Acids and bases are fundamental topics for any chemistry class. Learning how to properly identify, store and use these substances in the laboratory and in everyday life is essential for anyone. Although they seem harmful, there are plenty of acids and bases working in the human body.
How is an acid different from a base?
Various definitions for acids and bases exist. In a general sense, acids are substances with a pH level lower than 7, while a level higher than this is indicative of bases. People can also use indicators – from cheap litmus paper to reagents like phenolphthalein – to quickly distinguish between the two.
What is an Acid?
The exact definition of an acid changes slightly depending on which theory is used. As a rule of thumb, an acid refers to anything with a pH reading that is lower than 7.
These substances possess a sour taste, and many weak acids, such as the acetic acid in vinegar, are frequently consumed. Contact with an acid turns the color of both litmus paper and methyl orange into red, while changing phenolphthalein into a colorless substance. Acids also react with metals, emitting hydrogen gas in the process.
By the Brønsted-Lowry model, acids “donate” electrons in water. The Arrhenius concept stresses that these electrons have to form hydronium ions for the substance to be considered an acid. The Lewis theory simply considers an acid as something that accepts electron pairs.
What is a Base?
Bases, in the simplest sense, refer to substances with a higher pH level than 7.
People who have eaten soap as children can attest to bases tasting bitter. They can also be slippery when handled. A base will cause a strip of litmus paper to become blue, while methyl orange would turn yellow. Phenolphthalein added to a base in water will cause the solution to turn bright pink.
A base will “accept” electrons in water according to the Brønsted-Lowry concept. Its definition in the Arrhenius theory states that it adds hydroxyl (OH-) ions in an aqueous solution. It is described in the Lewis concept as something that gives electron pairs.
Differences Between an Acid and a Base
The pH scale can be used to determine whether a substance is acidic or basic based on the concentration of hydrogen ions (H+) in the solution.
Any substance with a pH under 7 is considered an acid. Likewise, anything with a pH over 7 is a base. Substances whose pH is 7 itself are neutral, such as distilled water.
Dissociation in Aqueous Solutions
Acids and bases both break up – or dissociate – in an aqueous solution.
An acid added to water will dissociate into their respective anions, as well as hydrogen ions (H+). However, these ions will almost instantaneously react with water molecules (H2O) to produce hydronium ions (H3O+).
A base will instead break up into cations and hydroxyl ions (OH-).
Result with Litmus Paper
Litmus paper is among the most common indicators used to distinguish acids and bases. Soaked in an acid, a piece of litmus turns red. In a base, the paper will become blue.
The litmus test is not totally specific. The paper will turn red when an acid’s pH level is 4.5 or below; weaker acids will only induce a lighter pink color. Likewise, the color blue is achieved when a base’s pH is 8.3 or higher. Weak bases produce a faded color.
Result with Methyl Orange
Methyl orange is a powdery, orange substance in its original form. When added to acids with a pH of 3.1 or less, it will quickly cause the solution to turn bright red. Substances above pH 4.4 reacting with this indicator turn yellow.
It is a common tool for titrating acids due to its sensitivity in detecting low pH levels. Conversely, since it turns yellow at pH 4.4, it can incorrectly identify weak acids as bases.
Result with Phenolphthalein
Phenolphthalein – or Hln, for short – is a white powder that becomes a colorless and slightly acidic solution in water. From a pH of 0.0 to below 8.3, any solution with Hln remains colorless. Substances with a pH greater than 8.3, however, become vividly pink, making Hln a useful indicator for bases.
Some acids are so strong that their pH level is below zero. Exposure to such substances causes an Hln solution to turn red.
Reaction to Metals
Most metals can react with acids through a displacement reaction, in which the metallic compound trades atoms with the acid compound. The products of such a reaction are a metal salt and hydrogen gas.
In contrast, bases only react with a few metals, such as aluminum or zinc. This process also releases hydrogen gas.
Weak acids and bases can be discerned through their taste, although this is not a recommended method for identifying them.
Acids will taste sour, as is the case for citric acid in lemons. Most bases, on the other hand, have a bitter taste.
People are familiar with many acids and bases in daily life.
Some common acids include Vitamin C or ascorbic acid, the acetic acid in vinegar, the formic acid that can be smelled in some ants, and uric acid in urine.
Household bases come as sodium hydroxide in most soaps, magnesium hydroxide in laxatives and antacids, and ammonia in bleach.
Comparison Chart: Acids Vs Bases
|pH Level||Below 7.0||Above 7.0|
|Dissociation||Breaks up into anions and hydrogen ions (H+), which can form hydronium ions (H3O+) with water||Dissociates into cations and hydroxyl ions (OH-)|
|Litmus Paper Result||Below pH 4.5: Red - pH 4.6-6.9: Light red or pink||Above pH 8.3: Blue - pH 7.1-8.2: Light blue|
|Methyl Orange Result||Below pH 3.3: Red||Above pH 4.4: Yellow|
|Phenolphthalein Result||Below 8.3: Colorless||Above 8.3: Pink|
|Reaction to Metals||Reacts with most metals, producing metal salt and hydrogen gas||Reacts with few metals, emitting hydrogen gas|
|Taste||Weak acids taste sour||Weak bases usually taste biter|
|Household Sources||Vitamin C, vinegar, lemons, some ant species, urine||Soaps, toothpaste, antacids, laxatives, cleaning products|
How are Acids and Bases similar?
Despite having mostly contrasting properties, acids and bases are the same in some aspects.
They are both able to react with water and dissociate into ions, which also means many acids and bases are at least slightly soluble in aqueous solutions. By dissolving into ions, acids and bases in water are also good conductors of electricity.
Both substances also react with metals, although bases do so with fewer types. Acids and bases can react with each other to form salts and water.
What kind of reaction occurs between acids and bases?
A neutralization reaction occurs when acids and bases in contact with each other. This phenomenon is also known as a double displacement or salt metathesis reaction. In this kind of reaction, parts are exchanged between each compound to produce new species.
As a very common example, hydrochloric acid (HCl) and lye (sodium hydroxide or NaOH) – both very harmful substances – combine into table salt (sodium chloride or NaCl) and water (dihydrogen monoxide or H2O).
Which acid is present in sour milk?
Milk naturally contains a sugar called lactose. In room temperature, any bacteria present in milk can convert this lactose into lactic acid through fermentation.
Lactic acid is responsible for the flavor of sour milk while also causing it to thicken, coagulating into curds. This reaction is harnessed to create yoghurts and cheeses.
The acid can also build up in the muscles during strenuous exercise, with its acidity damaging the tissue to cause the stinging feeling that occurs when muscles are overexerted.
Acids and bases oppose each other in many ways.
Their foremost difference stems from their pH level – acids have a pH less than 7.0, while bases have a pH over 7.0. Bases decrease the H+ concentration in aqueous solutions, while acids increase it by adding hydrogen ions.
Indicators help in distinguishing acids and bases. An acidic solution turns red with methyl orange, clear with phenolphthalein, and red with litmus. Contrastingly, basic solutions turn yellow with methyl orange, pink with Hln and blue with litmus.
Both of these substances react to metals, albeit bases react with far fewer metals.
Weak acids give off a sour taste. Bases are mostly bitter.
Acids and bases are commonplace – the former is present in vinegar, lemons and urine, and the latter in soaps and cleaning products.