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Acids and Bases are two different types of chemical substances that interact with each other to form Salts and Water. Acids and Bases form the basis of Chemistry and are used in numerous daily-life items.
- Acids are ionic compounds that donate a Hydrogen Ion (H+) to another compound, usually a base.
- Bases are substances that are H+ acceptors, i.e. they accept the proton and donate the electron.
- Acids and Bases can be differentiated using a pH scale.
- pH refers to Power of Hydrogen representing the concentration of hydrogen ions to determine Acids and Bases.
- Acids have a pH of less than 7 and Bases have a pH greater than 7.
Read More: NCERT Solutions for Class 10 Science Acids, Bases and Salts
Key Terms: Acids, Bases, Hydrogen Ions, Lewis Acids, Lewis Bases, pH, Proton, Electron, Litmus, Salts, Hydroxide Ions
What are Acids and Bases?
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Acids and Bases are ionic compounds that get dissociated into their constituent ions when dissolved in water.
- Acid is any hydrogen-containing substance that is capable of sharing a proton (H+ Ion) with another substance.
- Base is a substance that is capable to accept a hydrogen ion from an acid.
- Acids have a sour taste while Bases are characterized by a bitter taste.
- Bases have a slippery texture while acids have a sticky texture.
- Acids turn blue litmus red while Bases turn red litmus blue.
- Acids and Bases combine together to form Salts and Water.
What are Acids?
Acids are simply those substances that contribute to a proton or accept an electron pair. Acids have been defined in several contexts over the years.
- Arrhenius defined acids as compounds that ionize to produce hydrogen ions.
- As per Lowry-Bronsted, an acid is a proton donor and a base is a proton acceptor.
- As per Lewis's definition, acids are substances capable of coordinating with unshared electron pairs.
- Acids have a pH of less than 7.0 and turn blue litmus red.
- Examples of Acids are Hydrochloric Acid, Nitric Acid, Sulphuric Acid, etc.
What are Bases?
Bases are chemical substances that donate electrons, release hydroxide ions (OH– Ions), and accept protons (H+ Ions).
- According to Arrhenius, bases are compounds which ionize to produce hydroxide ions.
- As per Lowry-Bronsted definition, a base is a proton acceptor.
- As per Lewis's definition, bases are molecules having unshared electron pairs available for sharing with acids.
- Bases have a pH value greater than 7 and turn red litmus paper blue.
- Examples of Bases are Sodium Bicarbonate, Calcium Carbonate, Sodium Hydroxide, Ammonia, etc.
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Properties of Acids and Bases
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The properties of acids and bases are listed below in detail:
Properties of Acids
- Acids are good conductors of electricity.
- They are corrosive in nature.
- The pH value of acids is always less than 7.
- They produce hydrogen gas when reacted with metals.
- Acids are sour in taste.
Properties of Bases
- Bases have a bitter taste and are slippery in texture.
- Bases have a soapy texture when touched.
- They release hydroxide ions (OH– ions) when they are dissolved in water.
- Bases act as good conductors of electricity in their aqueous solutions.
- The pH value of Bases is always greater than 7.
- Bases turn red litmus paper blue.
Neutral Substances
Neutral Substances are those substances that are neither acidic nor basic. They have the same amount of hydrogen and hydroxyl ions and do not change the colour of the litmus paper.
- Neutral substances do not have acidic or basic properties.
- They have a pH value of around 7.
- They do not have any effect on litmus paper.
- Examples of Neutral Substances are Water, Common Salt (NaCl), etc.
How to Identify Acids and Bases?
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In order to identify whether a substance is acidic or basic, a pH scale is used.
- pH refers to the Power or Potential of Hydrogen.
- pH scale is a logarithmic scale that has a value ranging from 0 to 14.
- Depending on the concentration of hydronium ions present in a substance, the pH scale classifies it as acidic or basic (alkaline).
- When there is a high concentration of hydronium ions, it results in a low value on the pH scale thus categorizing it as acidic.
- When there is a low concentration of hydronium ions, it tends to be basic with high pH values.
pH of Acids and Bases
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pH Scale is used to find the numeric value of the level of acidity or basicity of a substance. pH scale is most commonly used to measure how acidic or basic a substance is.
- When the pH values are between 0 to 7, the solution is said to be acidic.
- When the pH values are between 7 to 14, the solution is said to be basic.
