Background
What are elements?
Elements are the primary substance from which all other materials are built. A substance that cannot be broken down into simpler substances by chemical means is called an element. Over 100 different elements have been identified. Most of these elements exist on the earth in different amounts. Gold, silver, oxygen, and sodium are just of few of the many important elements that you may already be familiar with. The periodic table provides a visual representation and listing of the known elements. The smallest part of an element that still has all of the properties of the element is an atom.
What are compounds?
A compound is a pure substance that is composed of two or more elements that have been chemically joined. A compound can only be broken down into its elements by chemical means. Water, sugar, table salt, and carbon dioxide are just a few of the compounds that you might have experienced in your life. The smallest part of a compound that still has all of the properties of the compound is a molecule.
What are chemical properties?
Chemical properties describe how one kind of matter behaves in the presence of another kind of matter. These properties describe what happens when one substance reacts with another substance. For instance, when hydrogen gas reacts with oxygen gas a large explosion occurs. Reacting explosively with oxygen is a chemical property of hydrogen gas.
What are physical properties?
The physical properties of a substance can be described by using any of your senses. Color, texture, and odor all describe the physical properties of a material. Other physical properties include hardness, density, solubility, taste, melting and boiling points of a substance.
What is a physical change?
A physical change is any change to a substance that does not result in the formation of a new substance. Physical changes can change the substance, they just cannot change the nature of the substance. For example, chopping a log into smaller pieces changes the log. However, the log is still a log. The chemical nature of the log did not change in the process.
What is solubility?
Solubility is a physical property that describes whether a substance will or will not dissolve in the presence of a solvent. Dissolving is a process in which the solute (material that is being dissolved) becomes separated and insulated from other solute particles by the solvent particles. The combination of a solute with a solvent is called a solution. The solute particles spread out and the space between the particles is filled in with solvent particles. Sodium chloride (NaCl), commonly referred to as table salt, is soluble in water. The water molecules move the salt particles apart. The salt appears to disappear; however, they have just been separated into smaller pieces that cannot be seen with the naked eye.
Solubility is a physical property because the solute is not chemically changed. The original solute material can be recovered by completing a phase change of the system. An example would be boiling a salt water solution to leave behind pure sodium chloride, which has identical properties to the original salt placed in the solution.
What is a chemical change?
A chemical change takes place when one kind of matter is changed into another kind of matter. Chemical changes involve chemical reactions. Iron is a hard, gray-colored material. When iron combines with oxygen a chemical reaction takes place forming iron (III) oxide, commonly called rust. Rust is a flaky, brownish-red material with very different properties from iron.
How do substances react with an acid?
Acids react with carbonates (CO3)-2 to form water, carbon dioxide gas, and a salt. The carbon dioxide gas emerges from the reaction in the form of bubbles. The following example shows how hydrochloric acid (HCl) reacts with a carbonate called calcium carbonate.
|
2 HCl
|
+
|
CaCO3
|
--->
|
CaCl2
|
+
|
H2O
|
+
|
CO2
|
A simple test to determine if an unknown substance contains carbonate is to add some acid to the sample. If bubbles develop, the substance may contain carbonate.
How do substances react with iron (III) chloride?
Iron (III) chloride contains the iron (III) ion. Some substances can form complexes with the iron (III) ion. Complexation is when one species is joined and surrounded by multiple other species. A complex occurs in the case of iron (III) chloride when an iron (III) ion becomes surrounded by multiple phenol groups from different molecules. The formation of these complexes can result in easily seen color changes. Substances with a phenol group (-OH) turn purple in color when they complex with the iron (III) ion. Salicylic acid, a precursor to aspirin, and acetaminophen, the active ingredient in Tylenol®, both contain the phenol group.
Common substances that do not contain the phenol group do not change color in the presence of iron (III) chloride. Substances with small amounts of the phenol group or with parts that may be converted to the phenol group during a reaction, such as aspirin, display lighter shades of the purple color.
What is pH?
The pH scale is a scale of numbers used to measure the strength of an acid or a base. The scale ranges from 0 to 14, with pH 7 being a neutral system. Substances with a pH less than 7 are classified as acids. Moving from 7 to 0 the acid increases in strength. Substances with a pH between 7 and 14 are bases. Moving from 7 to 14 the strength of a base increases. The table below lists the pH values for some common substances.
|
Substance
|
pH
|
|
Aspirin
|
2.5
|
|
Tylenol®
|
6.2
|
|
Sodium hydrogen carbonate (baking
soda)
|
8.3
|
|
Alka Seltzer®
|
6.4
|
|
Sodium chloride (table salt)
|
7
|
Indicators, such as universal solution, are used to measure the pH value of unknown substances. Indicators change color based on the pH of the system.
