To identify a chemical property, we look for a chemical change. (credit a: modification of work by Tony Hisgett credit b: modification of work by “Atoma”/Wikimedia Commons) (a) One of the chemical properties of iron is that it rusts (b) one of the chemical properties of chromium is that it does not. Nitroglycerin explodes when impacted neon poses almost no hazard because it is very unreactive. Iron, for example, combines with oxygen in the presence of water to form rust chromium does not oxidize ( Figure 3). Examples of chemical properties include flammability, acidity, reactivity (many types), and heat of combustion. The change of one type of matter into another type (or the inability to change) is a chemical property. (credit a: modification of work by “95jb14”/Wikimedia Commons credit b: modification of work by “mjneuby”/Flickr) (b) Steam condensing inside a cooking pot is a physical change, as water vapor is changed into liquid water. (a) Wax undergoes a physical change when solid wax is heated and forms liquid wax. In each of these examples, there is a change in the physical state, form, or properties of the substance, but no change in its chemical composition. Other examples of physical changes include magnetizing and demagnetizing metals (as is done with common antitheft security tags) and grinding solids into powders (which can sometimes yield noticeable changes in color). We observe a physical change when wax melts, when sugar dissolves in coffee, and when steam condenses into liquid water ( Figure 2). The different phase changes that matter can undergo. A physical change is a change in the state (Figure 1) or properties of matter without any accompanying change in its chemical composition (the identities of the substances contained in the matter), such as the melting and freezing described above. Other physical properties, such as the melting temperature of iron or the freezing temperature of water, can only be observed as matter undergoes a physical change. We can observe some physical properties, such as density and color, without changing the physical state of the matter observed. Familiar examples of physical properties include density, color, hardness, melting and boiling points. A physical property is a characteristic of matter that is not associated with a change in its chemical composition. The characteristics that enable us to distinguish one substance from another are called properties. So as heat flows into a substance its temperature will rise. The faster they move, the “hotter” it is. The cooling that occurs as sweat evaporates from our skin is an endothermic change of state from liquid water in the sweat to gas.Īn important distinction is that heat is energy that flows due to a temperature difference, while temperature is an indication or measure of the average kinetic energy of the molecules in a substance. These processes are less familiar to us, though some occur commonly. When heat is needs to be supplied to make a process occur the change is called endothermic. Reactions such as those that involve the combustion of fuels are familiar examples of exothermic reactions. Heat can be released during a change, which is called exothermic. When matter undergoes change, the process is often accompanied by a change in energy. Note that energy itself is not matter, but comes in a variety of forms such as heat, light, sound, or in the kinetic energy of moving matter. Recall that chemistry is the study of matter, its properties, the changes that matter undergoes and the energy associated with these changes.
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