- When the pH value is 7, the solution is said to be neutral.
Thus, a higher concentration of Hydrogen ions than pure water would yield a lower pH value and a lower concentration of Hydrogen ions than pure water would yield a higher pH value.
Read More: Importance of pH in Everyday Life
Calculation of pH Values
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pH Scale is a logarithmic scale that is used to measure the strength of acids and bases. A unit increase in the pH value corresponds to a ten-fold increase in the [H+] concentration. It is given by the negative of the base 10 log of the molar concentration of hydrogen ions.
Thus it can be expressed as,
| pH = -log10[H+] |
Where [H+] represents the concentration of Hydrogen ions
- An acid lowers the pH by increasing the concentration of [H+] ions by donating a hydrogen ion through dissociation.
- A base increases the pH value by donating hydroxide ions [OH-] ions which take up the excess [H+] ions thus reducing its concentration.
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Identification Tests for Acids and Bases
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Acids and Bases can be determined by a number of methods. The most common method is the use of Litmus paper. Litmus paper is an indicator that changes its physical properties such as colour when brought into contact with acids or bases.
Litmus Indicator
Litmus is available in variants of red and blue paper strips.
- In acidic conditions, blue litmus paper turns red and red litmus paper remains red.
- In basic conditions, red litmus paper turns to blue and blue litmus paper remains blue.
- In neutral conditions, the litmus paper turns purple.
Methyl Orange Indicator
It is a synthetic pH indicator that is often used in the titration process. It undergoes a vivid colour change when exposed to acidic and basic solutions.
- In an acidic medium, methyl orange shows red colour.
- In a basic medium, methyl orange shows yellow colour.
- In a neutral solution, methyl orange shows orange colour.
Phenolphthalein Indicator
Phenolphthalein is an organic compound that is often used in the acid-base titration process as an indicator.
- For acidic solutions, phenolphthalein remains colourless.
- For basic solutions, phenolphthalein turns pink.
Theories of Acids and Bases
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Acids and Bases can be described using three different theories:
- Arrhenius Concept of Acids and Bases: It describes that acids generate H+ ions in a solution whereas bases produce an OH– ion in their solution.
- Bronsted Lowry Theory of Acids and Bases: It describes an acid as a proton donor and a base as a proton acceptor.
- Lewis Concept of Acids and Bases: This theory describes acids as electron-pair acceptors and bases as electron-pair donors.
Arrhenius Concept of Acids and Bases
Svante August Arrhenius, a Swedish scientist, defined acids as substances that increase the H+ ion concentration of water when dissolved in them.
- These protons continue to form hydronium ions (H3O+) by combining with water molecules.
- It defines bases as substances that when dissolved in water, increase the concentration of OH– ions in it.
- One of the pros of this theory is that it successfully explains the reaction between acids and bases that form salts and water.
- One limitation of Arrhenius's concept is that it fails to explain how substances lacking hydroxide ions form basic solutions when dissolved in water like NO2– and F–.
Bronsted Lowry Theory of Acids and Bases
Bronsted-Lowry Theory is another important theory of acids and bases.
- It states an acid as a donor of protons.
- A base is referred to as a proton acceptor (or H+ ion acceptor) by this theory.
- Bronsted acids undergo dissociation to give in protons and therefore increase the concentration of H+ ions in the solution.
- Whereas, Bronsted bases accept protons from water (the solvent) to yield hydroxide ions.
- A merit of the Bronsted-Lowry definition of acids and bases is its ability to explain the acidic or basic nature of ionic species.
- One limitation of the theory is that it doesn’t explain how compounds lacking hydrogen exhibits acidic properties like BF3 and AlCl3.
Read More: Acids Bases and Salt Revision Notes
Lewis Concept of Acids and Bases
Lewis Acids and Bases are other important classifications of acids and bases.
- Lewis Acids are species that have a vacant orbital and has the ability to accept an electron pair.
- Lewis Bases are species that hold a lone pair of electrons and act as an electron-pair donor.
- Lewis theory does not involve the hydrogen atom in its definition.
- Lewis Acids are electrophilic while Lewis Bases are nucleophilic.
- Cu2+, BF3, and Fe3+ are some Lewis Acids while F–, NH3 and C2H4 are Lewis Bases.
- One merit of this theory is that many compounds can be defined as acids or bases by it.
- One limitation of this theory is that it fails to explain the acid-base reactions that do not involve the formation of a coordinate covalent bond.
