When a solution has an equal number of hydrogen and hydroxide ions The solution is considered?

Let's look at the whole picture now. There is a scale for acids and bases just like everything else. Here are a couple of definitions you should know:

Acid: A solution that has an excess of H+ ions. It comes from the Latin word acidus, which means "sharp" or "sour".

Base: A solution that has an excess of OH- ions. Another word for base is alkali.
Aqueous: A solution that is mainly water. Think about the word aquarium. AQUA means water.
Strong Acid: An acid that has a very low pH (0-4).
Strong Base: A base that has a very high pH (10-14).
Weak Acid: An acid that only partially ionizes in an aqueous solution. This means that not every molecule breaks apart. Weak acids usually have a pH close to 7 (3-6).
Weak Base: A base that only partially ionizes in an aqueous solution. This means that not every molecule breaks apart. Weak bases usually have a pH close to 7 (8-10).
Neutral: A solution that has a pH of 7. It is neither acidic nor basic.
We told you about that guy Arrhenius and his ideas about concentrations of hydrogen and hydroxide ions. You're also going to learn about Brønsted-Lowry ideas. These two chemists from Denmark and England looked at acids as donors and bases as acceptors. What were they donating and accepting? Hydrogen ions. It's a lot like the first definition we gave, where an acid breaks up and releases/donates a hydrogen ion. This newer definition is a little bit more detailed. Scientists used the new definition to describe more bases, such as ammonia (NH3). Since bases are proton acceptors, when ammonia was seen accepting an H+ and creating an ammonium ion (NH4+), it could be labeled as a base. You didn't have to worry about hydroxide ions anymore. If it got the H+ from a water molecule, then the water (H2O) was the proton donor. Does that mean the water was the acid in this situation? Yes.

A chemist named Lewis offered a third way to look at acids and bases. Instead of looking at hydrogen ions, he looked at pairs of electrons (remember our pictures with dot structures in Atoms and Elements?). In Lewis' view, acids accept pairs of electrons and bases donate pairs of electrons. We know that both of these descriptions of acids and bases use completely opposite terms, but the idea is the same. Hydrogen ions still want to accept two electrons to form a bond. Bases want to give them up. Overall, Lewis' definition was able to classify even more compounds as acids or bases.
What really happens in those solutions? It gets a little tricky here. Let's look at the breakup of molecules in aqueous (water-based) solutions one more time for good measure. Acids are compounds that dissociate (break) into hydrogen (H+) ions and another compound when placed in an aqueous solution. Remember that acetic acid example? Bases are compounds that break up into hydroxide (OH-) ions and another compound when placed in an aqueous solution. We'll talk about baking soda in a few paragraphs.

Let's change the wording a bit. If you have an ionic/electrovalent compound and you put it in water, it will break apart into two ions. If one of those ions is H+, the solution is acidic. The strong acid hydrogen chloride (HCl) is one example. If one of the ions is OH-, the solution is basic. An example of a strong base is sodium hydroxide (NaOH). There are other ions that make acidic and basic solutions, but we won't be talking about them here.

That pH scale we talked about is actually a measure of the number of H+ ions in a solution. If there are a lot of H+ ions, the pH is very low. If there are a lot of OH- ions compared to the number of H+ ions, the pH is high.

Think about this idea for a second: Why would a liquid with high levels of NaOH be very basic, yet dangerous at the same time? The Na-OH bond breaks in solution and you have sodium ions (positive) and hydroxide ions (negative). The sodium ions don't really pose a danger in solution, but there are a huge number of hydroxide ions in solution compared to the hydrogen ions that might be floating around as H3O+ (a hydronium ion).All of those excess OH- ions make the pH super high, and the solution will readily react with many compounds. The same thing happens on a less dangerous scale when you add baking soda to water. During the dissociation, OH- ions and carbonic acid are released in the solution. The number of OH- ions is greater than the number of H3O+ ions (H+ and H2O), and the pH increases. It's just not as strong a difference as in sodium hydroxide. That's basically it. (Ha ha! Get it?)

More information in part one.

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Learning objectiveS

Define and differentiate the terms acid and base
Define the terms pH, neutral, acidic, and basic (or alkaline)
Define the term buffer, and compare the response of a regular solution with a buffer solution to the addition of acid or base

An acid is a substance or compound that releases hydrogen ions (H+) when in solution. In a strong acid, such as hydrochloric acid (HCl), all hydrogen ions (H+), and chloride ions (Cl-) dissociate (separate) when placed in water and these ions are no longer held together by ionic bonding. In a weak acid, such as carbonic acid (H2CO3), only some of the ions dissociate into hydrogen ions (H+) and bicarbonate ions (HCO3-), while others are still held together by ionic bonding.

