Which statement best explains why a hot-air balloon rises when the air in the balloon is heated? a) According to Charles's law, the temperature of a gas is directly related to its volume. Thus the volume of the balloon increases, making the density smaller. This lifts the balloon. b) Hot air rises inside the balloon, an this lifts the balloon. c)The temperature of a gas is directly related to its pressure. The pressure therefore increases, and this lifts the balloon. d) Some of the gas escaped form the bottom of the ballon, thus decreasing the mass of the gas in the balloon. The decreases the density of the gas in the balloon, which lifts the balloon. e) Temperature is related to the root mean square welocity of the gas molecules. Thus the molecules are moving faster, hitting the balloon more, and thus lifting the balloon. 

Expert Answers

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The statement that best explains why a hot air balloon rises is A. As the air inside the balloon is heated its volume increases, inflating the balloon more fully. Having a greater volume makes it less dense than the surrounding air so it floats upward. 

Interestingly, Jacques Charles for whom Charles' law was named was a balloonist. He launched the first unmanned hydrogen balloon in the late 1700s. The hydrogen balloon rose in the atmosphere for the same reason, it was less dense than the surrounding air but due to hydrogen's lighter mass rather than its temperature. 

Charles, a French chemist, experimented with gas volume and temperature by filling balloons with different gases to the same volume then lowering the temperature of all of the balloons by the same amount. He found that the volumes of all of the gases decreased by the same amount. 

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