Acids, Bases and Salts

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Acids, Bases, and Salts in a Solution

Earlier compounds were classified into three categories on the basis of their taste: Acids, bases, and salts.

  1. Acids are the substances which have a sour taste. Example, lemon juice, vinegar, tomatoes etc.
  2. Bases are the substance which has bitter taste. Example washing soda, baking soda etc.
  3. Salts are the compounds formed from acid and bases. Example sodium chloride, silver nitrate etc.

Acids and bases in laboratory

Acid-base indicators are the substances which help us identify whether a substance is acidic or basic. These show one color in acidic medium and different color in basic medium. Indicators can either be natural or synthetic. Litmus and turmeric (video) are natural indicators. Litmus paper turns red when dipped in an acidic solution and turn blue when dipped in a basic solution. Similarly, turmeric becomes reddish-brown in basic solution. This is the reason why clothes stained by curries (containing turmeric) turn reddish-brown on washing with soap.

The indicator which gives different odor in an acidic and basic medium are called olfactory indicators. Example onion, clove, and vanilla can be used as olfactory indicators.

Know More: Acid-Base Indicators

How do acids react with metals?

Acids react with active metals like Zinc, magnesium etc. to form salts with the evolution of hydrogen gas.Screen Shot 2017-02-16 at 1.15.47 PMFor example: Reaction of zinc with dilute sulfuric acidScreen Shot 2017-02-16 at 1.15.51 PMIn this reaction zinc replaces hydrogen from the acid and combines with SO4 and forms a compound called salt.

How do bases react with metals?

Some bases like sodium hydroxide and potassium hydroxide react with active metals like Zinc and aluminum to liberate hydrogen gas along with the formation of salts.Screen Shot 2017-02-16 at 1.15.55 PMFor example: reaction of sodium hydroxide with zincScreen Shot 2017-02-16 at 1.16.09 PMSodium zincate is the salt formed in this reaction. But such reaction is not possible with all the metals. Only very reactive metals show such type of reactions with bases.

How do metals carbonates and hydro carbonates react with acid?

Acids react with metal carbonates and bicarbonates to give out carbon dioxide with effervescence and forming corresponding salt and waterScreen Shot 2017-02-16 at 1.17.28 PMFor example:

Reaction between sodium carbonate and sulfuric acidScreen Shot 2017-02-16 at 1.17.32 PMReaction between Sodium hydrogen carbonate and hydrochloric acidScreen Shot 2017-02-16 at 1.17.38 PMMaking Calcium hydrogencarbonate (Lime Cycle)

On passing carbon dioxide through lime water, white precipitate of calcium carbonates are formedScreen Shot 2017-02-16 at 1.17.42 PMLimestone, chalks and marbles are different forms of calcium carbonates. On passing excess of carbon dioxide following reaction takes place:Screen Shot 2017-02-16 at 1.17.47 PMAll metal carbonates and hydrogen carbonates react with acids to give corresponding salts.Screen Shot 2017-02-16 at 1.17.52 PM

The Lime Cycle
The Lime Cycle

Know More: The Lime Cycle

How acid and bases react with each other?

Neutralization is defined as a process in which an acid reacts with a base to form salt and water i.e.Screen Shot 2017-02-16 at 1.17.57 PMFor example: Neutralization reaction between sodium hydroxide and hydrochloric acid.Screen Shot 2017-02-16 at 1.18.00 PMKnow More: Neutralization Reaction and Net Ionic Equations for Neutralization Reactions

Reaction of metallic oxide with acids

Metal oxides react with acid to give salt and water.Screen Shot 2017-02-16 at 1.18.06 PMFor example, the reaction of copper oxide and HCl:Screen Shot 2017-02-16 at 1.18.10 PMJust like acids react with base, metallic oxide reacts with acids to gives salt and water. Hence, metallic oxides are also knows as basic oxides.

Reaction of a non-metallic oxide with bases

Reaction between carbon dioxide and calcium hydroxide is the best example to understand the reactions of non-metallic oxide with bases.Screen Shot 2017-02-16 at 1.18.23 PMCalcium hydroxide which is base reacts with carbon dioxide to give salt and water. This is similar to the reaction between acid and base thus we can conclude that non-metallic oxide are acidic in nature.

What do all acids and bases have in common?

