DENSITY, VISCOSITY, and BUOYANCY Adapted from TAKS Study Guide IPC 7A

O.K., what is density? Density is a measure of a substance’s mass per unit of volume. A dense object has much more mass in a given space than an object that isn’t very dense. IPC 7A

Density example #1: Aluminum isn’t very dense. That’s why empty aluminum soda cans don’t have much mass. If soda cans were made out of a dense metal, such as gold or lead, they would be much heavier than aluminum cans, even if they were the same size. IPC 7A

Density example #3: Suppose you are going to buy a gold necklace at a discount jeweler. The clerk claims that the necklace is made of pure gold. How could you tell if he is telling the truth? Calculate the density of the necklace and compare it to the density of pure gold. IPC 7A

How do I solve density problems? You can calculate an object’s density by dividing its mass by its volume. Density has 2 different units: g/mL or g/cm3 g= grams (mass) mL= milliliters (volume) cm3= cubic centimeters (volume) IPC 7A

Density calculation #1 A graduated cylinder containing 20 milliliters of mineral oil has a mass of 98.2 grams. The mass of the empty cylinder is 79.8 grams. What is the density of the mineral oil? Given: a. Empty Cylinder Mass 79.8 g b. Mass of cylinder of mineral oil 98.2 g c. Mass of mineral oil (b-a) 18.4 g d. Volume of mineral oil 20 mL Unknown: D = ? Equation: D = m/v Substitute: D = 18.4 g/20mL Solution: D = 0.92 g/mL IPC 7A

Viscosity Viscosity is the resistance of a liquid to flow. Water is less viscous than honey, for example, because water flows more easily. More viscous Less viscous IPC 7A

Viscosity Two factors that help determine a liquid’s viscosity are its cohesiveness and the size of its molecules. The tendency of water molecules to stick together—water’s cohesiveness—makes water more viscous than it would be if it were composed of nonpolar molecules. One reason that water is less viscous than honey, however, is that the molecules in honey are much larger than water molecules and flow past one another less easily. IPC 7A

And what about buoyancy? What’s that? When an object is submerged in water, the water exerts a force on all sides of the object. This force increases with depth, so the force at the top of the object is lower than the force at the bottom of the object. This means that the overall direction of the force is upward. This upward force is called the buoyant force. IPC 7A

The force at the bottom of a submerged object is greater than the force at the top. The net force – the buoyant force – is upward. When wood floats, the buoyant force on the wood is greater than the force of gravity on the wood. IPC 7A

When the buoyant force pushing up on the object is greater than the force of gravity pulling down on the object, the object rises to the surface. If the buoyant force is less than the force of gravity, the object sinks to the bottom. IPC 7A

IPC 7A

How do we know which is greater, the force of gravity or the buoyant force? We can determine which force is greater by comparing the density of the object to the density of water. If the density of the object is greater than the density of water, the force of gravity on the object will be greater than the buoyant force, and the object will sink. If the density of the object is less than the density of water, the force of gravity on the object will be less than the buoyant force, and the object will float. IPC 7A

Buoyant Force or Force of Gravity The greater force? Buoyant Force or Force of Gravity For example, vegetable oil with a density of 0.93 g/cm3 will float on water, which has a density of 1.0 g/cm3. 0.93 g/cm3 < 1.0 g/cm3 IPC 7A

Buoyant Force or Force of Gravity The greater force? Buoyant Force or Force of Gravity For example, wood with a density of 0.4 gram per cubic centimeter will float on water, which has a density of 1.0 gram per cubic centimeter. 0.4 g/cm3 < 1.0 g/cm3 IPC 7A