PAX Prep NLN Verbal Comprehension Passages

PAX Prep NLN Verbal Comprehension Passages The way Things Work It was the ancient Greeks who first had the idea that everything is made up of elements. They conjured up just four of them-earth, fire, wind and water. As it turned out, the idea was right but the elements were wrong. Modern elements are less evocative but more numerous. They make up just over one hundred basic substances. Some are commonplace, like hydrogen, oxygen, iron and carbon; others are rare and precious, such as mercury, uranium and gold. Purely by the power of reason, the Greeks also made another fundamental discovery, which is that all things consist of particles called atoms. Elements are substances that contain only one kind of atom. All other substances are compounds of two or more elements in which the atoms group together to form molecules.The way molecules behave governs the workings of many machines, in particular machines such as ships, airplanes, pumps, refrigerators and combustion engines, all of which harness the ancient elements and set molecules to work. The idea that everything is made of particles takes some imagination to understand. For example, as you read this, molecules of oxygen and nitrogen traveling at supersonic speed are bombarding you from all directions. The reason that you are unaware of this is that the molecules (which along with those others gases, make up the air) are on the small side. You could get about 400 million million of them into an empty matchbox. In fact, it would be truer to say that you could get these millions of molecules out of the matchbox, because the molecules of gases are so hyperactive that they will fill any space open to them. Like five-year-olds, they dash about in all directions. With unflagging energy, crashing into any obstacle they meet. In liquids, the molecules are less energetic and go haphazardly about ion small groups, rather like drunken dancers prone to colliding with the walls of the dance hall. The molecules in solids are the least energetic; they just huddle together like a flock of sheep shuffling around in a field. However invisible molecules might be, their existence does explain the properties and behavior of materials that are put to use in machines. In a solid, the molecular bonds are strong and hold the molecules firmly together so that the solid is hard and rigid. The weaker bonds between liquid molecules pull them together to give the liquid a set volume, but the bonds are sufficiently weak to allow the liquid to flow. The bonds between gas molecules are weaker still. They enable the molecules to move apart so the gas expands and fills any space. In all materials, the molecules urge to stick together or spread apart is put to use in machines and devices as different as the rocket, the toilet tank and the aqualung.