Why does 70° water feel cool while 70° air feels warm? Also, why does paper turn yellow when it ages? asks Markus Sher, a student in Brookville, NY.
Take a dip in a 70-degree ocean, and you’re likely to gasp. The water feels shockingly cold. And yet the same 70 degrees F. can feel comfortably warm on a summer or fall day. What’s up with that?
The strange difference we feel between water and air at the same temperature is due to the movement of heat. How? Heat naturally flows from warmer objects to cooler objects. (That’s why an oven set at 400 degrees radiates heat into the kitchen, warming the air.) And your body is a small furnace, its thermostat set at about 98.6 degrees F. So in 70-degree air or water, your nearly 100-degree body begins to lose heat.
Water, it turns out, is much better than air at conducting heat. When you wade into a 70-degree ocean, heat escapes through your skin much faster than it would in 70-degreee air. (Which is what make air such a good insulator between layers of clothing in winter.) Meanwhile, water has a greater capacity to hold heat energy than does air. There are far more molecules of water packed into the layer surrounding your skin than there are gas molecules in thin air. So while your body rapidly loses heat, the water temperature around you doesn’t change much. The water stays cool, and you feel extra-cold.
For the same reason, the water in a hot tub set at 102 degrees F. — about 3 or 4 degrees above normal body temperature — will feel hotter than 102-degree air. Your sitting-still body will heat up faster in hot water than in hot air.
And if you pick up an old book to read while you’re relaxing in your hot tub, you may notice that the pages are yellowed. This time, it’s air that’s the culprit.
Earth’s air is a mix of gases — about 77 percent nitrogen and 21 percent oxygen, plus traces of argon, carbon dioxide, helium, neon, krypton, xenon, carbon monoxide, and others. Humans and other animals are fond of oxygen, since it’s the gas that keeps our bodies running.
But oxygen has another, less friendly side. When oxygen from the air combines with other elements, such as hydrogen and carbon, there’s a reaction called oxidation. Oxidation makes organic molecules, which are the basis of life, fall apart.
At normal temperatures, oxygen combines slowly with other elements. The reaction produces heat, in amounts too small for us to notice. But oxidation can happen quickly if the temperature is raised. Strike a match, and the friction between the match head and the strip of sandpaper on the matchbook heats up the match. Oxidizing very quickly, the match bursts into flames.
When book pages turn yellow over time, they are oxidizing — doing a slow burn. We can’t feel the heat, but we can see the dingy result. In the end, the pages darken to brown, as if they’d been singed in a fire, and crumble to dust.
