A physical property is any characteristic of a material that can be observed or measured without changing the composition of the substances in the material. Viscosity, conductivity, malleability, hardness, melting point, boiling point, and density are examples of physical properties.
Suppose you knock over an open bottle of vinegar (C2H4O2) and an open bottle of honey at exactly the same time. In the time it takes for the vinegar bottle to empty, the honey will scarcely start to flow. The tendency of a liquid to keep from flowing - its resistance to flowing - is called its viscosity. The greater the viscosity, the slower the liquid moves. Thick liquids, such as corn syrup and honey, have a high viscosity. Thin liquids, such as vinegar, have a low viscosity.
The viscosity of a liquid usually decreases when it is heated. For example, a spoonful of cooking oil will spread more quickly across the bottom of a heated frying pan than across the bottom of a cold pan.
Why is the viscosity of a liquid so important? Consider the motor oil used to keep the parts of an automobile engine from wearing away as they move past one another. The motor oil must not be too thick in cold weather or too thin in hot weather.
Which spoon should you choose for stirring a pot of soup heating on the stove - a metal spoon or a wooden spoon? If one end of a metal object is heated, the other end will soon feel hot. A material's ability to allow heat to flow is called conductivity.
Materials that have a high conductivity, such as metals, are called conductors. If a material is a good conductor of heat, it is usually also a good conductor of electricity. Wood is not a good conductor of heat. You can stir hot soup with a wooden spoon without worrying about burning your hand because the wooden spoon stays cool to the touch.
Ancient gold (Au) objects were found in a tomb in Greece. A goldsmith made the medallions by tapping gold with a small hammer and punch. Gold can be shaped in this way because it is malleable. Malleability is the ability of a solid to be hammered without shattering. Most metals are malleable. By contract, an ice cube (H2O) breaks into small pieces when struck with a hammer. So does ordinary glass (SiO2) when hit by a fast-moving object such as a baseball. Solids that shatter when struck are brittle.
One way to compare the hardness of two materials is to see which of the materials can scratch the other. The blade of a typical kitchen knife, for example, can scratch a copper (Cu) sheet because stainless steel is harder than copper. The stainless steel in a knife blade is a hard solid that can be shaped into a sharp cutting edge. The material used to sharpen the blade must be harder than stainless steel. Diamond (C) is the hardest known material. Some of the grinding wheels used to sharpen steel contain small grains of diamond.
Melting and boiling pointsEdit
If you leave a tray of ice cubes on your kitchen counter, the ice cubes will melt. The temperature at which a substance changes from solid to liquid is its melting point. For water, this change normall occurs at 0°C. If you heat water to cook pasta, the water will normaly start to boil at 100°C. The temperature at which a substance boils is its boiling point.
Melting and boiling points of some substancesEdit
|Substance||Melting Point||Boiling Point|
|Acetic acid (C2H4O2)||16.6°C||117.9°C|
|Table salt (NaCl)||800.7°C||1465°C|
Density can be used to test the purity of a substance. Density is the ratio of the mass of a substance to its volume. At room temperature, silver (Ag) has a density of 10.5g/cm3. If a coin has a density of 9.9 g/cm3 at room temperature, either the coin is not made from silver or the coin contains substances in addition to silver. Density can be used to test the purity of methanol (CH4O). Methanol is a fuel burned in some racing motorcycles. The American Motorcycle Association (AMA) requires racers to use fuel that is at least 99.65 percent pure. Race officials may collect a sample of fuel and measure its temperature and density. Then they compare the measured density to the expected density of methanol at that temperature. These spot checks keep racers from adding substances to the fuel that will give them an unfair advantage in a race.