All the acids generate H2 gas on reaction with metals. However, all compounds containing hydrogen are not acidic. Acids contain ions, a cation and an anion. It is seen that acids are able to conduct electricity. However, hydrogen containing compounds such as glucose and alcohol do not conduct electricity. This shows that glucose and alcohol do not contain ions.

Since the acid contains H+ cations, this suggests that acid produce H+ ions which are responsible for their acidic properties. Similarly, alkalis such as sodium hydroxide, calcium hydroxide etc. will be able to conduct electricity because of presence of ions (Na+, Ca2+, etc.). Hence, acids and bases have ions in common and hence, they can conduct electricity.

Electric Circuit using a acidic solution
Electric Circuit using a acidic solution

Acids and bases in water solution

Acids acts as acids, i.e. they release their H+ ions only in the presence of water. HCl without water will not release H+ ions. This is because H+ ions can not exist alone. In presence of water, HCl dissociates into H+ and Cl.Screen Shot 2017-02-16 at 1.18.29 PMAfter dissociation of dry HCl in water, a number of water molecules remains attached to H+ and Cl-. Hence H+ ion combines with water molecules to form ion, called hydronium ion (H3O+).Screen Shot 2017-02-16 at 1.18.32 PMSimilarly, bases generate OH ions in the presence of water. The bases that are soluble in water are called alkalis.Screen Shot 2017-02-16 at 1.18.35 PMScreen Shot 2017-02-16 at 1.18.39 PMHence, the neutralization reaction can be written as:Screen Shot 2017-02-16 at 3.19.16 PMIn these reaction H+ ions from acid combines with the OH ions given by the base. Hence the effect of acid is nullified by base and vice-versa.

Special care must be taken while dissolving acids and bases with water. The process of dissolving acids and bases with water is a highly exothermic reaction. Strong acids such as sulphuric acid, nitric acid must be added slowly to water while constant stirring. Never add water to concentrated acid as the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive heating.

The process of mixing acids with water is called dilution and the solution is said to be diluted. Adding water decreases the concentration of ions (H3O+ / O) per unit volume.

How strong are acids and bases?

The process of mixing acids with water is called dilution and the solution is said to be diluted. Adding water decreases the concentration of ions (H3O+ / O) per unit volume. We also know that the concentration of H+ is higher in an acidic solution as compared to a basic solution. Bases have lower concentration of H+. Also, different acids have different concentration of H+ depending on which we call it a strong acid or weak acid. Hence, if we create a universal indicator that can check the concentration of H+ in the solution, we will be able to identify, strong acids, weak acids, strong bases and weak bases. A scale was developed which measures the hydrogen ion concentration in a solution and was named as pH scale. The pH scale consist of universal indicators (mixture of number of indicators) and shows different color at different pH values.

  • The pH value of a neutral solution is 7
  • If the pH value is less than 7 then the solution is acidic
  • If the pH value is more than 7 then the solution is basic
pH scale 4
pH Scale

The concentration of H+ is highest at pH 1 and decreases as we go to a higher pH. Also, the concentration of OH is the least at pH 1 and is the highest at pH 14. At pH 7, the concentrate of H+ and OHis equal and the solution is said to be neutral.

The strength of acidic or basic solution depends upon the number of H+ and OH ions produced. Higher the hydronium ion concentration, lower is the pH value. Acids that give more H+ ions are considered as strong acids and vice-versa and hence acids have lower pH values.

For example, when the pH values of HCl and acetic acid are measured, we will find out that pH value of acetic acid is more than HCl because HCl releases more H+ as compared to acetic acid and hence, HCl is a strong acid.

Know More: Check pH of various solutions

Importance of pH in everyday life

Are plants and animal pH sensitive?

Most of the reaction taking place in our body is in the narrow pH range of 7.0 to 7.8. If the pH falls below 7.0 or rises above 7.8, the survival of living organism becomes difficult.

For example the pH of rain water is less than 7.0 (as it is acidic due to dissolution of CO, SO etc of the air). When the pH of rain water falls below 5.6, it becomes more acidic, it is called acid rain. Acid rain reduces the pH of water bodies which makes it difficult for the survival of aquatic animals.