Conjugate Acids and Bases
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Bronsted-Lowry Concept describes an acid as a substance that can donate H+ and a base as a substance that can accept H+. Acids and Bases that differ by proton form conjugate acid and base pair.
- Conjugate Acid forms when a proton is added to a base.
- Conjugate Base forms when a proton is removed from an acid.
Example: Assume the reaction given below:
CH3COOH + H2O ⇋ CH3COO– + H3O+
Here,
- Acid: CH3COOH; Conjugate Base : CH3COO–
- Base: H2O; Conjugate Acid: H3O+
Conjugate acids are formed by adding an H+ ion to the base while Conjugate bases are formed by removing the H+ ion from the acid.
Read More: Acids Bases and Salt Important Questions
Difference Between Acids and Bases
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Acids and Bases are two totally different chemical compounds in Chemistry. The key difference between acids and bases is as follows:
| Acids | Bases |
|---|---|
| Acid donates hydrogen ions when dissolved in water. | Bases provide hydroxyl ions when dissolved in water. |
| They turn blue litmus paper into red. | They turn red litmus paper into blue. |
| They have a sour taste and sticky texture. | They have a bitter taste and a soapy texture. |
| The pH value of acids ranges from 1 to 7. | The pH value of bases ranges from 7 to 14. |
| Examples of Acids are HCl, H2SO4, etc. | Examples of Bases are NaOH, KOH, etc. |
Uses of Acids and Bases
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The important uses of various acids and bases are as follows:
Uses of Acids
- Citric Acid is an important component of lemon and orange juices and is also used in food preservation.
- Sulphuric acid is used in batteries that are used to start the engines of automobiles.
- Sulphuric acid and nitric acid are used in the industrial production of explosives, dyes, and fertilizers.
- Phosphoric acid is used in a lot of soft drinks.
- Vinegar is a diluted version of acetic acid and has numerous household applications.
Uses of Bases
- Sodium Hydroxide is a base used for the manufacturing of soap and paper.
- It is also used for manufacturing rayon.
- Magnesium Hydroxide is used as a laxative and antacid as it helps to reduce excess acidity.
- Ammonium Hydroxide is an important chemical reagent.
- Calcium Hydroxide (Ca(OH)2), also known as slaked lime is used in bleaching powder.
- It is also used in dry mixes used in painting or decoration.
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Things to Remember
- Acids are substances that donate H+ ions when they are dissolved in water.
- Bases are substances that donate hydroxide (OH-) ions when dissolved in water.
- pH scale is used to measure the acidity or basicity of acids and bases.
- Acids have pH value below 7, bases have pH value above 7, and 7 is neutral.
- pH value is calculated using the formula pH = -log10[H+].
- Acids turn blue litmus paper red while Bases turn red litmus paper blue.
- Arrhenius Theory, Bronsted-Lowry Theory, and Lewis Theory are the three important theories of acids and bases.
Sample Questions
Ques. What are Bases and Alkalies? (3 Marks)
Ans. Bases are chemical substances that donate electrons and accept protons (H+ Ions).
- Bases ionise to give OH– ions in an aqueous solution.
- Bases may be soluble or insoluble in water. Alkalis are defined as those bases which are soluble.
- Thus all alkalies are bases but all bases are not alkalies.
Examples: NaOH and Cu (OH)2 both are bases, but, since NaOH is soluble in water, it is an alkali. On the other hand, since Cu(OH)2 is insoluble in water, it is not an alkali. Other examples of alkalies are KOH, Ca (OH)2 and NH4OH.
Ques. What are Acids? State the properties of Acids. (3 Marks)
Ans. Acids are defined as ionic compounds that when dissolved in water, lead to the production of positive hydrogen ions (H+). The properties of Acids are as follows:
- Acids are sour in taste.
- They are usually very sticky.
- They conduct electricity and react with metals to produce hydrogen gas.
- Acids turn blue litmus red and phenolphthalein turns colourless with acids.
Ques. State the major difference between Acids and Bases. (3 Marks)
Ans. Acids and Bases are two integral chemical compounds that form Salts in combination with each other.
- Acids are ionic compounds that release hydrogen ions (H+) when dissolved in water while Bases are ionic compounds that produce hydroxide ions(OH-) when dissolved in water.
- Acids have a pH value between 0 and 7 while Bases have a pH value between 7 and 14.
