Chemical and Physical Changes

States of Matter

Matter can be classified into three states, although we might think of examples that do not fit neatly into any of the three categories. In the solid state, substances are rigid and have definite shapes. Volumes of solids do not vary much with changes in temperature and pressure. In crystalline solids, the individual particles that make up the solid occupy definite positions in the crystal structure. The strengths of interaction between the individual particles determine how hard and how strong the crystals are. In the liquid state, the individual particles are confined to a given volume. A liquid flows and assumes the shape of its container up to the volume of the liquid because the molecules are randomly oriented and have weaker forces of attraction between them relative to solids.

Liquids are very hard to compress because their molecules are very close together. Gases are much less dense than liquids and solids. A gas occupies all parts of any vessel in which it is confined. Gases are capable of indefinite expansion and are highly compressible. We conclude that gases consist primarily of empty space, meaning that the individual particles are quite far apart.

Chemical and Physical Properties

To distinguish among samples of different kinds of matter, we determine and compare their properties. We recognize different kinds of matter by their properties. We can broadly classify these into chemical properties and physical properties. Chemical properties are exhibited by matter as it undergoes changes in composition. These properties of substances are related to the kinds of chemical changes that the substances undergo. For instance, the combination of metallic magnesium with gaseous oxygen forms magnesium oxide, a white powder. A chemical property of magnesium is that it can combine with oxygen, releasing energy in the process. A chemical property of oxygen is that it can combine with magnesium.

All substances also exhibit physical properties that can be observed in the absence of any change in composition. Color, density, hardness, melting point, boiling point, and electrical and thermal conductivities are physical properties. Some physical properties of a substance depend on the conditions, such as temperature and pressure, under which they are measured. For instance, water is a solid (ice) at low temperatures but is a liquid at higher temperatures. At still higher temperatures, it is a gas (steam). As water is converted from one state to another, its composition is constant. Its chemical properties change very little. On the other hand, the physical properties of ice, liquid water, and steam are very different.

Chemical and Physical Changes

In any chemical change, one or more substances are used up (at least partially), one or more new substances are formed, and energy is absorbed or released. As substances undergo chemical changes, they demonstrate their chemical properties. A physical change, on the other hand, occurs with no change in chemical composition. Physical properties are usually altered significantly as matter undergoes physical changes. In addition, a physical change may suggest that a chemical change has also taken place. For instance, a color change, a warming, or the formation of a solid when two solutions are mixed could indicate a chemical change.

After-reading

1. What states can matter be classified into?

2. What states of matter do the following substances have under normal conditions?
(a) mercury; (b) potassium; (c) gasoline; (d) carbon dioxide; (e) hydrated copper (II) sulphate; (f) ammonium sulphate; (g) bromine; (h) neon; (i) cryolite; (j) methane; (k) dolomite; (l) phosphine.

3. Give examples of physical and chemical properties of matter.

4. Find information about the physical properties of the following substances.

5. Give synonyms to the highlighted words in the text.

6. Use a dictionary to find definitions to the words in bald.

7. Classifying from general to specific.

Classifying from specific to general.

or

8. Make up sentences to classify the following notion.

1) copper, iron, gold (metals)

2) salt (inorganic compound)

3) azurite, labradorite, hematite (minerals)

4) graphite, diamond (carbon)

5) coal, oil (fuel)

6) deoxyribonucleic acid (nucleic acid)

7) calcium fluoride, iron(II) sulfate, zinc phosphate (ionic compounds)

Date: 2016-01-14; view: 1040