17.2 and 17.3 Classifying Matter Liquids. Liquids

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1 17.2 and 17.3 Classifying Matter Liquids Read p in book Liquids Liquids have an indefinite shape, but a definite volume. the same shape as their container. particles that are close together, but mobile. particles that move slowly. 2

2 Density Density compares the mass of an object to its volume. is the mass of a substance divided by its volume. Density Expression Density = mass = g or g = g/cm 3 volume ml cm 3 Note: 1 ml = 1 cm 3 3 Densities of Common Substances (at 4 C) 4

3 Density and Temperature For most substances, as temperature increases, density decreases. D Why? Which is less dense, gas or a liquid? Gas In which state of matter do molecules move faster? Molecules move faster in gas When temperature is increased, molecules in a liquid move faster, making it less dense (more like a gas).

4 Exception: Water Solid water (ice) is less dense than liquid water. This is the result of the crystal structure of ice. It has lots of open space in it. Buoyancy of Fluids The forces from pressure acting on the bottom of this golf ball are greater than those on the top. This produces a net force called the buoyant force that acts upward on the ball.

5 Buoyant Force What is the effect of buoyancy on the apparent weight of an object? Buoyancy is the ability of a fluid to exert an upward force on an object placed in it. Buoyancy results in the apparent loss of weight of an object in a fluid. Archeimedes Principle According to Archimedes principle, the buoyant force on an object is equal to the weight of the fluid displaced by the object. A submerged object pushes aside, or displaces, a volume of fluid equal to its own volume. A floating object displaces a volume equal to the volume of the part of the object that is submerged.

6 Density and Buoyancy How can you determine if an object will float or sink in a fluid? If an object is less dense than the fluid it is in, it will float. If the object is more dense than the fluid it is in, it will sink. When the buoyant force is equal to the weight, an object floats or is suspended. When the buoyant force is less than the weight, the object sinks. Suspended An object that has the same density as the fluid it is submerged in will be suspended (it will float at any level) in the fluid. The buoyant force acting on the suspended object exactly equals the object s weight. Submarines and some fish are able to suspend themselves in water partly by adjusting their density.

7 Sinking When a ship s weight becomes greater than the buoyant force acting on it, the ship will sink. As a sinking ship takes on water, the ship displaces less water, and the buoyant force decreases. Floating A solid piece of steel sinks in water. A heavy steel ship floats because of the shape of its hull. The hull is shaped so that it displaces a large volume of water, creating a large buoyant force.

8 Assessment Question An unknown substance is suspended in water. What can be concluded about the substance s density? a. The substance s density is greater than water s density. b. The substance s density is less than water s density. c. The substance s density is equal to water s density. d. Nothing can be concluded about the substance s density. Assessment Question An unknown substance is suspended in water. What can be concluded about the substance s density? a. The substance s density is greater than water s density. b. The substance s density is less than water s density. c. The substance s density is equal to water s density. d. Nothing can be concluded about the substance s density. ANS: C

9 Volume by Displacement A solid completely submerged in water displaces its own volume of water. The volume of the object is calculated from the difference in volume ml ml = 9.5 ml = 9.5 cm 3 Copyright 2009 by Pearson Education, Inc. 17 Learning Check What is the density (g/cm 3 ) of 48.0 g of a metal if the level of water in a graduated cylinder rises from 25.0 ml to 33.0 ml after the metal is added? 1) 0.17 g/cm 3 2) 6.0 g/cm 3 3) 380 g/cm ml 33.0 ml object 18

10 Solution Given: 48.0 g Volume of water = 25.0 ml Volume of water + metal = 33.0 ml Need: Density (g/ml) Plan: Calculate the volume difference in cm 3 and place in density expression ml ml = 8.0 ml 8.0 ml x 1 cm 3 = 8.0 cm 3 Set Up Problem: 1 ml Density = 48.0 g = 6.0 g = 6.0 g/cm cm 3 1 cm 3 19 Sink or Float Ice floats in water because the density of ice is less than the density of water. Aluminum sinks because its density is greater than the density of water. Copyright 2009 by Pearson Education, Inc. 20

11 Learning Check Which diagram correctly represents the liquid layers in the cylinder? Karo (K) syrup (1.4 g/ml); vegetable (V) oil (0.91 g/ml); water (W) (1.0 g/ml) V W K W K V K V W 21 Solution 1) V W K vegetable oil 0.91 g/ml water 1.0 g/ml Karo syrup 1.4 g/ml 22