Chemistry 2014-2015 CHEMISTRY SECTION V. Many of the substances we know about can be sorted into general categories according to their properties, such.

Chemistry2014-2015

CHEMISTRY SECTION V

Many of the substances we know about can be sorted into general categories according to their properties, such as solubility (the ability to dissolve) melting point (the temperature at which a substance melts) and conductivity (the ability to conduct electricity).

CLASSIFYING SUBSTANCES

Some substances, like sugar, sodium chloride, and ammonium sulfate, dissolve in water to make an aqueous solution (a term we learned a few weeks ago). Gold, however, does not dissolve in water (but aqua regia, which we learned about in our first unit, can dissolve gold). A substance that dissolves in water is said to be soluble in water, and one that does not dissolve is said to be insoluble. Therefore sodium chloride is soluble in water, while gold is insoluble in water.

Oil and water are insoluble.

The liquid on the left is insoluble in water.The liquid on the right is soluble in water.

The property of conductivity is related to a substance’s ability to conduct electricity. Electrical conductivity requires the movement of ions or electrons. Copper wire is an excellent conductor; the human body can also act as a conductor. Rubber is a poor conductor.

Electrical conductivity can be tested by setting up a simple electrical circuit—if the substance being tested is part of the circuit, it will either lead to the flow or blockage of an electrical current. This is easy to test if a light bulb is also part of the circuit; if the bulb lights up, the substance conducts electricity.

We can classify substances into four categories using these properties.IonicMolecular covalentMetallicNetwork covalent

TYPES OF SUBSTANCES

Ionic substances are soluble in water, conduct electricity, and have high melting points.

Examples: sodium chloride, copper sulfateTypes of atoms: ionic compounds (a

metal and a nonmetal, or polyatomic ions) in aqueous solution

Type of bond: IonicElectrons are transferredDescription: brittle solids

IONIC

Ex. sodium chloride, which can form large “halite” crystals like the one on the right

Copper sulfate (left) and cobalt chloride (right). Ionic compounds contain a metal and a nonmetal, and/or polyatomic ions. Many are white crystals, but some are colored.

Molecular covalent substances are soluble in water, do not conduct electricity, and have low melting points.

Examples: table sugar/sucrose, C12H22O11; carbon dioxide, CO2

Types of atoms: nonmetals; often C, H, and OType of bond: molecular covalentElectrons are shared between atoms in a

molecule.Often liquids or gases, sometimes solids

MOLECULAR COVALENT

In a covalent bond, electrons are shared, like in the bond between two oxygen atoms on the right.

Molecular covalent molecules often contain carbon, hydrogen, and/or oxygen. Look at the three molecular covalent compounds below—they contain these elements.

Metallic substances are insoluble in water, conduct electricity, and have high melting points.

Examples: gold, copper, aluminumTypes of atoms: metalsType of bond: metallicFree-flowing “sea of electrons”Bendable, malleable, shiny solids

METALLIC

Malleable—can be hammered into a thin sheet

Metals can often be bent, pressed, hammered, and/or stretched into wires

Remember—some of the electrons in metals are free-moving; they are no longer anchored to atoms, but they can move around the entire metal

Network covalent substances are insoluble in water, do not conduct electricity, and have high melting points.

Examples: sand/glass (silicon dioxide), diamonds (one form of carbon)

Types of atoms: nonmetalsType of bond: network covalentElectrons are shared throughout

substanceExtremely hard solids

NETWORK COVALENT

Sand, quartz, and glass are all silicon dioxide, SiO2. Note that it contains two nonmetals bonded together in a large network.

Predict whether the following substances will dissolve in water, and whether they will conduct electricity.

Leadsoluble / insoluble conduct / not conduct

Potassium bromidesoluble / insoluble conduct / not conduct

PACKET 3 EXAMPLE 1: PREDICTING PROPERTIES

Lead is a metal—look at the properties of metals

“Potassium bromide”—contains a metal and a nonmetal; ionic

Ionic substances only conduct electricity when they are dissolved in water

Chemists call the attraction that holds atoms together a chemical bond. Several types of bonds exist, and they all involve electrons in some way.  

We’ve seen that most substances can be divided into four categories based on their physical properties. These categories can be explained by different models of bonding. You can see these in the table on the previous page.

BONDING

Carbon dioxide exists as separate CO2 molecules; silicon dioxide exists as a network of silicon and oxygen.

Some properties are directly related to the type of bonds the atoms in the substances have. Therefore, it is possible to match the bonding with the physical properties observed in different substances.

BONDS AND PROPERTIES

Conduction requires the movement of charged particles. Ionic substances in aqueous solution contain free-moving cations and anions, so they conduct electricity.

Metallic substances conduct electricity because their valence electrons are free to move within the solid.

Network covalent substances and molecular covalent substances do not contain ions or transfer electrons, so they do not conduct electricity.

Melting point depends on the attractive forces between the particles.The higher the melting point, the stronger the attractive forces.

Substance

Type of Bond

Soluble in water?

Conducts electricity?

Electrons are…

Description

Magnesium chloride, MgCl2

Rubbing alcohol, C3H8O

Packet 3 Example 2: Identifying Types of BondsDetermine the type of bond in each of the following substances. Then decide the physical properties each substance would have.

Ionic (metal + nonmetal)

Yes Yes—when dissolved in water

Transferred

Brittle solids

Molecular covalent (C, H, and O)

Yes No Shared between atoms

Liquid

Most metals are dug out of the ground as ionic compounds (ores); in other words, they cannot be found in nature in their pure forms. Through the ages, people have struggled to extract the pure metals from these ores; some are easier to purify than others.

ELECTROPLATING METALS

This is a piece of gold ore—obviously, it’s not pure gold. Gold and other metals can be extracted from ores using a variety of methods.

Despite being the third most common element in Earth’s crust, aluminum was one of the most diffi cult metals for scientists to isolate. It was first purified in 1827 by a German chemist named Friedrich Wöhler. Processing aluminum was still diffi cult and ineffi cient, making it more expensive per ounce than gold for quite some time.

ALUMINUM

This is bauxite, the ore aluminum can be extracted from.

Napoleon III famously let only his favorite guests use his aluminum cutlery, while the rest had to use gold.

The Washington Monument was also capped with aluminum which (at the time) was as expensive as silver.

It was not until the advent of electrolysis (running an electric current through aluminum ore), that the isolation of aluminum became more effi cient, therefore dropping the price of aluminum.

Electricity can be used to extract metal from compounds by “giving” electrons back to metal ions, which converts them to neutral metal atoms. This process is called electroplating. Ex. Copper metal can be

extracted from a copper sulfate solution by running an electrical current through the solution.

ELECTROPLATING

ELECTROPLATING WITH GOLD