Like animals, plants also require an optimal range of pH for their survival. Soil pH is a very important factor in plant health – if the soil is too acidic or too alkaline, plants will be unable to absorb nutrients properly, and your garden won’t grow. Different types of plants have a different pH requirement. However, it is ideal to maintain a soil pH of 5-6.5.

pH in our digestive system

Our stomach produces hydrochloric acid which is helpful in digestion of food. This HCl does not harm the stomach as it has a special lining of protective material. The acid kills the harmful bacteria that may enter in stomach along with that we eat. However, at times the amount of HCl produced inside the stomach increases which causes pain and irritation in stomach. It can be cured by taking medicines called ‘antacids’. The most commonly used antacid contains magnesium hydroxide (milk of magnesia) which is a weak base and neutralizes the excess acid.

Know More: Baking Soda, Washing Soda And Plaster Of Paris

pH change as the cause of tooth decay

Tooth Decay
Tooth Decay

The enamel of our teeth is the hardest substance in our body. Chemically it contains calcium phosphate which is insoluble in water. But if the pH of the mouth falls below 5.5, i.e. our mouth is moderately acidic; the dissolution of calcium phosphate starts, i.e. tooth decay begins. Bacteria in the mouth produce acids by degradation of sugar and food particle remaining in the mouth after eating. The best way of preventing tooth decay is to clean your teeth every day.

 

Self-defense of animals and plants through chemical warfare

Insects such as honey bee or yellow ant use acids for their self-defense. They inject acids into the skin of the enemy which causes pain and irritation. To get relief, we apply the solution of a mild base like baking soda. Similarly, neetle plants have leaves with stinging hairs.

Honey Bee Sting
Honey Bee Sting

 

Salts

Salts are the ionic compounds consisting of two parts, one part carrying a positive charge (cation) and other part consisting of negative charge (anion). The positive and negative ions are present in such a manner that total positive charge is equal to total the negative charge.

Salts have been categorized into different families. Salts having the same positive or negative radicals are said to belong to the same family.

Example:

Chloride family: Sodium chloride and potassium chloride,

Sulphate family: Sodium Sulphate, Calcium Sulphate, Potassium Sulphate, etc.

Know More: Salts

pH of salts

Neutralization reaction is the reaction between acid and bases to form salt and water. Salts of strong acid and strong bases are neutral with pH value of 7. On the other hand, salts of strong acid weak base are acidic with pH value less than 7. Salts of strong base and weak acid are basic in nature, with pH value of more than 7.

Chemicals from common salts

Common salt is sodium chloride with the formula, NaCl. This is the salt that is used in food. Seawater contains many salts dissolved in it including sodium chloride. Seawater is one of the main source of common salt. Solid salt deposits are also found in several parts of the world. These large crystal, also called rock salt are often brown because of impurities. Rock salt is mined like coal.

Common salt is used as a raw material for the manufacturing of many chemicals that we use in our day to day life such as sodium hydroxide, baking soda, washing soda, bleaching powder, etc.

Sodium Hydroxide (Caustic soda)

Sodium hydroxide is manufactured by electrolysis of an aqueous solution of sodium chloride called brine. This method is called chloralkali process because the products of electrolysis are chlorine (chlor) and sodium hydroxide (alkali).Screen Shot 2017-02-16 at 1.18.54 PMChlorine moves towards the anode (positive charge) where they lose an electron to form Cl atoms. These Cl atoms combine to form chlorine gas (Cl2).

Hydrogen moves towards the cathode (negative charge). Here, they gain an electron to form hydrogen atoms which combine to form hydrogen gas (H2). NaOH solution is formed at the cathode.

Bleaching Powder

Bleaching powder is generally represented by the formula CaOCl2, called calcium oxychloride. The chlorine produced during the manufacture of caustic soda is used for the manufacture of bleaching powder. It is produced by the action of chlorine gas on dry slaked lime, Ca(OH)2. The following reaction takes place:Screen Shot 2017-02-16 at 1.19.00 PMUses of Bleaching powder

  • The most important use of bleaching powder is in textile industries for bleaching cotton and linen, in paper industries for bleaching wood pulp and in laundry for bleaching washed clothes.
  • Used for disinfecting drinking water by killing germs.
  • As an oxidizing agent in chemical industries.

