Chapter Twelve: Properties of Matter 12.1 Properties of Solids 12.2 Properties of Fluids 12.3 Buoyancy

12.1 Properties of Solids Different kinds of matter have different characteristics. Characteristics that can you observe directly are called physical properties. Physical properties include color, texture, density, brittleness, and state (solid, liquid, or gas). Ex. Iron is solid at room temp.

12.1 Properties of Solids A physical change is any change in the size, shape, or phase of matter in which the identity of a substance does not change. For example, when water is frozen, it changes from a liquid to a solid.

12.1 Properties of Solids Properties that can only be observed when one substance changes into a different substance are called chemical properties. Any change that transforms one substance into a different substance is called a chemical change. Ex. If you leave a nail outside, it rusts.

12.1 Properties of Solids The density of a solid material depends on two things: the individual mass of each atom or molecule, how closely the atoms or molecules are packed together. Carbon atoms in diamond are packed very tightly.

12.1 Properties of Solids Paraffin wax is also mostly carbon, but its density is only 0.87 g/cm3. Paraffin’s carbon atoms are mixed with hydrogen atoms in long molecules that take up more space. The density of paraffin is low compared to diamond.

12.1 Properties of Solids The atoms or molecules in a solid are arranged in two ways. If the particles are arranged in an orderly, repeating pattern, the solid is crystalline. If the particles are arranged in a random way, the solid is amorphous.

12.1 Properties of Solids Examples of crystalline solids include salts, minerals, and metals.

12.1 Properties of Solids Metals don’t look like “crystals” because solid metal is made from very tiny crystals fused together in a jumble of different orientations.

12.1 Properties of Solids The atoms or molecules in amorphous solids are randomly arranged. Examples of amorphous solids include rubber, wax, and glass.

12.1 Mechanical properties “Strength” describes the ability of a solid object to maintain its shape even when force is applied.

12.1 Mechanical properties Tensile strength is a measure of how much stress a material can withstand before breaking.

12.1 Mechanical properties Hardness measures a solid’s resistance to scratching. How might you compare the hardness of these two metals?

12.1 Mechanical properties Elasticity describes a solid’s ability to be stretched and then return to its original size. Brittleness is defined as the tendency of a solid to crack or break before stretching very much.

12.1 Mechanical properties A ductile material can be bent a relatively large amount without breaking. The ductility of many metals, like copper, allow then to be drawn into wire.

12.1 Mechanical properties Malleability measures a solid’s ability to be pounded into thin sheets. Aluminum is a highly malleable metal.

12.1 Mechanical properties Almost all solid materials expand as the temperature increases. The increased vibration makes each particle take up a little more space, causing thermal expansion. Sidewalks and bridges have grooves that allow for thermal expansion.

12.2 Properties of Fluids A fluid is defined as any matter that flows when force is applied. Liquids like water or silver are kinds of fluid.

12.2 Pressure A force applied to a fluid creates pressure. Pressure acts in all directions, not just the direction of the applied force.

12.2 Forces in fluids Forces in fluids are more complicated than forces in solids because fluids can change shape.

12.2 Units of pressure The units of pressure are force divided by area. One psi is one pound per square inch.

12.2 Units of pressure The S.I. unit of force is the pascal. One pascal (unit of force) is one newton of force per square meter of area (N/m2).

12.2 Pressure If your car tires are inflated to 35 pounds per square inch (35 psi), then a force of 35 pounds acts on every square inch of area inside the tire. What might happen if you over-inflate a tire?

12.2 Pressure On the microscopic level, pressure comes from collisions between atoms. Every surface can experience a force from the constant impact of trillions of atoms. This force is what we measure as pressure.

12.2 Pressure In a car engine high pressure is created by an exploding gasoline-air mixture.

12.2 Energy conservation and Bernoulli’s Principle Streamlines are imaginary lines drawn to show the flow of fluid. Bernoulli’s principle tells us that the energy of any sample of fluid moving along a streamline is constant.

12.2 Bernoulli’s Principle Bernoulli’s principle says the three variables of height, pressure, and speed are related by energy conservation.

12.2 Three Variables and Bernoulli’s Principle If one variable increases along a streamline, at least one of the other two must decrease. For example, if speed goes up, pressure goes down.

12.2 The air foil One of the most important applications of Bernoulli’s principle is the airfoil shape of wings on a plane. When a plane is moving, the pressure on the top surface of the wings is lower than the pressure beneath the wings.

12.2 Viscosity Viscosity is the property of fluids that causes friction. Viscosity is determined in large part by the shape and size of the particles in a liquid.

12.2 Viscosity and temperature As the temperature of a liquid increases, the viscosity of a liquid decreases. Increasing the kinetic energy of the substance allows the particles to slide past one another more easily.

12.3 Buoyancy is a force Buoyancy is a measure of the upward force a fluid exerts on an object that is submerged. The water in the pool exerts an upward force that acts in a direction opposite to the boy’s weight.

12.3 Volume and buoyancy The strength of the buoyant force on an object in water depends on the volume of the object that is underwater. As you keep pushing downward on the ball, the buoyant force gets stronger and stronger. Which ball has more volume underwater?

12.3 Weight and buoyancy Weight is a force, like any other pushing or pulling force, and is caused by Earth’s gravity. It is easy to confuse mass and weight, but they are not the same. Weight is the downward force of gravity acting on mass. What is the rock’s weight? What is the rock’s mass?

12.3 Weight and buoyancy In the third century BC, a Greek mathematician named Archimedes realized that buoyant force is equal to the weight of fluid displaced by an object. A simple experiment can be done to measure the buoyant force on a rock with a spring scale when it is immersed in water.

12.3 Weight and buoyancy In air the buoyant force on the rock is 29.4 N. When the rock was submerged, the scale read 19.6 N. The difference is a force of 9.8 N, exactly the amount of force the displaced water exerts.

12.3 Weight and buoyancy These blocks are the same total volume. Which block has more buoyant force acting on it? Which block weighs more in air?

12.3 Weight and buoyancy Buoyancy explains why some objects sink and others float. Whether an object sinks or floats depends on how the buoyant force compares with the weight.

12.3 Density and buoyancy If you know an object’s density you can quickly predict whether it will sink or float. Which ball will sink in water? Which ball will float in water?

12.3 Density and buoyancy Average density helps determine whether objects sink or float. An object with an average density GREATER than the density of water will sink. An object with an average density LESS than the density of water will float.

12.3 Density and buoyancy What can you say about the average density of these blocks?

12.3 Density and buoyancy When they are completely underwater, both balls have the same buoyant force because they displace the same volume of water.

12.3 Boats and average density Use your understanding of average density to explain how steel boats can be made to float.

12.3 Boats and average density If you have seen a loaded cargo ship, you might have noticed that it sat lower in the water than an unloaded ship nearby. This means a full ship must displace more water (sink deeper) to make the buoyant force large enough to balance the ship’s weight.