Matter & Energy


Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Physical Properties of Liquids

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G. Osmosis

When a substance dissolves in a liquid, the resulting mixture is called a solution. Osmosis occurs when molecules of the initial liquid pass through a membrane, but molecules of the dissolved substance do not. The molecules of the initial liquid can pass through the membrane because they are relatively small. Osmosis tends to equalize the concentration of the solutions on both sides of a membrane. The membrane in this case is called semipermeable, because it allows one part of the mixture to pass through but not another. Cells in living organisms consist mostly of water, and they are surrounded by a watery environment. If the concentration of a dissolved substance, such as sugar or salt, differs inside and outside a cell, osmosis causes water to pass through the cell’s membrane from the area of lower concentration to the area of higher concentration, until the concentration on each side of the membrane is equal. Osmosis makes sugar and salt good food preservatives. When harmful bacteria encounter sugary or salty foods, water flows from the area of lower concentration—the cells of the bacteria—to the area of higher concentration—the food. The flow of water out from the bacteria’s cells dehydrates the bacteria, which kills it.