles are distributed in a stream. Since sand particles are smaller and have less mass they are carried to areas with the weakest current and are deposited there. That is where beaches are formed. Silt and other very fine materials are carried still further. Silt tends to build up at the mouths of rivers where the current deposits it. This silt forms a geologic formation known as a delta. The Natural Sorting of Earth Materials We have discussed the sorting of materials by size and density as it applies to streams and the Earth, but particles are sorted by density and size in other settings as well. If you have ever seen a construction crew move Earth around to build a home or a road, you may have noticed that there are layers in the soil. These layers are the result of the same processes tht separate materials in a stream or in the Earth s interior. As materials are deposited, wherever that may be, those materials are sorted by size and density. Of course there are exceptions where the separation has not been complete or some factor has limited sorting, but the concept is present in every part of the Earth that we live on. Turn back to page 10 and look at the jar of Earth sediments and you can see how these sediments settle into a correct density placement. Now take a look at the next picture and explain why it has the rocks and pebbles distributed as it does. This picture is also from the google internet images:
Page 12 As the water in the river can no longer carry the more dense stones, they are dropped off the river load and pile up on the river bank. Rivers can carry a lot of sediments, small rocks and even larger rocks when the force of the water in the river is strong. Rivers can deposit sand and pebbles, and even large rocks if the force of the water can carry them. Remember as each stone becomes too dense for the water flow, the sediments will drop off and become part of the river bank or larger ones can be found in the middle of the river. Summary of this section and more to come Different types of seismic waves behave differently in different materials. Their behavior can tell scientist about the material they travel through. Density is a measure of how closely molecules are packed together. The closer together they are, the greater the density. Since air is a gas, the molecules can pack tightly or spread out in a certain volume. The density of air varies from place to place. Air density depends on several factors. One is temperature. Like other materials, warm air is less dense than cool air. Since warmer molecules have more energy, they are more active. The molecules bounce off each other and spread apart. Earth s materials will sort according to densities also. The most dense at the bottom and least dense towards the top as long as something does not disturb those materials. STANDARD 2: I can analyze how density affects Earth s structure. Terms to know: Atmosphere, Crust, Inner Core, Mantle, Outer core
Density and Earth s Structure Page 13 The picture above shows 4 layers. From Google Earth s structure IS affected by density! Do you remember our discussion on mixtures? If you do then you remember that a mixture is a combination of different materials that are mixed together but not chemically combined. In some mixtures the different parts mix together and move evenly throughout the container. A salt water solution is a good example of this. As the salt dissolves in the water, the salt molecules diffuse (move around) until the number of salt particles is the same in every part of the solution. Other mixtures are not like solutions and the particles tend to separate. The particles in mixtures separate according to particle size and the density of the particles in the
Page 14 mixture. When particles separate according to density, the more dense particles will always be on the bottom and the least dense particles will always be on top. Think back to our demonstrations with liquids. Whatever object is MORE dense than the liquid it is put into... it will sink! Whatever object is LESS dense than the liquid it is put into... will float! Remember these facts. What s below our feet? What s way below? If we could cut Earth open, we d see the inner core at the center, then the outer core, the mantle in the middle and the crust on the outside. If you are talking about plates, though, there s the brittle lithosphere riding on the plastic asthenosphere... Whew!!! That is 9 th grade so let s not go too far and get off on a tangent!! Sticking to density and Earth s layers!! Layers by Composition The layers scientist recognize are pictured in the top Earth on page 13. The bottom Earth is a cross section showing the following layers: (1) crust (2) mantle (3a) outer core (3b) inner core (4) lithosphere (5) asthenosphere (6) outer core (7) inner core. Core, mantle, and crust are divisions based on composition: The materials that make up the earth follow all of the principles of density that we have discussed. The more dense materials in the Earth tend to be located closest to the center, or core, of the Earth. The least dense materials are on top of the Earth. The air, which is the least dense Earth material, is located above the Earth. The Atmosphere The least dense layer of the Earth is the atmosphere. The atmosphere is the layer of the Earth that contains all of the oxygen and other gases in the air around us. It extends several miles above the Earth s surface. The reason our atmosphere is above the Earth is because it is far less dense than each of the other layers. The atmosphere has a density of less than.0013 g/cm3. Remember, the densest materials in the Earth tend to be the closest to the core. Likewise, the least dense materials tend to be located on the surface and above the surface of the Earth. The Crust The crust is less than 1% of Earth by mass. The crust is the least dense of Earth s solid layers. The crust is the upper part of the Earth where most life exists. The crust ranges in thickness from about 5 to 100 km. The thickest sports are on land and are called continental crust. The thinner parts of the crust are under the ocean, called oceanic crust.
