The Basics of Life

By Enger, E.D., Ross, F.C., Bailey, D.B.

Edited by Paul Ducham


One of the important scientific laws, the law of conservation of energy, or the first law of thermodynamics, states that energy is never created or destroyed. Energy can be converted from one form to another, but the total energy remains constant. All living things obey this law. One kind of energy change is between kinetic and potential energy. An object that appears to be motionless does not necessarily lack energy. An object on top of a mountain may be motionless but still may contain significant amounts of potential energy, or “energy of position.” It can be released as kinetic energy if the object rolls down the mountain. Keep in mind that potential energy also increases whenever things experiencing a repelling force are pushed together. You experience this every time you use kinetic energy to “click” your ballpoint pen, which compresses the spring inside. This gives the spring more potential energy, which is converted back into kinetic energy when the spring expands as the ink cartridge is retracted into its case. The kinetic energy for compressing the spring comes from your biological ability to harvest energy from food molecules. Potential energy also increases whenever things that attract each other are pulled apart. An example of this occurs when you stretch a rubber band. That increased potential energy is converted to the snapping back of the band when you let go. Again, the kinetic energy needed to stretch the rubber band comes from you. It is important to understand that energy is not used up in these processes. The same amount of energy is released when the rubber band is snapped back as was stored when you stretched the band.


There are five forms of energy, and each can be either kinetic or potential: (1) mechanical, (2) nuclear, (3) electrical, (4) radiant, and (5) chemical. All organisms interact in some way with these forms of energy. Mechanical energy is the energy most people associate with machines or things in motion. A track athlete displays potential mechanical energy at the start line; the energy becomes kinetic mechanical energy when the athlete is running (figure 2.2). Nuclear energy is the form of energy from reactions involving the innermost part of matter, the atomic nucleus. In a nuclear power plant, nuclear energy is used to generate electrical energy. Electrical energy, or electricity, is the flow of charged particles. All organisms use charged particles as a part of their metabolism. Radiant energy is most familiar as heat and visible light, but there are other forms as well, such as X-radiation and microwaves. Chemical energy is a kind of internal potential energy that is stored in matter and can be released as kinetic energy when chemicals are changed from one form to another. For example, the burning of wood involves converting the chemical energy of wood into heat and light. A slower, controlled burning process, called cellular respiration, releases energy from food in living systems.

Figure 2.2


Atoms are the smallest units of matter that can exist alone. Elements are fundamental chemical substances made up of collections of only one kind of atom. For example, hydrogen, helium, lead, gold, potassium, and iron are all elements. There are over 100 elements. To understand how the atoms of various elements differ from each other, we need to look at the structure of atoms (How Science Works 2.1).
Atoms are constructed of three major subatomic particles: neutrons, protons, and electrons. A neutron is a heavy subatomic (units smaller than an atom) particle that does not have a charge; it is located in the central core of each atom. The central core is called the atomic nucleus. The mass of the atom is concentrated in the atomic nucleus. A proton is a heavy subatomic particle that has a positive charge; it is also located in the atomic nucleus. An electron is a light subatomic particle with a negative electrical charge that moves about outside the atomic nucleus in regions known as energy levels (figure 2.3). An energy level is a region of space surrounding the atomic nucleus that contains electrons with certain amounts of energy. The number of electrons an atom has determines the space, or volume, an atom takes up.
All the atoms of an element have the same number of protons. The number of protons determines the element’s identity. For example, carbon always has 6 protons; no other element has that number. Oxygen always has 8 protons. The atomic number of an element is the number of protons in an atom of that element; therefore, each element has a unique atomic number. Because oxygen has 8 protons, its atomic number is 8. The mass of a proton is 1.67 ×10-24 grams. Because this is an extremely small mass and is awkward to express, 1 proton is said to have a mass of 1 atomic mass unit (abbreviated as AMU) (table 2.1).

