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Elements and Compounds.When sodium metal and chlorine gas are heated together, a white solid called sodium chloride is formed.Compounds are pure substances that are composed of two or more elements combined in definite proportions.It consists of nitrogen, oxygen, and argon gases, but also contains solid particles of pollen and dust.The action of an electric current, called electrolysis, is one method that can be used to decompose both water and molten table salt into their constituent elements.The individual chunks (particles) of ice and salt are clearly visible, and the particles are so large, they are not evenly dispersed.An element is a pure substance that cannot be decomposed into simpler substances by ordinary chemical reactions.They can be homogeneous or heterogeneous depending on the state of subdivision of the components.On the ordinary scale of observation we cannot detect any chemical or physical differences between adjacent regions of the mixture.The properties of a homogeneous mixture vary since they depend on the percent composition.Chemical properties most often are descriptions of reactions that a substance undergoes when brought in contact with other substances.A pure substance is a form of matter that has a definite composition and distinct properties.Just as each individual person has a set of characteristics, such as fingerprints and color of eyes and hair, each pure substance has characteristic properues.A number of common elements and their symbols are listed in the textbook in Table 1.1.The original crystals of salt have dissolved and are dispersed evenly.A mixture that has the same composition throughout is said to be a homogeneous mixture.And indeed the individual particles of their components can often be seen by the unaided eye.Any mixture, whether

it be

homogeneous or heterogeneous, can be separated into its pure components by physical means.Mixtures.ust.

Elements and Compounds. A pure substance is a form of matter that has a definite composition and distinct properties. Examples are water, table salt, and iron. Just as each individual person has a set of characteristics, such as fingerprints and color of eyes and hair, each pure substance has characteristic properues. There are two types of pure substance: elements and compounds. An element is a pure substance that cannot be decomposed into simpler substances by ordinary chemical reactions. Elements are the building blocks of which all compounds are composed. Nitrogen, oxygen, and iron are examples of elements. A number of common elements and their symbols are listed in the textbook in Table 1.1.

Compounds are pure substances that are composed of two or more elements combined in definite proportions. Compounds can be broken down into the elements of which they are composed by chemical means. Water and table salt, mentioned earlier, are compounds. The action of an electric current, called electrolysis, is one method that can be used to decompose both water and molten table salt into their constituent elements. Pure

water consists of 89 percent oxygen and 11 percent hydrogen by mass. Pure salt contains 39 percent sodium and

61 percent chlorine.

Mixtures. Pure substances can be brought together to form mixtures. Mixtures are combinations of two

or more substances. They can be homogeneous or heterogeneous depending on the state of subdivision of the components. Salt water is a uniform mixture of table salt (NaCl) and water. The original crystals of salt have dissolved and are dispersed evenly. On the ordinary scale of observation we cannot detect any chemical or physical differences between adjacent regions of the mixture. The particles of salt are too small to observe. A mixture that has the same composition throughout is said to be a homogeneous mixture. The properties of a homogeneous mixture vary since they depend on the percent composition. For example, the hardness of steel. a solid mixture of iron and carbon, depends on the percentage of carbon that is added to iron. Homogeneous mixtures are also called solutions.
Heterogeneous mixtures are not uniform in composition. And indeed the individual particles of their components can often be seen by the unaided eye. For example, when preparing home-made ice cream, you use a mixture of ice and rock salt. This is a heterogeneous mixture. The individual chunks (particles) of ice and salt are clearly visible, and the particles are so large, they are not evenly dispersed. The composition of this mixture varies from place to place within the mixture itself. Natural air is actually a heterogeneous mixture. It consists of nitrogen, oxygen, and argon gases, but also contains solid particles of pollen and dust. ust. Any mixture, whether

it be

homogeneous or heterogeneous, can be separated into its pure components by physical means.

Properties of Matter. Physical properties are those properties that can be measured and observed without

changing the identity or composition of the substance. Physical properties include color, hardness, solubility. density, specific heat, melting point, and boiling point. Physical changes are those that take place with no change in chemical composition Changes of a substance from one state of matter to another do not change its chemical composition and are examples of physical changes The three forms of water we call ice, liquid water. and steam are all the same substance, just different physical states.

Chemical properties most often are descriptions of reactions that a substance undergoes when brought in contact with other substances. In a chemical reaction the original substance or substances are changed into new substances. When sodium metal and chlorine gas are heated together, a white solid called sodium chloride is formed.