Ques. What are the practical applications of Neutralization Reaction? (3 Marks)
Ans. Neutralization is a chemical reaction in which an acid and base quantitatively react together to form salt and water as products. In this reaction, acids and bases neutralize each other. The applications of Neutralization Reactions are as follows:
- Being alkaline in nature, cold milk is used to neutralise the acidity produced by HCl present in the gastric juice in the stomach.
- Astronauts in spaceships use this reaction to neutralise the dangerous levels of CO2.
- Farmers add slaked lime (calcium hydroxide) to reduce the acidity of the soil.
- The sting of ants and bees contains formic acid. This can be neutralised by rubbing soap, which contains free sodium hydroxide.
- Persons suffering from acidity are given antacid tablets, containing magnesium hydroxide which neutralises excess HCl produced, in the stomach. Alternately, they are advised to sip cold milk, which neutralises HCl.
Ques. Why common indicators cannot determine the pH Value of a solution? (3 Marks)
Ans. Common indicators, such as litmus paper methyl orange and phenolphthalein can easily tell whether a solution is acidic or alkaline, but they cannot easily tell how much a given acidic solution is stronger than another acidic solution. It means they cannot really tell us the pH value of different acidic or alkaline solutions.
- In the case of litmus, we cannot tell about pH values of 5,6 and 7.
- Similarly, in the case of methyl orange pH values between 3 and 8 will not be very accurate.
Ques. Explain the importance of Acids in Human Body. (2 Marks)
Ans. Acids play a very important role within the human body. Hydrochloric acid is present in the stomach and helps in digestion by breaking down large and complex food molecules. Amino acids are used for protein synthesis which further helps to grow and repair body tissues.
Ques. What will be the action of the following substances on Litmus Paper?
(I) Dry HCl Gas
(II) Moistened NH3 Gas
(III) Lemon juice
(IV) Carbonated Soft Drink
(V) Curd
(VI) Soap Solution (3 Marks)
Ans. (I) Dry HCl gas will not have any effect on the litmus paper.
(II) Moistened NH3 gas will turn red litmus paper blue.
(III) Lemon juice will turn blue litmus paper red.
(IV) Carbonated Soft Drink will turn blue litmus paper red.
(V) Curd will turn blue litmus paper red.
(VI) Soap solution will turn red litmus paper into blue.
Ques. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid? (3 Marks)
Ans. Diluting an acid should be done by adding concentrated acid to water gradually with stirring and not by adding water to concentrated acid. The heat is evolved gradually when a concentrated acid is added to water for diluting an acid and a large amount of water is easily absorbed. If, however, water is added to concentrated acid to dilute it, a large amount of heat is evolved at once. The heat generated may cause the mixture to splash the acid on our face or clothes and cause acid burns.
Ques. What is meant by Alkaline? Is it true that all bases are alkaline? (3 Marks)
Ans. Alkaline can be referred to as a basic solution sometimes, although they are not the same. Bases are chemical compounds that can accept hydrogen ions.
- As all bases are not soluble in water, they are not alkalis.
- Alkalis are basically the bases that dissolve in water to generate the hydroxyl ion (OH-), and they are all Arrhenius bases.
- Water-soluble alkalis, for instance, barium carbonate, usually only become soluble in water after reacting with an acidic solution that contains water.
Ques. Is pure water Acidic or Alkaline? (3 Marks)
Ans. Pure water is neither acidic nor alkaline and is neutral.
- Pure water has a pH of 7 which indicates that it is neutral.
- pH is referred to as the negative log of hydrogen ion concentration.
- Pure water is neutral due to the number of positive hydrogen ions produced equals the number of negative hydrogen ions produced.
Ques. Define pH. (3 Marks)
Ans. pH is a logarithmic scale that gives the measurement of the alkalinity or acidity present in a solution. pH scale ranges from values 0 to 14.
- Solution with a pH value of 0 to 6 is acidic in nature.
- Solution with a pH value of 8 to 14 is basic in nature.
- Solution with a pH value of 7 is neutral.
Ques. How can Citric Acid solution be prepared? (2 Marks)
Ans. In order to make the citric acid solution, combine citric acid crystals with 1 or 2 quarts of distilled boiling water for every pound of citric acid. Then, place the citric acid crystals in a non-metal dish and slowly pour in the boiling water, stirring constantly with a non-metal spoon.
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