Figure 2.3

Table 2.1



Although all atoms of the same element have the same number of protons and electrons, they do not always have the same number of neutrons. In the case of oxygen, over 99% of the atoms have 8 neutrons, but others have more or fewer neutrons. Each atom of the same element with a different number of neutrons is called an isotope of that element. Since neutrons have a mass very similar to that of protons, isotopes that have more neutrons have a greater mass than those that have fewer neutrons. Elements occur in nature as a mixture of isotopes. The atomic weight of an element is an average of all the isotopes present in a mixture in their normal proportions. For example. of all the hydrogen isotopes on Earth, 99.985% occur as an isotope without a neutron and 0.015% as an isotope with 1 neutron. There is a third isotope with 2 neutrons, and is even more rare. When the math is done to account for the relative amounts of these three isotopes of hydrogen, the atomic weight turns out to be 1.0079 AMU. The sum of the number of protons and neutrons in the nucleus of an atom is called the mass number. Mass numbers are used to identify isotopes. The most common isotope of hydrogen has 1 proton and no neutrons. Thus, its mass number is 1 (1 proton + 0 neutrons = 1) also called protium. A hydrogen atom with 1 proton and 1 neutron has a mass number of 1 + 1, or 2, and is referred to as hydrogen-2, also called deuterium. A hydrogen atom with 1 proton and 2 neutrons has a mass number of 1 + 2, or 3, and is referred to as hydrogen-3, also called tritium (figure 2.4).

Figure 2.4


Subatomic particles were named to reflect their electrical charge. Protons have a positive (+) electrical charge. Neutrons, the second type of particle in the atomic nucleus, are neutral, since they lack an electrical charge (0). Electrons have a negative (-) electrical charge. Because positive and negative particles are attracted to one another, electrons are held near the nucleus. However, their kinetic energy (motion) keeps them from combining with the nucleus. The overall electrical charge of an atom is neutral (0) because the number of protons (positively charged) equals the number of electrons (negatively charged). For instance, hydrogen, with 1 proton, has 1 electron; carbon, with 6 protons, has 6 electrons. You can determine the number of either of these two particles in an atom if you know the number of the other particle.
Scientists’ understanding of the structure of an atom has changed since the concept was first introduced. At one time, people thought of atoms as miniature solar systems, with the nucleus in the center and electrons in orbits, like satellites, around the nucleus. However, as more experimental data were gathered and interpreted, a new model was formulated.


In contrast to the “solar system” model, electrons are now believed to occupy certain areas around the nucleus, the energy levels. Each energy level contains electrons moving at approximately the same speed; therefore, electrons of a given level have about the same amount of kinetic energy. Each energy level is numbered in increasing order, with energy level 1 containing electrons closest to the nucleus, with the lowest amount of energy. The electrons in energy level 2 have more energy and are farther from the nucleus than those found in energy level 1. Electrons in energy level 3 having electrons with even more energy are still farther from the nucleus than those in level 2 and so forth.
Electrons do not encircle the atomic nucleus in twodimensional paths. Some move around the atomic nucleus in a three-dimensional region that is spherical, forming cloudlike or fuzzy layers about the nucleus. Others move in a manner that resembles the forming fuzzy regions that look like dumbbells or hourglasses (figure 2.5). The first energy level is full when it has 2 electrons. The second energy level is full when it has 8 electrons; the third energy level, 8; and so forth (table 2.2). Also note in table 2.2 that, for some of the atoms (He, Ne, Ar), the outermost energy level contains the maximum number of electrons it can hold. Elements such as He and Ne, with filled outer energy levels, are particularly stable.
All atoms have a tendency to seek such a stable, filled outer energy level arrangement, a tendency referred to as the octet (8) rule. (Hydrogen and helium are exceptions to this rule and have a filled outer energy level when they have 2 electrons.) The rule states that atoms attempt to acquire an outermost energy level with 8 electrons through processes called chemical reactions. Because elements such as He and Ne have full outermost energy levels under ordinary circumstances, they do not normally undergo chemical reactions and are therefore referred to as noble (implying that they are too special to interact with other elements) or inert. Atoms of other elements have outer energy levels that are not full; for example, H, C, Mg, and Ca will undergo reactions to fill their outermost energy level in order to become stable. It is important for chemists and biologists to focus on electrons in the outermost energy level, because it is these electrons that are involved in the chemical activities of all life.

Figure 2.5

Table 2.2


Because atoms tend to fill their outer energy levels, they often interact with other atoms. A molecule is the smallest particle of a chemical compound that is a definite and distinct, electrically neutral group of bonded atoms. Some atoms, such as oxygen, hydrogen, and nitrogen, bond to form diatomic (di = two) molecules. In our atmosphere, these elements are found as the gases H2, O2, and N2. The subscript indicates the number of atoms of an element in a single molecule of a substance. Other elements are not normally diatomic but exist as single, or monatomic (mon = one), units—for example, the gases helium (He) and neon (Ne). These chemical symbols, or initials, indicate a single atom of that element.
When two different kinds of atoms combine, they form compounds. A compound is a chemical substance made up of atoms of two or more elements combined in a specific ratio and arrangement. The attractive forces that hold the atoms of a molecule together are called chemical bonds. Molecules can consist of two or more atoms of the same element (such as O2 or N2) or of specific numbers of atoms of different elements.
The formula of a compound describes what elements it contains (as indicated by a chemical symbol) and in what proportions they occur (as indicated by the subscript number). For example, pure water is composed of two atoms of hydrogen and one atom of oxygen. It is represented by the chemical formula H2O. The subscript “2” indicates two atoms of the element hydrogen, and the symbol for oxygen without a subscript indicates that there is only 1 atom of oxygen present in this molecule.


