fig. 11-1 Lavoisier's experiment showed that tin, upon heating, combines with a gas from the air. (a) Before heating; (b) after heating. The tin is partly changed to a white powder, and the water level rises until only four-fifths as much air is left as there was at the start. Further heating causes no additional change.

fig. 11-2 Composition of dry air near ground level. A variable amount of water vapor is usually present also.

fig. 11-3 (a) A book raised above a table has more PE than the same book lying on the table because the attractive force the earth exerts on the book is greater when the book is closer to the earth. (b) The outer electron in an Na atom has more PE than the same electron has when it is attached to a Cl atom to form a Cl^- ion because the electron is more strongly attracted to the nucleus of the Cl^- ion (see Fig. 9-12). (c) The electron in a H atom has more PE than the same electron has when it is part of a H molecule because in the H molecule the electron is attracted by two protons rather than one proton.

fig. 11-4 Activation energy. The potential energy of the car will be converted into kinetic energy if it moves down in to the valley. However, the car requires initial kinetic energy in order to climb the hill between it and the valley, analogous to the activation energy required in many exothermic reactions.

fig. 11-5 Energy contents of various fuels. Shown are the number of kilojoules of energy liberated when 1 g of each fuel is burned. Carbohydrates provide much of the energy in our diets (see Sec. 12-11). All these fuels produce carbon dioxide and water when burned.

fig. 11-7 Silver plating. The bath is a solution of silver nitrate, AgNO₃. The nitrate ions remain in solution because Ag atoms lose electrons at the anode more readily than NO₃^- ions do.

fig. 11-8 The lead-acid storage battery. The reactions shown are those that occur at each electrode when the battery provides current. These reactions are reversed when the battery is being charged.

fig. 11-9 A hydrometer is often used to measure the density of the acid in a car's storage battery. By squeezing and then releasing the rubber bulb, the acid is drawn into a glass tube that contains a smaller sealed tube with a weight at its bottom. The higher the sealed tube floats, the denser the acid. A fully charged battery typically has an acid density of 1.28 g/cm³, which drops to about 1.15 g/cm³ when the battery is completely discharged.