Know More: Bleaching Powder

Bleaching powder used in water treatment
Bleaching powder used in water treatment

Baking Soda

The soda commonly used in the kitchen for making tasty crispy pakoras is baking soda. Baking soda is sodium hydrogen carbonate with the formula, NaHCO3. It is also called sodium bicarbonate. Baking soda is manufactured from sodium chloride as one of the raw materials. The method is known as Solvay process or ammonia-soda process.Screen Shot 2017-02-16 at 1.19.11 PMNaHCO3 is a mild non-corrosive base. If solid sodium hydrogencarbonate is heated or its aqueous solution is boiled, it gives out carbon dioxide. The carbon dioxide evolved helps in making breads rise up and become fluffy.Screen Shot 2017-02-16 at 1.19.16 PMUses of baking soda

  • The excess acid formed in the stomach due to eating of spicy food is easily neutralized by sodium hydrogen carbonate. Hence, it is used as an ingredient of antacid medicines.
  • It is also used in soda-acid fire extinguishers.

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Making Baking Powder

Baking powder is a mixture of sodium hydrogen carbonate and a mild edible acid like tartaric acid or citric acid. In making certain food items such as bread, cake etc.Screen Shot 2017-02-16 at 1.19.22 PM

Washing Soda

Washing soda is sodium carbonate containing ten molecules of water of crystallization, i.e. it is sodium carbonate decahydrate. Thus, its molecular formula is Na2CO3.10H2O. It is also a basic salt.

Recrystallization of sodium carbonates gives washing soda. Hence, sodium carbonate which can be obtained by heating baking soda when recrystallized gives washing soda.Screen Shot 2017-02-16 at 1.19.28 PMUses of washing soda

  • It is used in the laundry for cleaning of clothes.
  • It is used for removing permanent hardness of water.
  • It is used in the manufacture of a number of useful products like glass, soap, paper, borax etc.
  • It is used as a laboratory reagent. For example, its standard solution can be prepared to carry out the acid-base titration.

Know More: The story of washing soda

Chemistry of crystals

The chemical formula of washing soda is Na2CO3.10H2O. Thus, one formula unit contains 10 water molecules. Similarly, there are many other crystalline salts containing water molecules. For example crystalline copper sulfate contain 5 water molecules (CuSO4.5H2O) and gypsum contains 2 water molecules (CaSO4.2H2O). The fixed number of water molecules present in one formula unit of the salt is called water of crystallization. Hence, a crystallized salt may look dry, but it contain water in the form of water of crystallization.

Plaster of Paris

Chemically, plaster of Paris (P.O.P) is calcium sulfate hemihydrates, i.e., it containing half molecule of water crystallization. It is represented by formula CaSO4.1/2H2O. A half molecule of water of crystallization in P.O.P signifies that one water molecule is shared by two formula units of CaSO4.

P.O.P is prepared by heating gypsum. At high temperature, gypsum loses water molecule and becomes calcium sulfate hemihydrates (CaSO4.1/2H2O). This is called Plaster of Paris. On mixing P.O.P with water it changes to gypsum which is a hard solid mass.Screen Shot 2017-02-16 at 1.19.35 PMUses of Plaster of Paris

  • In medical science, it is used for setting fractured bones in the right position and in making casts in dentistry.
  • Making toys, casts for statues, decorative material etc
  • Making chalks for writing.
  • Making fire proof material.

Know More: Baking Soda, Washing Soda And Plaster Of Paris

See Also

  1. Acids
  2. Harmful effects of acid rain
  3. Prevention of tooth decay
  4. What is a Hydronium Ion? 
  5. How to dilute an Acid?
  6. The strengths and weaknesses of acids and bases
  7. Antacids
  8. Video Summary: Acids, Bases, and Salts
  9. More about gypsum

Exam preparation

  1. MCQs: Acids, Bases, and Salts
  2. Quiz: Acids, Bases, and Salts
  3. Extra Questions 1: Acids, Bases, and Salts
  4. HOTS: Acids, Bases, and Salts
  5. NCERT Solutions: Acids, Bases, and Salts
  6. MCQs, Short, and Long Questions: Acids, Bases and Salts
  7. Extra Questions 2: Acids, Bases, and Salts

References

  1. Acid Rains
  2. Advantages of HCl in our stomach
  3. Brine
  4. How plaster of Paris is formed?
  5. How to find water of crystallization?
  6. Electrolysis of Brine
  7. How Do Acids And Bases React With Metals
  8. Turmeric as indicator
  9. Acid-Base Indicators
  10. The Lime Cycle
  11. Neutralization Reaction and Net Ionic Equations for Neutralization Reactions
  12. Check pH of various solutions
  13. pH level of Human Body
  14. Salts
  15. Bleaching Powder
  16. The story of washing soda
  17. Baking Soda, Washing Soda, And Plaster Of Paris

 

 

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