Common experience shows that all matter has a certain amount of kinetic energy. For instance, if you were to open a bottle of perfume in a closed room with no air movement, it wouldn’t take long for the aroma to move throughout the room. The kinetic molecular theory explains this by saying that the molecules diffuse, or spread, throughout the room because they are in constant, random motion. This theory also predicts that the rate at which they diffuse depends on the temperature of the room—the higher the air temperature, the greater the kinetic energy of the molecules and the more rapid the diffusion of the perfume.
Temperature is a measure of the average kinetic energy of the molecules making up a substance. The two most common numerical scales used to measure temperature are the Fahrenheit scale and the Celsius scale. When people comment on the temperature of something, they usually are making a comparison. For example, they may say that the air temperature today is “colder” or “hotter” than it was yesterday. They may also refer to a scale for comparison, such as “the temperature is 20°C [68°F].”
Heat is the total internal kinetic energy of molecules. Heat is measured in units called calories. A calorie is the amount of heat necessary to raise the temperature of 1 gram of water 1 degree Celsius (°C). The concept of heat is not the same as the concept of temperature. Heat is a quantity of energy. Temperature deals with the comparative hotness or coldness of things. The heat, or internal kinetic energy, of molecules can change as a result of interactions with the environment. This is what happens when you rub your hands together. Friction results in increased temperatures because molecules on one moving surface catch on another surface, stretching the molecular forces that are holding them. They are pulled back to their original position with a “snap,” resulting in an increase of vibrational kinetic energy. Heat (measured in calories) and temperature (measured in Celsius or Fahrenheit) are not the same thing but are related to one another. The heat that an object possesses cannot be measured with a thermometer. What a thermometer measures is the temperature of an object. The temperature is really a measure of how fast the molecules of the substance are moving and how often they bump into other molecules, a measure of their kinetic energy. If heat energy is added to an object, the molecules vibrate faster. Consequently, the temperature rises, because the added heat energy results in a speeding up of the movement of the molecules. Although there is a relationship between heat and temperature, the amount of heat, in calories, that an object has depends on the size of the object and its particular properties, such as its density, volume, and pressure.
Why do we take a person’s body temperature? The body’s size and composition usually do not change in a short time, so any change in temperature means that the body has either gained or lost heat. If the temperature is high, the body has usually gained heat as a result of increased metabolism. This increase in temperature is a symptom of abnormality, as is a low body temperature.


There are implications to the kinetic molecular theory. First, the amount of kinetic energy that particles contain can change. Molecules can gain or lose energy from their surroundings, resulting in changes in their behavior. Second, molecules have an attraction for one another. This force of attraction is important in determining the phase in which a particular kind of matter exists.
The amount of kinetic energy molecules have, the strength of the attractive forces between molecules, and the kind of arrangements they form result in three phases of matter: solid, liquid, and gas (figure 2.6). A solid (e.g., bone) consists of molecules with strong attractive forces and low kinetic energy. The molecules are packed tightly together. With the least amount of kinetic energy of all the phases of matter, these molecules vibrate in place and are at fixed distances from one another. Powerful forces bind them together. Solids have definite shapes and volumes under ordinary temperature and pressure conditions. The hardness of a solid is its resistance to forces that tend to push its molecules farther apart. There is less kinetic energy in a solid than in a liquid of the same material.
A liquid (e.g., the watery component of blood and lymph) has molecules with enough kinetic energy to overcome the attractive forces that hold molecules together. Thus, although the molecules are still strongly attracted to each other, they are slightly farther apart than in a solid. Because they are moving more rapidly, and the attractive forces can be overcome, they sometimes slide past each other as they move. Although liquids can change their shape under ordinary conditions, they maintain a fixed volume under ordinary temperature and pressure conditions—that is, a liquid of a certain volume will take the shape of the container into which it is poured, but it will take up the same amount of space regardless of the container’s shape. This gives liquids the ability to flow, so they are called fluids.
A gas (e.g., air and the components of air that are present in the blood) is made of molecules that have a great deal of kinetic energy. The attraction the gas molecules have for each other is overcome by the speed with which the individual molecules move. Because gas molecules are moving faster than the molecules of solids or liquids, their collisions tend to push them farther apart, so a gas expands to fill its container. The shape of the container and the pressure determine the shape and volume of the gas. The term vapor is used to describe the gaseous form of a substance, that is normally in the liquid phase. Water vapor, for example, is the gaseous form of liquid water.

