It is amazing to realize that millions of chemical reactions occur in your body at every moment. What is even more remarkable is that they happen without your even knowing it. We know that chemicals are critical to life because humans and other animals can live only a few weeks without food, several days without water, and a few minutes without oxygen. In most cases, the difference between health and illness depends on the proper functioning of chemical reactions. When reactions occur as they should, an organism is healthy. Medical experts identify faulty reactions and use drugs and other treatments to correct problems.
Chemistry is the study of matter. Matter is composed of tiny particles called atoms that contain protons, neutrons, and electrons. Elements are pure substances that are made up of a single kind of atom. The number of protons in the nucleus, called the atomic number , determines the identity of an atom.
Atoms are represented by symbols, such as C for carbon and Cl for chlorine. The periodic table lists the elements in order of increasing atomic number ( Figure 1 ).
Table 1 summarizes how common atoms are illustrated.
Molecules are combinations of two or more atoms held together by forces of attraction called chemical bonds. Molecules that contain atoms of one element only are called molecular elements.
Oxygen in the air, O SUBSCRIPT 2(g), is a molecular element.
Molecules that contain atoms of different elements are called compounds. Water, H SUBSCRIPT 2 O SUBSCRIPT (1), and carbon dioxide, CO SUBSCRIPT 2(g), are examples of compounds ( Figure
2 ).
The subscripts (1), (s), (g), and (aq) refer to the state of the substance--liquid, solid, gas, or aqueous solution. The (aq) symbol means that the solute is dissolved in water.
Atoms form chemical bonds by gaining and losing electrons or by sharing electrons. An ion is an atom that becomes electrically charged by gaining or losing electrons. Bonds formed when oppositely charged ions come together are called ionic bonds .
Sodium chloride (table salt) is a common ionic compound ( Figure

3 ).
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Covalent bonds form when two atoms share electrons. Molecules may have single, double, or triple covalent bonds. Hydrogen (H
SUBSCRIPT 2) has a single covalent bond, oxygen (O SUBSCRIPT 2) has a double covalent bond, and nitrogen (N SUBSCRIPT 2) has a triple covalent bond ( Figure 4 ). The shared electrons in a covalent bond may be equally or unequally shared. In a hydrogen chloride molecule, HCl, the electron pair is unequally shared because chlorine has a larger, more positive nucleus (17 protons) than hydrogen (1 proton). This causes the hydrogen side of the molecule to have a partial positive charge (LOWER CASE
DELTA SUPERSCRIPT +) and the chlorine side to have a partial negative charge (LOWER CASE DELTA SUPERSCRIPT -) ( Figure 5 ). A covalent bond in which positive and negative charges are separated because of unequally shared electron pairs is called a polar covalent bond .
Equally shared electrons form a nonpolar covalent bond , as in hydrogen, H SUBSCRIPT 2(g). Polar molecules are molecules that have a positively charged end and a negatively charged end. Nonpolar molecules do not have charged ends.
Covalent bonds are the strongest forces of attraction that hold the atoms of a molecule together. They are known as intramolecular bonds (bonds within a molecule). However, molecules are attracted to other molecules by a group of much weaker forces called intermolecular bonds (bonds between molecules). A single intermolecular bond is weak, but when they form between the atoms of very large molecules, they add up to a large total force of attraction ( Figure 6 ).
The strongest intermolecular bond is the hydrogen bond. Hydrogen bonds form between highly polar molecules containing various combinations of hydrogen, oxygen, and nitrogen atoms. Water molecules form hydrogen bonds.
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Water is probably the most important molecule on Earth. Water covers three-quarters of Earth's surface and makes up about twothirds of your body's mass. Water is a polar molecule composed of one oxygen atom bonded to two hydrogen atoms by single polar covalent bonds. Water molecules are polar and form hydrogen bonds with other water molecules ( Figure 7 ).
Hydrogen bonds between water molecules are difficult to break and are responsible for water's high boiling point and strong surface tension ( Figure 8 ).
A solution is a homogeneous (uniform) mixture of two or more pure substances. The pure substance in greater quantity is called the solvent, and the substance in lesser quantity is the solute. Thus, if 10 mL of alcohol is dissolved in 100 mL of water, the alcohol is the solute and the water is the solvent.
When predicting whether or not a substance will dissolve in water, scientists use the saying "like dissolves like." This means that polar solutes dissolve only in polar solvents and nonpolar solutes dissolve only in nonpolar solvents. Oil molecules (nonpolar) do not dissolve in water molecules (polar) because they cannot form hydrogen bonds with water. Instead, the water molecules form hydrogen bonds with one another, causing the nonpolar oil molecules to be excluded. For this reason, nonpolar molecules are described as hydrophobic (literally,
"water-fearing"). Oil floats on water because it is hydrophobic and less dense than water ( Figure 9 ).
Substances that are attracted to water are described as hydrophilic (literally,
"water-loving"). Hydrophilic substances include polar molecules such as alcohols and sugars, and ions such as sodium ions (Na
SUPERSCRIPT +) and chloride ions (Cl SUPERSCRIPT -).
The concentration of a solution is a ratio of the quantity of solute to the quantity of solution. This value may be expressed as a percentage. A vinegar label indicating 5% acetic acid means that there are 5 mL of pure acetic acid (a liquid) dissolved in every 100 mL of vinegar solution. When solids are dissolved in liquids, the concentration is usually given in units of grams of solute per litre of solution (g/L), or grams of solute per hundred millilitres of solution (g/100 mL). For example, if 10 g

