CHEMISTRY

MODULE TWO

Two Types of Substances

• Of the known elements, only about 90 occur naturally on Earth. The remainder are synthesized, usually in barely detectable amounts, in high-energy nuclear experiments.

• Less than half of the 90 naturally occurring elements are abundant enough to play a significant role in the chemistry of everyday stuff.

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Organizing the Elements

• The periodic table organizes elements in a way that provides a wealth of chemical information—much more than is evident to you now. It shows the chemical symbols for the elements.

• Their symbols usually correspond to their names in Latin.

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Periodic Table of the Elements 2 2

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Compounds Are More Than One Element

• A more complete definition is that a compound is a chemical combination of two or more different elements joined together in a fixed proportion with a unique set of chemical and physical properties.

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• More than 10 million compounds are known and the number keeps growing.

Compounds Are More Than One Element

• The properties of the compound are different from the properties of the elements that compose the compound.

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silver + bromine = silver bromide

Compounds Are More Than One Element

• More than 10 million compounds are known and the number keeps growing.

• New compounds are discovered and isolated from natural chemical sources such as plants and colonies of bacteria and are synthesized in laboratories for many different uses.

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Compounds Are More Than One Element

• A formula is a combination of the chemical symbols that show what elements make up a compound and the number of atoms of each element.

Compound Formula

Caffeine C8H10N4O2

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States of Matter• Some substances are described as volatile,

which means that they change to a gas easily at room temperature.

• Alcohol and gasoline are more volatile than water.

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• Density is the amount of matter (mass) contained in a unit of volume.

• Styrofoam has a low density or small mass per unit of volume.

States of Matter

• Stones have a large density or a large mass per unit of volume.

• In science, the density of solids and liquids is usually measured in units of grams (mass) per milliliter (volume) or g/mL.

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Chemical Properties

• Chemical properties are those that can be observed only when there is a change in the composition of the substance.

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• Rusting is a chemical reaction in which iron combines with oxygen to form a new substance, iron oxide.

• Inability to react is also a chemical property.

Chemical Changes

chemical change, the change of one or more substances into other substances.

• A chemical property always relates to a

• Another term for chemical change is chemical reaction.

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Chemical Changes

• All matter is made of atoms, and any chemical change involves only a rearrangement of the atoms. Atoms do not just appear. Atoms do not just disappear.

• This is an example of the law of conservation of mass, which says that in a chemical change, matter is neither created nor destroyed. It would be equally correct to call this the law of conservation of matter.

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Chemical Reactions and Energy

• All chemical changes also involve some sort of energy change.

• Energy is either taken in or given off as the chemical change takes place. Energy is the capacity to do work.

• Work is done whenever something is moved.

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Chemical Reactions and Energy• Many reactions give off energy.

• For example, burning wood is a chemical change in which cellulose, and other substances in the wood, combine with oxygen from the air to produce mainly carbon dioxide and water.

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Chemical Reactions and Energy

• Energy is also produced and released in the form of heat and light.

• Chemical reactions that give off heat energy are called exothermic reactions.

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• Chemical reactions that absorb heat energy are called endothermic reactions.

Compound or mixture?

A. sand

B. water

C. juice

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Physical or chemical property?

A. density

B. reactivity

C. color

D. melting point

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End of Module 2Take Module 2 Test

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