Figure 2.6


Any positively or negatively charged atom or molecule is called an ion. Ionic bonds are formed after atoms transfer electrons to achieve a full outermost energy level. Electrons are donated or received in the transfer, forming a positive and a negative ion, a process called ionization. The force of attraction between oppositely charged ions forms ionic bonds, and ionic compounds are the result. Ionic compounds are formed when an element from the left side of the periodic table (those eager to gain electrons) reacts with an element from the right side (those eager to donate electrons). This results in the formation of a stable group, which has an orderly arrangement and is a crystalline solid (figure 2.7).
Ions and ionic compounds are very important in living systems. For example, sodium chloride is a crystal solid known as table salt. A positively charged sodium ion is formed when a sodium atom loses 1 electron. This results in a stable, outermost energy level with 8 electrons. When an atom of chlorine receives an electron to stabilize its outermost energy level, it becomes a negative ion. All positively charged ions are called cations and all negative charged ions are called anions (figure 2.8). When these oppositely charged ions are close to one another, the attractive force between them forms an ionic bond. The dots in the following diagram represent the electrons in the outermost energy levels of each atom. This kind of diagram is called an electron dot formula.

Image 1

When many ionic compounds are dissolved in water, the ionic bonds are broken and the ions separate, or dissociate, from one another. For example, solid sodium chloride dissociates in water to become ions in solution:

NaCl → Na+ + Cl-

Any substance that dissociates into ions in water and allows the conduction of electric current is called an electrolyte (How Science Works 2.2).

Figure 2.7

Figure 2.8

Work 2.2


Most substances do not have the properties of ionic compounds, because they are not composed of ions. Most substances are composed of electrically neutral groups of atoms that are tightly bound together. As noted earlier, many gases are diatomic, occurring naturally as two of the same kinds of atoms bound together as an electrically neutral molecule. Hydrogen, for example, occurs as molecules of H2 and no ions are involved. The hydrogen atoms are held together by a covalent bond, a chemical bond formed by the sharing of a pair of electrons. In the diatomic hydrogen molecule, each hydrogen atom contributes a single electron to the shared pair. Hydrogen atoms both share one pair of electrons, but other elements might share more than one pair.
Consider how the covalent bond forms between two hydrogen atoms by imagining two hydrogen atoms moving toward one another. Each atom has a single electron. As the atoms move closer and closer together, their outer energy levels begin to overlap. Each electron is attracted to the oppositely charged nucleus of the other atom and the overlap tightens. Then, the repulsive forces from the like-charged nuclei stop the merger. A state of stability is reached between the 2 nuclei and 2 electrons, because the outermost energy level is full and an H2 molecule has been formed. The electron pair is now shared by both atoms, and the attraction of each nucleus for the electron of the other holds the atoms together (figure 2.9).
Dots can be used to represent the electrons in the outer energy levels of atoms. If each atom shares one of its electrons with the other, the two dots represent the bonding pair of electrons shared by the two atoms. Bonding pairs of electrons are often represented by a simple line between two atoms, as in the following example:

Image 2

A covalent bond in which a single pair of electrons is shared by two atoms is called a single covalent bond or, simply, a single bond. Some atoms can share more than one electron pair. A double bond is a covalent bond formed when two pairs of electrons are shared by two atoms. This happens mostly in compounds involving atoms of the elements C, N, O, and S. For example, ethylene, a gas given off by ripening fruit, has a double bond between the two carbons (figure 2.10). The electron dot formula for ethylene is

Image 3

A triple bond is a covalent bond formed when three pairs of electrons are shared by two atoms. Triple bonds occur mostly in compounds with atoms of the elements C and N. Atmospheric nitrogen gas, for example, forms a triple covalent bond:

Image 4

Figure 2.9

Figure 2.10


A mixture is matter that contains two or more substances that are not in set proportions (figure 2.12). A solution is a liquid mixture of ions or molecules of two or more substances. For example, salt water can be composed of varying amounts of NaC1 and H2O. If the components of the mixture are distributed equally throughout, the mixture is homogenous. The process of making a solution is called dissolving. The amounts of the component parts of a solution are identified by the terms solvent and solute. The solvent is the component present in the larger amount. The solute is the component that dissolves in the solvent. Many combinations of solutes and solvents are posssible. If one of the components of a solution is a liquid, it is usually identified as the solvent. An aqueous solution is a solution of a solid, liquid, or gas in water. When sugar dissolves in water, sugar molecules separate from one another. The molecules become uniformly dispersed throughout the molecules of water. In an aqueous salt solution, however, the salt dissociates into sodium and chlorine ions.
The relative amounts of solute and solvent are described by the concentration of a solution. In general, a solution with a large amount of solute is “concentrated,” and a solution with much less solute is “dilute,” although these are somewhat arbitrary terms.

Figure 2.12


An oxidation-reduction reaction is a chemical change in which electrons are transferred from one atom to another and, with it, the energy contained in its electrons. As implied by the name, such a reaction has two parts and each part tells what happens to the electrons. Oxidation is the part of an oxidationreduction reaction in which an atom loses an electron. Reduction is the part of an oxidation-reduction reaction in which an atom gains an electron. When the term oxidation was first used, it specifically meant reactions involving the combination of oxygen with other atoms. But fluorine, chlorine, and other elements were soon recognized to participate in similar reactions, so the definition was changed to describe the shifts of electrons in the reaction. The name also implies that, in any reaction in which oxidation occurs, reduction must also take place. One cannot take place without the other. Cellular respiration is an oxidation-reduction reaction that occurs in all cells:

C6H12O6 + 6 O2 → 6 H2O + 6 CO2 + energy

sugar + oxygen → water +  carbon + energy

In this cellular respiration reaction, sugar is being oxidized (losing its electrons) and oxygen is being reduced (gaining the electrons from sugar). The high chemical potential energy in the sugar molecule is released, and the organism uses some of this energy to perform work. In the photosynthesis reaction, water is oxidized (loses its electrons) and carbon dioxide is reduced (gains the electrons from water). The energy required to carry out this reaction comes from the sunlight and is stored in the product, sugar.


Dehydration synthesis reactions are chemical changes in which water is released and a larger, more complex molecule is made (synthesized) from smaller, less complex parts. The water is a product formed from its component parts (H and OH), which are removed from the reactants. Proteins, for example, consist of a large number of amino acid subunits joined together by dehydration synthesis:

Image 5

The building blocks of protein (amino acids) are bonded to one another to synthesize larger, more complex product molecules (i.e., protein). In dehydration synthesis reactions, water is produced as smaller reactants become chemically bonded to one another, forming fewer but larger product molecules.


Hydrolysis reactions are the opposite of dehydration synthesis reactions. In a hydrolysis reaction, water is used to break the reactants into smaller, less complex products:

Image 6

A more familiar name for this chemical reaction is digestion. This is the kind of chemical reaction that occurs when a protein food, such as meat, is digested. Notice in the previous example that the H and OH component parts of the reactant water become parts of the building block products. In hydrolysis reactions, water is used as a reactant, and larger molecules are broken down into smaller units.


A phosphorylation reaction takes place when a cluster of atoms known as a phosphate group

Image 7

is added to another molecule. This cluster is abbreviated in many chemical formulas in a shorthand form as P, and only the P is shown when a phosphate is transferred from one molecule to another. This is a very important reaction, because the bond between a phosphate group and another atom contains the potential energy that is used by all cells to power numerous activities. Phosphorylation reactions result in the transfer of their potential energy to other molecules to power the activities of all organisms (figure 2.13).

              high               low                  low                    high
             potential        potential       potential            potential
             energy            energy            energy              energy

             Q–P        +      Z          →      Q        +       Z–P

This type of reaction is commonly involved in providing the kinetic energy needed by all organisms. It can also take place in reverse. When this occurs, energy must be added from the environment (sunlight or another phosphorylated molecule) and is stored in the newly phosphorylated molecule.

Figure 2.13


Acid-base reactions take place when the ions of an acid interact with the ions of a base, forming a salt and water. An aqueous solution containing dissolved acid is a solution containing hydrogen ions. If a solution containing a second ionic basic compound is added, a mixture of ions results. While they are mixed together, a reaction can take place—for example,

          H+Cl-     +   Na+OH-     → Na+Cl-      + HOH
    hydrochloric +   sodium        → sodium     + water
         acid              hydroxide       chloride         (H2O)

In an acid-base reaction, the H from the acid becomes chemically bonded to the OH of the base. This type of reaction frequently occurs in organisms and their environment. Because acids and bases can be very harmful, reactions in which they neutralize one another protect organisms from damage.