of sugar is dissolved in enough water to form 100 mL of solution, the concentration may be given as 0.1 g/mL.
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Frequently, you may need several concentrations of the same solution. If you begin with a solution of known concentration (a stock solution), you may dilute the solution (decrease its concentration) by adding more solvent (usually water).
Calculating the concentration of the diluted solution is straightforward if you know the concentration of the original solution and the volume of the final solution. For example, if water is added to 6% hydrogen peroxide until the total volume is doubled, the concentration becomes one-half the original value, or 3%.
Aqueous solutions may be classified as acidic, basic, or neutral, depending on their physical and chemical properties.
Table 2 lists the properties of acids, bases, and neutral solutions, and provides some examples.
Acidic solutions contain high concentrations of hydrogen ions (H
SUPERSCRIPT +). Hydrogen ions are responsible for the properties of acids. Basic solutions contain high concentrations of hydroxide ions (OH SUPERSCRIPT -). Hydroxide ions are responsible for the properties of bases. Neutral solutions contain equal numbers of hydrogen ions and hydroxide ions, and do not display the properties of acids or bases.
When an acid reacts with a base, water and a salt are produced.
This is called a neutralization reaction. Neutralization results in a neutral solution.
HCl SUBSCRIPT (aq) (acid) + NaOH SUBSCRIPT (aq) (base) APPROACHES H
SUBSCRIPT 2 O SUBSCRIPT (l) (water) + NaCl SUBSCRIPT (aq) (salt)
[neutral salt solution]
The acidity of a solution may be expressed in terms of pH. The pH scale was devised as a convenient way to distinguish between an acidic solution, a basic solution, and a neutral solution

( Figure 10 ).
The pH of pure water is 7. Any aqueous solution with pH = 7 is neutral. Solutions with pH greater than 7 are basic, indicating that there are more hydroxide ions than hydrogen ions in the solution. A solution with pH less than 7 is acidic. This indicates that there are more hydrogen ions than hydroxide ions in the solution.
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The components of living cells are sensitive to pH levels. Most cellular processes operate best in a neutral environment where pH = 7. However, cells have to contend with acids and bases all the time. Many of the reactions that occur within cells produce acids or bases. Many of the foods you eat are acidic. When your digestive system absorbs acidic nutrients, such as the acids in fruits, many vegetables, and salad dressings, the pH of your blood may be lowered. Living cells use buffers to resist such changes in pH. A buffer is a mixture of chemicals that can neutralize small amounts of acid (hydrogen ions) or base
(hydroxide ions) that may be accidentally added to the mixture.
In living organisms, buffers usually consist of equal concentrations of an acid and a base. The most important buffer in human extracellular fluid and blood is the carbonic acid— bicarbonate ion buffer. In this buffer, the acid is carbonic acid (H SUBSCRIPT 2 CO SUBSCRIPT 3) and the base is bicarbonate ions (HCO SUBSCRIPT 3 SUPERSCRIPT -). The carbonic acid component will neutralize small amounts of base that may enter the solution, and bicarbonate ions will neutralize small amounts of acid ( Figure 11 ).