For thousands of years people have known that vinegar, lemon juice and many other foods taste sour. However, it was not until a few hundred years ago that it was discovered why these things taste sour – because they are all acids. The term acid, in fact, comes from the Latin term acre, which means “sour”. While there are many slightly different definitions of acids and bases.
In the seventeenth century, the Irish writer and amateur chemist Robert Boyle first labeled substances as either acids or bases (he called bases alkalies) according to the following characteristics:
Acid (often represented by the general formula HA) is generally a chemical compound that when dissolved in water will produce a solution with a pH less than 7. In the modern definition, the acid is a substance that can give a proton (H +) to other substances (called bases), or can accept a lone pair of bases. An acid reacts with a base in a neutralization reaction to form salts. Examples of acids are acetic acid (found in vinegar) and sulfuric acid (used in batteries or car battery) acid is generally taste sour, acidic taste however mainly concentrated acid can be dangerous. In general, acids have the following properties:
Taste: Sour when dissolved in water.
Touch: Acid stung when touched, especially a strong acid.
Reactivity: acid reacts violently with most metal, which is corrosive to metal.
Acids taste sour, are corrosive to metals, change litmus (a dye extracted from lichens) red, and become less acidic when mixed with bases.
General definition of the base is a chemical compound that absorbs hydronium ions when dissolved in water. Tongues is the opposite of acid, which is devoted to the elements / compounds that have a pH greater than 7. Bases are compounds that when dissolved in water produces OH-ions.
Generally speaking tongue has properties as follows:
Taste: Not sour when dissolved in water.
Touch: Does not feel the sting when touched.
Reactivity: Most do not react to metal.
feel slippery, change litmus blue, and less basic Become mixed with acids.
While Boyle and others tried to explain why acids and bases behave the way they do, the first reasonable definition of acids and bases would not be proposed until 200 years later.
In the late 1800s, the Swedish scientist Svante Arrhenius proposed that water can dissolve many compounds by separating them into their individual ions. Arrhenius suggested that acids are compounds that contain hydrogen and can dissolve in water to release hydrogen ions into solution.
Arrhenius defined bases as substances that dissolve in water to release hydroxide ions (OH-) into solution. For example, a typical base according to the Arrhenius definition is sodium hydroxide (NaOH):
NaOH ———— Na+(aq) + OH-(aq)
The Arrhenius definition of acids and bases explains a number of things. Arrhenius theory explains why all acids have similar properties to each other (and, conversely, why all bases are similar): because all acids release H+ into solution (and all bases release OH-). The Arrhenius definition also explains Boyle’s observation that acids and bases counteract each other. This idea, that a base can make an acid weaker, and vice versa, is called neutralization.
In 1923, the Danish scientist Johannes Bronsted and the Englishman Thomas Lawry published independent yet similar papers that refined Arrhenius’theory. In Brønsted’s words, “… acids and bases are substances that are capable of splitting off or taking up hydrogen ions, respectively.” The Brønsted-Lowry definition broadened the Arrhenius concept of acids and bases.
The Brønsted-Lowry definition of acids is very similar to the Arrhenius definition, any substance that can donate a hydrogen ion is an acid (under the Brønsted definition, acids are often referred to as proton donors because an H+ ion, hydrogen minus its electron, is simply a proton).
The Brønsted definition of bases is, however, quite different from the Arrhenius definition. The Brønsted base is defined as any substance that can accept a hydrogen ion. In essence, a base is the opposite of an acid. NaOH and KOH, as we saw above, would still be considered bases because they can accept an H+ from an acid to form water. However, the Brønsted-Lowry definition also explains why substances that do not contain OH- can act like bases. Baking soda (NaHCO3), for example, acts like a base by accepting a hydrogen ion from an acid as illustrated below:
Acid Base Salt
HCl + NaHCO3 ———— H2CO3 + NaCl
In this example, the carbonic acid formed (H2CO3) undergoes rapid decomposition to water and gaseous carbon dioxide, and so the solution bubbles as CO2 gas is released.
pH or acidity level is used to declare same or base which is owned by a substance, solution or objects. normally have a pH value of 7 while when the pH value> 7 indicates alkaline substances have the property while the value of
pH <7 indicates acidity. pH of 0 indicates a high degree of acidity, and pH 14 showed the highest degree of alkalinity. Indicator generally used is a simple litmus paper turns red when the high acidity and blue when the acidity is low.
Besides using litmus paper, acid-base indicator can be measured with a pH meter that works on the principle of electrolyte / conductivity of a solution. PH measuring system has three parts, namely the measurement of pH electrode, reference electrode and high impedance measuring device. The term pH is derived from “p”, the symbol of the negative logarithm of mathematics, and “H”, the chemical symbol for the element Hydrogen. A formal definition of pH is the negative logarithm of Hydrogen ion activity. pH stands for power of Hydrogen.pH = -log[H+]
Understanding Degrees of acidity
To understand the basic terms of acidity are described briefly below about ionization.