Acids, bases, and salts are three classes of biologically important compounds (table 2.3). Their characteristics are determined by the nature of their chemical bonds. Acids are ionic compounds that release hydrogen ions in solution. A hydrogen atom without its electron is a proton. You can think of an acid, then, as a substance able to donate a proton to a solution. Acids have a sour taste, such as that of citrus fruits. However, tasting chemicals to see if they are acids can be very hazardous, because many are highly corrosive. An example of a common acid is the phosphoric acid—H3PO4—in cola soft drinks. It is a dilute solution of this acid that gives cola drinks their typical flavor. Hydrochloric acid is another example:

Image 8

A base is the opposite of an acid, in that it is an ionic compound, which, when dissolved in water, removes hydrogen ions from solution. Bases, or alkaline substances, have a slippery feel on the skin. They have a caustic action on living tissue by converting the fats in living tissue into a water-soluble substance. A similar reaction is used to make soap by mixing a strong base with fat. This chemical reaction gives soap its slippery feeling. Bases are also used in alkaline batteries. Weak bases have a bitter taste—for example, the taste of broccoli, turnip, and cabbage. Many kinds of bases release a group of hydrogen ions known as a hydroxide ions, or an OH-group. This group is composed of an oxygen atom and a hydrogen atom bonded together, but with an additional electron. The hydroxide ion is negatively charged; therefore, it will remove positively charged hydrogen ions from solution. A very strong base used in oven cleaners is sodium hydroxide, NaOH. Notice that ions that are free in solution are always written with the type and number of their electrical charge as a superscript.

Image 9

Acids and bases are also spoken of as being strong or weak (Outlooks 2.2). Strong acids (e.g., hydrochloric acid) are those that dissociate nearly all of their hydrogens when in solution. Weak acids (e.g., phosphoric acid) dissociate only a small percentage of their hydrogens. Strong bases dissociate nearly all of their hydroxides (NaOH); weak bases, only a small percentage. The weak base sodium bicarbonate, NaHCO3, will react with acids in the following manner:

            NaHCO3 + HCl → NaCl + CO2 + H2O

Notice that sodium bicarbonate does not contain a hydroxide ion but it is still a base, because it removes hydrogen ions from solution.
The degree to which a solution is acidic or basic is represented by a quantity known as pH. The pH scale is a measure of hydrogen ion concentration (figure 2.14). A pH of 7 indicates that the solution is neutral and has an equal number of H-ions and OH-ions to balance each other. As the pH number gets smaller, the number of hydrogen ions in the solution increases. A number higher than 7 indicates that the solution has more OH- than H+. Pure water has a pH of 7. As the pH number gets larger, the number of hydroxide ions increases. It is important to note that the pH scale is logarithmic—that is, a change in one pH number is actually a 10-fold change in real numbers of OH-or H+. For example, there is 10 times more H+ in a solution of pH 5 than in a solution of pH 6 and 100 times more H+ in a solution of pH 4 than in a solution of pH 6.

Image 11

Salts are ionic compounds that do not release either H+ or OH-when dissolved in water; thus, they are neither acids nor bases. However, they are generally the result of the reaction between an acid and a base in a solution. For example, when an acid, such as HCl, is mixed with NaOH in water, the H+ and the OH-combine with each other to form pure water, H2O. The remaining ions (Na+and Cl- ) join to form the salt NaCl:

Image 10

The chemical reaction that occurs when acids and bases react with each other is called neutralization. The acid no longer acts as an acid (it has been neutralized) and the base no longer acts as a base.
As you can see from figure 2.14, not all acids or bases produce the same pH. Some compounds release hydrogen ions very easily, cause low pHs, and are called strong acids. Hydrochloric acid (HCl) and sulfuric acid (H2SO4) are strong acids (figure 2.15a). Many other compounds give up their hydrogen ions grudgingly and therefore do not change pH very much. They are known as weak acids. Carbonic acid (H2CO3) and many organic acids found in living things are weak acids. Similarly, there are strong bases, such as sodium hydroxide (NaOH) and weak bases, such as sodium bicarbonate—Na(HCO3)-.

Figure 2.14

Figure 2.15

Outlook 2.2