#5 #7 #25 #26 #27 #28 #30 #525. Which of the following is not an element: hydrogen, oxygen, sodium, chlorine, or water?
Water is not an element. Unlike the other elements listed, water may be divided into its component elements of hydrogen and oxygen by chemical methods.
7. When the element mercury is heated in air, a red powder is formed and the mass increases. Is this powder an element or a compound? Explain your reasoning.
The idea of conservation of mass tells us that a quantity of an element would remain the same if heated, although it could melt or boil. It would not increase in mass. So, the red powder must be a compound formed by the heated mercury with something from the air. In our case, it mainly forms a compound with oxygen.
25. What are the units of pressure in terms of length, mass, and time?
Pressure is the force per unit area found at some surface, whether real or "imagined." So, pressure has units of Force/Area. Force has units of (mass)(length)/time/time. Thus, the units of pressure are (mass)/(length-time)/time. If we take force in Newtons and length in meters, the units of pressure are kg/(m-s)/s.
26. A cube and a spherical ball are made of the same material and have the same mass. Which exerts the larger pressure on the floor?
Remember our definition of pressure as force/area. Since the cube and ball have the same mass, they apply equal weights downward (W=mg) on the floor. But the sphere only touches the floor at one point while the cube normally rests on an entire face. Thus, the sphere exerts the larger pressure since its area of contact is smaller.
27. Describe how a gas exerts pressure on the walls of its container.
Individual gas molecules collide with the container walls, imparting an average impulse (Ft) with each collision. The total force on a wall is the number of such collisions per unit time multiplied by the average impulse. The pressure on a wall is then the total force divided by the area of the wall.
28. Why does the pressure in a tire increase as you add more air?
Adding air to a tire increases the number of air molecules inside the tire. The average impulse that each molecule imparts to the tire wall depends mainly on the tire temperature. So, when you add air to a tire you increase the number of tire wall collisions per unit time while the average impulse per collision stays the same. From the answer to (27) above, this will increase the pressure.
30. What happens to the average speed of the molecules of a gas as it is heated?
Adding heat energy to a gas increases the average kinetic energy of random motion of the gas molecules. Using the definition of kinetic energy for a single mass as one-half times the mass times the speed squared, the speed increases when heat energy is added.
52. How does an alcohol rub cool your body?
The liquid alcohol on your skin acquires heat energy from your body and quickly comes to your skin temperature. Alcohol molecules evaporate, leaving your skin and taking their kinetic energy with them. This reduces the amount of heat energy left in your body, cooling it.