Page 15 The crust and the very top layers of the mantle compose a layer called the lithosphere. The top layer of the mantle is included with the lithosphere because it is solid and does not flow like the asthenosphere. The lithosphere contains the tectonic plates. The most abundant elements in the crust are oxygen, silicon, and aluminum. Many other elements are present as well, but in lower quantities. The density of earth s crust is between 2.7 and 3.0 g/cm3. Since water has a density of 1.0 g/cm3, it sits on top of the crust and fills in any seams, cracks, and empty areas that may be present. Inside the Earth Earth layers from Google Remember, this is an average only. The Core The very center part of the Earth is called the core. The core is mostly iron metal. Earth s core is divided into the inner core and the outer core. The inner core of the Earth is a solid ball and is made up mostly of iron and nickel. These are very dense materials. The inner core is about 1,230 km in diameter. The outer core, like the inner core, is mostly iron and nickel, but the outer core is a liquid and is about 2,200 km
Page 16 thick. Together the core makes up about 31% of the Earth. Remember, the inner core is the densest layer of the Earth. This animation shows the layers by composition and by mechanical properties: http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_layers.html Water Crust Mantle Outer core Inner core Layer Approximate density 1.0 g/cm3 2.7-3.0 g/cm3 3.3-5.7 g/cm3 9.0-12.0 g/cm3 12.7-13.0 g/cm3 You are given the following materials and their densities and then asked to construct a model of the Earth consisting of a core, mantle, crust, water, and air. Based on density, which of the above materials would best represent the Earth s core? Cotton Glue Clay Aluminum foil A Nickel Substance 0.2 g/cm3 1.0 g/cm3 1.8 g/cm3 2.4 g/cm3 4.6 g/cm3 Approximate density Which did you choose? Should have been a nickel for the core. How deep can we go into Earth s Interior? Not very deep, that s for sure! The deepest a drill hole has gone was the Kola Superdeep Borehole. That hole got to 40,230 feet (12,262 m), about one-third of the way into the crust in that area. Learning About Earth s Interior If someone told you to figure out what is inside Earth, what would you do? How could you figure out what is inside our planet? How do scientist figure it out? Do they use
Page 17 density, temperatures, depth? Well let s give you some more information and then we will get to more answers. Density and Earth s Layers Atmosphere-above the Earth s surface of course! The density is less than.001 g/cm3 The common materials are: Nitrogen 78% Oxygen 21% Other gases 1%
Water Page 18 The density is 1.0 g/cm3 The Crust Where we walk. The density is 2.7-3.0 g/cm3 The common materials are: 46.6% Oxygen 27.7% Silicon 8.1 % Aluminum The Mantle The density is 3 9 g/cm3 The common materials are: Magnesium Silicon Oxygen Iron The Core (inner & outer) The density is 9-13 g/cm3 The common materials are: Iron Nickel FYI Temperature increases as you get closer to the center of the Earth. If both the inner and outer core are made of similar materials then why would the inner core be solid and the outer core liquid even though the inner core was hotter? In order for a solid to turn to a
Page 19 liquid it must be able to expand. The weight of all of the upper materials on the inner core is so intense that it cannot overcome the pressure and expand. Therefore, it must remain solid. If the pressure were to ease up, the inner core would expand and liquefy. The core of the Earth is tightly compacted. The density of the core ranges between 9 and 13 g/cm3. The core contains about 33% of Earth s total mass. Before we give you any more information about the layers of Earth, let s go back to the question of How do we know what is inside the Earth? Here is how: Seismic Waves Geologists study earthquake waves to see Earth s interior. Waves of energy radiate out from an earthquake s focus. These waves are called seismic waves (see the figure below from a google image). Seismic waves go different speeds through different materials. They change speed when they go from one type of material to another. The properties of seismic waves allow scientists to understand the composition of Earth s interior.