When an atom receives additional energy from outside, it will increase the atomic electron inetic energy.
It will move the electron energy level to a higher level. The electrons will move toward the outer skin that eventually if the received energy can be large enough to separate electrons from atoms.
Of this atom will get two particles, each of which positively and negatively charged particles. Atomic particles that releaseelectrons are called positive ions. Atom can also receive so will the excess electron electron. Such particles are also called ion but a negative ion.
The molecules of a substance in the solution can conduct electricity are called electrolytes. Negative ions move toward the anode, therefore the negative ions are called anions.
Positive ions move toward the cathode, therefore the positive ions are called cations
An electrolyte solution, molecule unrevel into ions. Pure water classified as a weak electrolyte. Some molecule unrevel into ions H+ and OH-.
H+ + OH——–H2O
From the equation above, 1 H+ ion and 1 OH- ions derived from the decomposition of H2O 1 molecule. Thus, the concentration of H+ ion concentration equal to ion OH- .Water solution as it was called with Neutral solution. Solution containing H+ ion concentration is greater than the concentration of OH- and solution acid, while the solution containing smaller H+ ion concentration of OH- ion concentration of the solution is called tongue. Acid solution can receive a free electron, while the base can provide a free electron.
The number of solutions that break down into ions is called the degree of ionization. The amount ranged from 0 to 1. A ionization of the electrolyte large degrees, close to 1 are called strong electrolytes, while small degree of ionization close to 0 are called weak electrolytes .Ionization have the equilibrium constant (K). Eg for water, the equilibrium can be calculated by the formula:
K= ((H^++)+ (OH^–))/H2O
Because the concentration of large H2Orelatif, then this equation can be rewritten as:
K(H2O) = (H+) . (OH-)
In the pure water with temperature 25°C, concentration H+=10-7 mol/liter, whereas the product of concentration H+ with OH-= 10- 14. concentration H+= concentration OH -=10 -7. To determine the acid or alkaline pH scale is required as follows:
Picture. pH scale for some every substances
Acid Tongue with indicator
a. litmus paper
Litmus paper is paper that is given a chemical compound that will show different colors when put in acid or alkaline solution. Litmus paper will change color according to the solution as listed in Table1.
Table 1. The color of litmus paper when subjected to a solution of acid-base
kind of litmus paper In solution is
Acid Base Neutral
Red Red Blue Red
Blue Red Blue Blue
Below are given some testing using litmus paper.
Table 2. The color of litmus paper if drops of sample solution.
Substance Red litmus Blue litmus
HCl 0,1 M
NaOH 0,1 M Still Red
Blue Still blue
From these data we can conclude that:
1) Water is neutral because it does not give a color change in litmuspaper.
2) HCl solution is acidic because it can redden blue litmus.
3) NaOH solution is alkaline because it can color blue litmus red.
Relationship electrical Conductivity, pH strength, with acid and base
In an electrolyte solution, there are two kinds of solutions, namely a strong solution and weak solution, strong electrolyte solution can be determined by the flame of the lamp and the gas bubbles. Adapaun weak electrolyte compounds, the lights remain on, but dim or no light. Nevertheless there is still a gas bubble. This same thing happened to a solution of acid / strong base which can be determined with the flame of the lamp and the presence of gas bubbles.
Strong acid reacts with water to give a solution containing hydronium ions (H3O+) and anions of the acid.
Acid-base strength can be determined from the pH of the solution with the same concentration. PH of strong acids smaller than the pH of a weak acid, while the pH of strong base is greater than the pH of a weak base strong acid is a compound electorit strong, in water of these compounds can have H+ ions are perfectly (so as to produce lights and bubbles of gas (many) strong acid have the value of a small pH range 1-2
The compound is a weak acid weak electrolyte. In the water. This compound produces H+ in sodium absorption ratio ion perfect that it can not turn on lights and a little gas bubbles. Weak acid has a pH value of which ranges between 3-5
Strong base is a weak electrolyte compound. In water, these compounds can hanve completely OH-ion. (Whole molecule alkaline ion) so as to produce flame and gas bubbles. Strong base also has a small pOH values (pH values ranging between 12-13)
Weak base compound is a weak electrolyte. In water, these compounds produce OH-ions are not perfect so it can not turn on the lights. Weak base has a pOH value (the price of a small pH range between 9-11). Price range depends on the concentration of compounds.
The solution does not produce the light and the gas bubbles is a non electrolyte. Because the solution can not produce ions- or ion+.
Chang, Raymond. 2003. General Chemistry: The Essential Concepts. Third Edition. Boston: Mc Graw-Hill.
Goldberg, David E. 2004. Fundamentals of Chemistry. Fourth Edition. New York The McGraw – Hill Companies, Inc.
Heyworth, Rex. 1990. Chemistry A New Approach. Hongkong: Macmillan Publishers (HK) Limited.