Once you have proved the existence of atoms, you naturally ask what they’re up to in different situations. And that leads directly to an understanding of what heat energy actually is. We often talk about heat as though it’s a fluid that can flow through or into or out of the objects around us. But really, it’s just movement energy, shared around as different objects come into contact with one another. Temperature is a direct measurement of that movement energy. We can control how the energy is shared by using materials that are good conductors of heat, like metals, or poor conductors of heat, like ceramics. When you look at the control of heat and cold in our society, one system stands out above all the others for the difference it makes to our lives. We humans spend a lot of time making sure that we keep warm. But when it comes to food and pharmaceutical drugs, we have a huge invisible infrastructure for keeping things cold. Let’s finish this chapter by having a look at fridges and freezers.
If a piece of cheese warms up, and its molecules speed up their dance, there is more energy in the system, and that means that there’s more energy available for chemical reactions. In the case of cheese, this means that any microbes sitting on the surface can rev up their internal factories and start the process of decay. This is why refrigeration is useful. If we cool the food down, the molecules slow down, and the energy needed for the microbes to get stuck in isn’t available. So cheese will last far longer in the fridge than at room temperature. Through a clever mechanism in the back, it cools the air inside itself by generating more hot air outside the fridge.§ Cold lets us preserve food, because it limits how much the molecules can change.
Just imagine what life would be like without refrigeration. It isn’t just that there would be no ice cream or cold beer. You’d have to go shopping far more often, because any vegetables you bought wouldn’t last. You’d have to live very close to a farm if you wanted milk or cheese or meat, and very close to the ocean or a river if you wanted fish. You’d only get fresh salad leaves when they were in season. We can preserve some foods by pickling, drying, salting, or canning, but that won’t help you if you want a fresh tomato in December.
Behind our supermarkets sits a vast chain of refrigerated warehouses, ships, trains, and aircraft. Blueberries grown in Rhode Island can be sold in California a week after they were picked, because from the moment they were plucked from the bush until the moment they were put on the supermarket shelf, they were never allowed to take enough energy from their surroundings to warm up. We can trust that our food is safe to eat, because it’s deprived of heat energy on its way to us. And it’s not just food. Many pharmaceutical drugs rely on being kept cool, too. Vaccines are especially vulnerable to damage if they’re allowed to heat up, and one of the issues with getting vaccines into the developing world is that they must be kept cool all the way. The freezers and fridges that we see in our kitchens and doctors’ offices are the last step in an unbroken chain of cold that stretches around our planet, connecting farms and cities, factories and consumers. When you or I heat up milk to make hot chocolate, that’s the first time that milk has been warm since it was pasteurized just after it left the cow. And so when we trust that it’s safe to drink, we are trusting the enormous chain of cold that brought it to us. The atoms in the milk have been deprived of thermal energy all the way along that chain, so the chemical reactions that would have spoiled the milk have been almost completely switched off. Preventing atoms having too much thermal energy is what keeps our food safe.
Next time you drop an ice cube into a drink, watch it melt and imagine tiny atomic vibrations sharing out energy, as heat flows from the water into the ice cube. Even though you can’t see the atoms themselves, you can see the consequences of what they’re up to all around you.
* An ion is just an atom that’s either given away some electrons or gained some extra ones. Here, the sodium atom has given the chlorine atom one electron, so the sodium becomes a positive ion and the chloride becomes a negative ion. It sounds perverse, but now that they’ve got opposite charge, they’ll attract each other.
? The reason the space taken up by the submerged part of the ice cube is exactly the same as the space needed to accommodate the melted liquid is to do with the way that buoyancy works. The rest of the water has to support the weight of whatever is in that hole. It doesn’t matter to the rest of the glass what’s actually in the hole, as long as it takes up that much space. Once the ice cube has filled that space, it’s got extra volume left over, and this is what sticks up above the surface.
? This is crown glass, in case you’d ever wondered where that phrase comes from. The blobby bit in the middle of very old pub windows is where the iron rod was attached. This was the cheapest section of glass, because the thickness was so uneven. Of course, these days that sort of “character” is considered a valuable trait. As my northern family would say, “You’d pay extra for that in a posh restaurant.” Or in this case, a posh pub.
§ This works by using the gas laws introduced in chapter 1, the ones that control how letting gases expand and contract affects their temperature. A fridge has a motor that pumps a fluid called a refrigerant around a loop that passes from outside to inside the fridge and back out again. First, the fluid expands and therefore cools. This cool fluid is passed through the back of the fridge to the interior, where heat energy moves from the air to the refrigerant, cooling the air. Then the fluid arrives outside the fridge again, where it’s compressed using a motor, and so heats up. The extra heat is lost to the air, the fluid is allowed to expand, and the cycle starts all over again.
CHAPTER 7
Spoons, Spirals, and Sputnik
ONE OF THE nice things about bubbles is that you know where to look for them: at the top. They’re either on their way there, wobbling upward through fish tanks or swimming pools, or nestled in with the crowd on top of champagne or beer. Bubbles reliably find their way to the highest point of the liquid they’re in. But next time you stir a mug of tea or coffee, have a look at what’s going on at the surface. The first odd thing that happens is that as you move the spoon around in circles, the surface of the tea develops a hole. As the liquid whirls around, the middle of the tea sinks and the edges rise up. And the second odd thing is that the bubbles in the tea are spinning quietly at the bottom of the hole. They’re not at the highest point of the tea, at the edges. They’re hiding at the lowest point on the surface, and they stay there. If you push them away, they find their way back. If you make new bubbles at the edges, they spiral into the center. Odd.