Matter, Atoms & Molecules

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1 Matter, Atoms & Molecules Matter is anything that has mass and takes up space. All matter is made of tiny particles called atoms, which are too small to see with the naked eye. Matter Matter is anything that has mass and takes up space. It can exist in any phase (solid, liquid, or gas). The parts that make up matter are too small to be seen without magnification. Everything that exists is made up of matter, including air. Air is a type of matter that constantly surrounds us. Even though it is all around us, we cannot see air because it is a colorless gas. However, we can see that it takes up space when we use it to fill balloons. We also know that it has mass because it can be measured. Atoms Atoms are the tiny particles that make up all matter. An atom cannot be broken down into smaller pieces using chemical reactions or a physical change. Both living and non-living things are made of atoms. A flower, for example is made up of many living cells, but each cell is made up of trillions of atoms. Atoms are not alive. Elements The cells in living organisms are made up of tiny particles called atoms. There are over 100 different kinds of atoms. A substance made up of only one kind of atom is called an element. For example, the element helium is made up of only helium atoms.

2 The element helium is a usually a gas. The particles in helium gas are helium atoms. Molecules If two or more atoms join together, they form a connection called a bond. Particles made up of two or more bonded atoms are called molecules. Sometimes, atoms of the same element are bonded together. Other times, atoms of different elements are bonded together. For example, each molecule of carbon dioxide contains exactly two oxygen atoms and one carbon atom. A drawing of the molecules of carbon dioxide gas is shown below. States of Matter Matter on Earth is generally found in one of three different states: solid, liquid, or gas. The state that a substance is in at any particular time depends on the arrangement of the particles, the attractions between particles, and the amount of energy the particles possess. A substance's state of matter is a physical property. The particle arrangement within each state solid, liquid, gas is represented in the diagrams below. Temperature and Particle Motion The particles that make up a substance are always in motion. The temperature of the substance is a measure of the average energy of motion, or kinetic energy, of the particles in

3 that substance. When the average motion of the particles of a substance decreases, the temperature of that substance decreases. Likewise, an increase in the average motion of the particles increases the temperature of the substance. For example, if a solid, room-temperature object is placed in a refrigerator, the substance will lose energy and the particles will begin to slow down. The temperature of the object will decrease. Likewise, if a solid, room-temperature object is placed on a hot plate, the substance will gain energy, and the particles will speed up. The temperature of the object will increase. Solids The particles of a solid are generally arranged very close together in an ordered, regular pattern. Solid particles remain in fixed positions, primarily due to the strong attractive forces that exist between them. Solids have definite shapes and definite volumes. This is because the particles in solids cannot move past each other; they cannot move very far at all. The solid phase of a particular substance generally has a higher density than the liquid phase of the same substance because the particles in the solid are closer together. Water is an important exception to this generalization. Ice is less dense than liquid water, which is why ice floats in water. This bar of solid gold metal holds its shape. Gold is very dense because its particles are packed tightly together. Many solids have a crystalline structure. A crystal is a form a solid takes on when a collection of atoms is repeated in the same arrangement over and over again throughout the substance. Liquids Liquids have definite volumes, but they do not have definite shapes. This is because the particles in a liquid are free to move and flow around one another. The particles in liquids are arranged in a less orderly way than the particles of solids. Liquid particles have fewer attractive forces between them, so they do not hold together as strongly as solids do. The particles in a liquid have more energy than particles of the same substance in the solid phase. This means that particles in a liquid can move farther and faster. Since the particles of a liquid are more spread out, a liquid is generally less dense than the same substance as a solid, but liquids are still denser than gases. Gases Gases do not have definite shapes or definite volumes. Gas particles are generally very far apart, and they move very rapidly. They spread out to fill all the space that is available. The particles in a gas have more energy than the particles of the same substance in the solid or liquid state. Gas particles have enough energy to completely overcome the attractive forces between their particles. This is why their particles are located so far apart. Even though they

4 are located far apart and move very rapidly, gas particles can still touch each other when they collide. Heat & Changes in Matter The properties of a substance can change when heat is added or removed. Thermal energy is the energy a substance has due to the kinetic energy of the particles that make up the substance. The temperature of a substance is a measurement of the average kinetic energy of these particles. Changes in thermal energy can be observed by measuring a substance's temperature. An increase in temperature indicates an increase in thermal energy, and a decrease in temperature indicates a decrease in thermal energy. When thermal energy is transferred from one object to another, it is called heat. If the addition or removal of heat causes the temperature of a substance to change enough, the substance may go through a phase change. Some of the processes by which a substance undergoes a change of state include melting, freezing, vaporizing, and condensing. Melting When enough heat is added to a solid substance, the substance can reach its melting point, which is the temperature at which it transitions from a solid to a liquid state. When in its solid state, a substance's particles occupy a fixed volume or nearly fixed volume. The particles cannot move and flow past one another. When in its liquid state, a substance's particles are able to move past one another, and they generally occupy a larger volume. The volume, however, is still somewhat fixed. Freezing Freezing is the reverse of the melting process. When heat is removed from a liquid substance, its particles lose energy and slow down.

