46 . Why is alizarin a good acid-base indicator, whereas phenol is not?

47. Why is the ability to absorb visible light useful to living organisms? In what way or ways is this ability especially useful to plants, and in what different way is it useful to animals?

48. One objection to the supersonic transport (SST) is that waste oxides of nitrogen emitted at high altitudes by the SST would combine with, and slowly destroy, the high-altitude ozone layer around the planet. Recent calculations have shown that a thermonuclear war would practically eliminate the ozone layer. Why would the destruction of the ozone layer be dangerous to life (in addition to the immediate hazards of the atomic war)?

49. What is the approximate wavelength range of the radiation that reaches the surface of the Earth at the present time? What absorbs the longer and shorter wavelengths and prevents them from reaching the surface?