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*****If the rate at which shoppers move from the first floor to the second is equal to the rate at which shoppers move from the second floor to the first, then the number of shoppers on each floor remains constant. Applying Concepts Is it necessary that an equal number of shoppers be on each floor? Explain.

**Molecules of SO2 and O2 react to give SO3 . Molecules of SO3 decompose to give SO2 and O2 . At equilibrium, all three types of molecules are present in the mixture.

**These graphs show how the concentrations of O2, SO2, and SO3 vary with time. Left: Initially, SO2 and O2 are present. Right: Initially, only SO3 is present. INTERPRETING GRAPHS a. Navigate Where on the graphs can you find the initial concentrations of the reactants and products? The equilibrium concentrations? b. Read Which gas is most abundant at equilibrium? c. Interpret How do the equilibrium concentrations of O2, SO2, and SO3 compare?

******The rapid exhalation of CO2 during and after vigorous exercise helps reestablish the bodys correct CO2:H2CO3 equilibrium. This keeps the acid concentration in the blood within a safe range.

*Dinitrogen tetroxide is a colorless gas; nitrogen dioxide is a brown gas. The flask on the left is in a dish of hot water; the flask on the right is in ice. Interpreting Illustrations How does an increase in temperature affect the equilibrium of a mixture of these gases?

*Pressure affects a mixture of nitrogen, hydrogen, and ammonia at equilibrium. a) The system is at equilibrium. b) Equilibrium is disturbed by an increase in pressure. c) A new equilibrium position is established with fewer gas molecules. Interpreting Diagrams What effect does a decrease in volume have on the number of gas molecules?

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