5 When enough heat is removed from a liquid substance, the substance freezes and becomes a solid. During this transition, the substance's particles move closer together until the attractions between them become great enough to hold the particles in place. They are then no longer able to move past one another at all. Vaporizing When enough heat is added to a liquid substance, it evaporates, changing from a liquid to a gas. In the gaseous state, the substance's particles move faster and farther than they did in the liquid state. Gaseous substances expand to occupy the volume of the container if the container they are in is closed. If the container is open, a gas will expand until it has diffused into the gases of the atmosphere. Condensing Condensing is the reverse process of vaporizing. Condensation occurs when enough heat is removed from a gaseous substance to cause the substance to change state and become a liquid. In the gaseous state, a substance's particles are not attached to each other, so the substance does not occupy a fixed volume. Once the substance condenses into the liquid state, its particles have to stay close together, so they occupy a fixed volume.

6 Force & Motion The motion of an object can be changed by an unbalanced force. The way that the movement changes depends on the strength of the force pushing or pulling and the mass of the object. Forces have both a direction and a magnitude (size). Force & Mass A force is a push or a pull that acts on an object and may change its motion. Forces are needed to start, stop, or change the direction of an object's motion. The more force that is applied to an object, the greater the change in the object's motion. In other words, the harder an object is pushed, the faster it will move. But, if the same amount of force is used to move two objects with different masses, the object with less mass will move faster. For instance, if the woman in the picture below pushed two different lawn mowers using the same amount of force, the lawn mower with less mass would move faster. Friction Friction is a force that opposes motion. If an object is already moving, friction can slow it down or make it stop. For example, when a box is slid across the ground, it will only travel a certain distance before it stops. Friction is the force that brings it to a stop. Objects on rough surfaces require more force to move. Objects on smooth surfaces require less force. Gravity Gravity is the force of attraction that exists between any two objects that have mass. Gravity keeps the planets orbiting around the Sun, and it pulls objects on Earth toward the Earth's center. For example, gravity causes fruit on a tree to fall to the ground. Gravity depends on the mass of the objects and the distance between them. The gravity we feel from Earth is very strong because we are close to the Earth and the mass of the Earth is very large.

7 Net Force A net force is the total unbalanced force acting on an object. A net force has a certain strength, or magnitude, and a direction. If the net force is not zero, then it will cause the object to speed up, slow down or change direction. Balanced forces have equal magnitudes (sizes) but opposite directions. When the forces on an object are balanced, the object does not accelerate because the net force is zero. When this happens, the object can either be moving at a constant velocity or at rest. The apple on the desk below is pulled downward by the force of gravity. This force equals the weight of the apple. The table simultaneously exerts a normal force that is equal in magnitude but opposite in direction pushing up on the apple. Since these forces are balanced, the apple does not accelerate in either direction. And since the apple was not in motion to begin with, it remains at rest. Unbalanced forces have unequal magnitudes and/or directions. Unbalanced forces result in net forces that are not zero. An unbalanced force will change an object's speed and/or direction. When forces are unbalanced, the change in movement will take place in the direction of the net force. Newton's First Law - Inertia Newton's First Law of Motion is also known as the law of inertia. It states that an object in motion stays in motion, and an object at rest remains at rest, unless acted on by an unbalanced outside force. Inertia is a property of matter that describes the tendency of an object to resist changes in its state of motion.

8 Newton's First Law of Motion According to Newton's first law of motion, if a ball is rolled across the floor, it should continue rolling across the floor in a straight line forever, unless acted upon by some other unbalanced force. From experience, we know that balls do not continue rolling forever. But, this is because they are acted upon by the unbalanced force of friction, which causes them to slow down and eventually stop. Or, according to Newton's 1st law, if a box is at rest on the floor, it will remain motionless on the floor forever, unless acted upon by some unbalanced force. So, if someone came along and pushed the box, the box would move because the person provided an unbalanced force. Inertia is dependent on the mass of an object. The more massive the object, the greater its inertia the more force is required to change its motion. It takes more force to change the motion of more massive objects. Newton's Second Law of Motion Newton's second law states that an unbalanced force will cause an object to accelerate according to the following equation: According to Newton's second law of motion, whenever the sum of the forces acting on an object is nonzero (meaning the forces acting on it are unbalanced), the object will accelerate in the direction of the net force. Acceleration is a change in an object's speed or direction of motion over time. So if an object is accelerating in a certain direction that is an indication that a force is acting on the object in the same direction as its acceleration. Newton's Third Law of Motion Newton's third law of motion states that for every action there is an equal and opposite reaction.

9 One way to think of Newton's third law is that whenever one object exerts a force on another object, the second object exerts an equal force on the first object but in the opposite direction. Newton's Third Law & Net Forces Since Newton's third law suggests that all forces act in pairs, it's reasonable to wonder how anything ever moves. For example, if a horse is pulling forward on a buggy and the buggy is pulling back on the horse with the same force, the forces are balanced. And if the forces are balanced, how can anything move? The short answer is that not all of the forces have been accounted for. If the only two forces involved were the horse pulling on the buggy and the buggy pulling back on the horse, neither would move. In this case, as in many others, it is very important to identify all of the forces and note which forces are acting on which objects in order to identify a net force. In this example, the force of the buggy isn't the only force acting on the horse. As the horse walks, it pushes backward against the ground, and the ground pushes forward on the horse. These forces are also balanced, but they are acting on different objects. The horse's push acts on the ground, and the ground's push acts on the horse. The result is that the two are accelerated away from each other. However, since the Earth is so incredibly massive compared to the horse and buggy, the effect of the horse's push against it has almost no effect, while the horse and buggy visibly move forward. Although the forces are balanced, the pushing forces between the horse and the ground allow each to move. The horse moves much, much more because it is much, much less massive.