A base is a substance that releases hydroxyl ions (OH-) when in solution. The hydroxyl ions (OH-) released will combine with any hydrogen ions (H+) in the solution to form water molecules (OH- + H+ = H2O), so we can also define a base as a substance that takes or accepts hydrogen ions (H+) already present in the solution.

Sodium hydroxide (NaOH) is a strong base because when placed in water, it dissociates completely into sodium ions (Na+) and hydroxyl ions (OH-), all of which are now released and dissolved in water.

Acids, bases and salts, dissociate (separate) into electrolytes (ions) when placed in water. Acids dissociate into H+ and an anion, bases dissociate into OH- and a cation, and salts dissociate into a cation (that is not H+) and an anion (that is not OH-).

Figure \(\PageIndex{1}\) (a) In aqueous (watery) solution, an acid dissociates into hydrogen ions (H+) and anions. Every molecule of a strong acid dissociates, producing a high concentration of H+. (b) In aqueous solution, a base dissociates into hydroxyl ions (OH–) and cations. Every molecule of a strong base dissociates, producing a high concentration of OH–.

When an acid and a base react (combine) releasing equal quantities of H+ ions and OH- ions, neutralization results. H+ ions and OH- ions combine (neutralize each other) to regenerate water.

Concepts, terms, and facts check

Study Questions Write your answer in a sentence form (do not answer using loose words)

1. What is an acid?
2. What is a base?

pH is a unit of measurement of the concentration of hydrogen ions (H+) and hydroxyl ions (OH-) in an aqueous (water) solution. Pure water is said to be neutral with a pH of 7, because there are very few H+ and OH- ions in equal concentrations (only 1 in 10,000,000 water molecules dissociate to H+ and OH-, which gives a pH of 7). Adding equal amounts of H+ and OH- to water will also be neutral with a pH of 7, because most of these ions combine to form water molecules and the remaining H+ and OH- ion concentration is equal and very low.

When H+ concentration is higher than OH- concentration, the solution is acidic, and the pH of the solution is indicated with a number below 7. Saliva, coffee, lemon juice, tomato juice, and the acid in a battery are all acidic, so in all of them the concentration of H+ is higher than the concentration of OH-. The more H+ in a solution the more acidic and the lower is its pH (See Figure \(\PageIndex{2}\) below).

When H+ concentration is lower than OH- concentration, the solution is basic or alkaline, and the pH of the solution is indicated with a number above 7. Blood, baking soda, ammonia and bleaches are all basic, so in all of them the concentration of H+ is lower than the concentration of OH-.

Figure \(\PageIndex{2}\) pH of various solutions. The lower the pH, the more
hydrogen ions (H+) the solutions has. The higher the pH, the less hydrogen ions the solution has.

Concepts, terms, and facts check

Study Questions Write your answer in a sentence form (do not answer using loose words)

1. What is pH? 2. What is a neutral solution? 3. What is an acidic solution?

4. What is a basic (or alkaline) solution?

Chemical reactions in the body, the food we eat, medication we take, and the effects of some diseases can add or remove hydrogen or hydroxyl ions in or from our body fluids. Levels of these ions, especially H+ since body cells are constantly producing H+ as a waste product of cell activity, must be maintained within a normal range (slightly alkaline pH between 7.35 and 7.45,). Then, all cells in our body depend on homeostatic regulation of acid-base balance to maintain pH within optimal living conditions.

There are several homeostatic mechanisms to maintain pH within optimal conditions. It can be regulated by the internal availability of substances (chemicals), by adjusting breathing rate, and by eliminating chemicals in urine. Chemical buffers in the body are substances that can absorb extra hydrogen ions preventing a change in pH. For example, during exercise muscle cells can produce excess lactic acid, which increases hydrogen ions (acids release hydrogen ions). These hydrogen ions tend to make our body fluids more acidic, but chemical buffers in the body absorb them preventing a pH change. See below a table comparing what happens when acid or base are added to a plain solution (no buffer), or to a solution that absorbs hydrogen ions or hydroxyl ions (buffer).

Table \(\PageIndex{1}\) Comparison between a regular solution without buffering properties and a buffer solution with buffering properties

	Regular solution (without buffering properties)	Buffer solution (with buffering properties)
When acid is added, it releases hydrogen ions and...	the pH drops and the solution becomes more acidic	the pH does not drop
When base is added, it absorbs hydrogen ions (or releases hydroxyl ions) and...	the pH rises and the solution becomes more basic (alkaline)	the pH does not drop

Bicarbonate, phosphates, and proteins work as a chemical buffer in our body fluids. They absorb extra hydrogen ions or extra hydroxyl ions released from the things we make or eat.

Concepts, terms, and facts check

Study Question Write your answer in a sentence form (do not answer using loose words)

1. What is a buffer? 2. What happens to the pH of a plain solution when acid is added to it?

3. What happens to the pH of a buffer solution when acid is added to it?