Page 20 This change from one type of material to another causes the waves to bend. Some seismic waves do not travel through liquids or gases. They just stop. Scientists use information from seismic waves to understand what makes up the Earth s interior. They can study the inside without having to go there! To understand what these waves are like, fill a tube with water and drop a rock into the water. When an earthquake occurs, the energy waves move out from the point of the quake in much the same way. There are two types of seismic waves, S waves and P waves. These waves act differently in solids and liquids. When P waves pass through liquids they slow down. They pick up speed when they reach a solid on the other side of the liquid. The S waves stop completely in liquids. Waves also travel faster in materials that are denser. Scientists look at data that they collect from earthquakes and can determine the composition of earth s interior by how the waves slow down, speed up, and disappear. Models of the Earth: Accurate or Inaccurate? Models are representations of a concept or process that help teach about the principle. They can be very effective ways to study the Earth and lean about the relationships of various parts of the Earth. You can study a map in order to learn more about how countries or landscapes are situated. A globe, which is another type of model, can help you to have a better understanding of how the Earth is organized. There are many types of maps and other Earth models that you can learn from. Models can have drawbacks as well. A flat map cannot correctly show exactly what a round world looks like. Likewise, a round globe can never show the detail that a flat section of a map can show. Basically, models can show one or two things very well, but in order to really understand, a series of different types of models would be best. It is always important to keep in mind the strengths and weaknesses of each type of model that you are using. Meterorites Scientists study meteorites to learn about Earth s interior. Meteorites formed in the early solar system. These objects represent early solar system materials. Some meteorites are made of iron and nickel. They are thought to be very similar to Earth s core. An iron meteorite is the closest thing to a sample of the core that scientist can hold in their hands! Density Earth s overall density is higher than the density of rocks on the crust, so the core must be made of something denser, like metal.
Magnetic Field Since Earth has a magnetic field, there must be metal within the planet. Iron and nickel are both magnetic. Continuing with the Layers of the Earth Page 21 If you recall, we covered the atmosphere, crust and the core on pages 14 and 15. We also gave you a small amount of information on the mantle. However, there is more on the mantle that you should know. Please read on. The Mantle The part of the Earth that lies between the core and the surface is the mantle. The mantle is about 2,900 km thick and is composed of upper and lower parts. About 67% of Earth s mass is located in the mantle. The mantle is located far enough below the crust that no one has been able to go there and study it. Scientists must rely on interpreting data that they have in order to study the mantle. Volcanoes offer some help in studying the mantle. Since magma from volcanoes, comes from the mantle, scientists can use lava flows and magma domes to see what materials are present in the mantle. From these sources we know that magnesium is one of the more common materials found in the mantle. The mantle is divided into the mesosphere and the asthenosphere. The mesosphere is about 2,550 km thick and is made of rock that flows very slowly. This rock can best be explained as a semi-solid. It is plasticky, having the consistency of peanut butter. Heat in the asthenosphere circulates in convection currents. Scientists believe that, since the Earth s plates sit on top of the asthenosphere, they ride these currents. It is these currents that are responsible for the movement in plate tectonics. The mantle is a fairly dense region of Earth s interior but nowhere near as dense as the core. The density of the mantle ranges between 3 and 9 g/cm3. Convection Currents
Page 22 Cooling material is pushed away. The material becomes more dense as it cools, causing it to sink. Once the material is down by the heat being generated by the core, it heats up and expands and becomes less dense which causes it to rise. At the top, the material cools again and becomes denser and again begins to sink back down. It repeats this cycle.
Page 23 To sum up the convection currents in the mantle, heated material rises because it is less dense than the cooler material around it. As the material rises, it cools and is pushed out by the hotter material rising below it. Eventually, it cools and sinks. As it sinks it is heated again and is pulled to the center to fill the space left by the rising heated material. This is how the tectonic plates on Earth are moving!! Glossary NOTE: You will need to copy each term down, number and underline each, and write the definition into your science journal under the date and title of Vocabulary for Earth s density. 1. Atmosphere: The various layers of air that surround Earth. The densest atmospheric layers are closer to Earth, and the least dense layers are farther away from Earth.
Page 24 2. Crust: The solid outer layer of Earth where life is found. This is the thinnest layer of Earth. 3. Density: A comparison of the Mass and Volume of an object. Density is calculated as mass divided by Volume. (Remember the heart drawing) A dense object or liquid tends to sink in a less dense liquid. 4. Inner Core: The centermost layer of Earth. This solid metal layer is the hottest and most dense layer. 5. Mantle: A semi-solid layer of Earth that takes up the most volume of Earth. This layer is found directly below Earth s crust. 6. Mixture: A combination or blend of two or more substances that have not chemically combined. Each substance maintains its own identity. 7. Outer Core: A liquid layer of Earth found near the center. 8. Inner core: This layer is made of metal. 9. Particle: A small piece of something. Typically used to represent a small part of matter. 10. Sorting: The process that separates particles based on differences in density and/or particle size.