Chapter 2 Acids,Bases and Salts

When an acid reacts with a base, a neutral salt is formed by the neutralisation process. As neutralisation process is an exothermic reaction, the temperature of the solution increases.

Note:

(i) An acid reacts with a base to form salt and water. This reaction is called the neutralisation reaction. Such a reaction is always exothermic in nature. Hence, the temperature of the solution increases.

(ii) In an exothermic reaction, heat is liberated thereby increasing the temperature.

The aqueous solution is a base as it turns red litmus solution blue. An acidic solution such as hydrochloric acid would reverse the change. In contrast, hydrochloric acid would turn blue litmus solution to red.

Other options such as baking powder, lime and ammonium hydroxide solution are basic compounds. So, these substances would not be able to reverse the change. These substances would only change red litmus solution to blue.

Calcium chloride is a good dehydrating agent. It absorbs moisture or water content from the gas making it dry. Calcium chloride is widely used as a desiccant for drying gases and liquid hydrocarbons in many industries due to its ability to absorb water.

Baking soda does not contain water of crystallization. The chemical formula of baking soda is NaHCO₃. Its formula does not show any association with water molecules.

Other three contain water of crystallization blue vitriol (CuSO₄ .5H₂O), washing soda (Na₂CO_{3 .}10H₂O) and gypsum (CaSO_{4 .}2H₂O).

Note:

The number of water molecules that are attached to a molecule of salt is called its water of crystallization. It is also known as water of hydration. This is necessary to maintain the crystalline structure of a salt. The salts that contain water of crystallization are called hydrated salts.

Sodium carbonate (Na₂CO₃) is the salt formed by the combination of a weak acid such as carbonic acid (H₂CO₃) and a strong base such as sodium hydroxide (NaOH).

Note: The salt formed by the combination of weak acid and a strong base is basic in nature. Basic salts have pH more than 7.

Calcium phosphate Ca₃(PO₄)₂ is basic salt, as it is formed by the combination of a weak acid (phosphoric acid) and slightly stronger base (calcium hydroxide). Bacteria feed on sugars commonly found in foods and produce acids in the mouth decreasing the pH level to 5.5 or lower thus eroding the enamel that is basic in nature.

Note:

A weak base turns the pH paper greenish-blue. Antacids usually contain weak base such as Mg(OH) _2. So, an antacid would change the colour of this pH paper to greenish- blue. Other options such as lemon juice and vinegar contain acids and common salt is a neutral salt.

The increasing order of acid strength is water < acetic acid < hydrochloride acid. Hydrochloride acid is a mineral acid and ionises completely in water. A solution hydrochloric acid will have a much higher concentration of H⁺ ions and hence lower the pH level. That’s why it is a strong acid. Acetic acid is an organic acid and ionises only partially in water. That’s why it is a weak acid. Water is almost neutral in nature. In pure water number of positive hydrogen ions is equal to the negative ions.

Wash the hand immediately with plenty of water as it helps to wash away most of the acid. Thereafter apply a paste of baking soda (NaHCO₃) to neutralise the little acid if any, left over the hand. Baking soda has weak basic properties that is used to neutralise the acid. Here, a strong base cannot be used to neutralise the acid as it can corrode the skin.

When sodium hydrogen carbonate is added to acetic acid, carbon dioxide (CO₂) gas is evolved.

NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂

(Sodium hydrogen carbonate + Acetic acid = Sodium acetate + water + carbon dioxide)

Carbon dioxide supports all the above three properties. CO₂ turns lime water milky, it is a non-supporter of combustion and is absorbed by strong alkalis such as NaOH.

Common salt (NaCl) is formed by reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH). It serves as an important raw material for the production of various chemical compounds such as caustic soda (sodium hydroxide), washing soda (sodium carbonate), baking soda (sodium bicarbonate) used in various industries. Chlorine gas obtained is used for making bleaching powder (calcium hypochlorite)

Baking powder is a mixture of baking soda NaHCO₃ (sodium hydrogen carbonate) and a mild edible acid such as tartaric acid. When baking powder is mixed with water, sodium hydrogen carbonate reacts with tartaric acid and accepts hydrogen ions from it. This process releases carbon dioxide gas which is trapped in the dough when baking powder is used to make bread and cake. The trapping the carbon dioxide makes the dough rise and hence the bread is soft and fluffy.

Toothpastes we generally contain mild bases such as sodium fluoride or sodium bicarbonate in their composition. The base reacts with the acid formed during tooth decay and neutralises its bad effects thus, preventing tooth decay.

Note:

When we eat sweet things, the pH of our mouth falls below 5.5 (moderately acid) as the oral bacteria releases lactic acid while acting on sugars. The acidic conditions are capable to corrode the enamel as these are made up of calcium phosphate. This makes the tooth decay.

When the pH of an aqueous solution is higher, acidic properties are weak. Thus it is a base. When the pH an aqueous solution is lower, basic properties are weak. Thus it is an acid.

Note:

A pH scale is used to measure the acidic and basic strength of an aqueous solution. This scale is marked from 0 to 14. A solution is neutral if its pH value is 7. An acidic solution has a pH less than 7. This means an acidic solution has more concentration of H⁺ ions. A basic solution has a pH more than 7. This means a basic solution has more concentration of OH^– ions.

When we eat, our stomach releases hydrochloric acid that helps in digestion. Acids always have pH less than 7. The pH of stomach varies between 1 and 5. The pH of the stomach and the acid present in the stomach need not necessarily be the same.

(i) When a small amount of acid such as hydrochloric acid is added to water, individual ions of the acid are dissociated. This process is known as ionisation. So hydrochloric acid separates into positively charged hydrogen ions and negatively charged chlorine ions.

(iii) Mixing of an acid with water is also called dilution. Addition water results in the decrease in the concentration of ions, (H₃O⁺) per unit volume.

Vanilla essence serves as an olfactory indicator. An olfactory indicator is a substance whose smell differs in when added to acids and bases respectively. Vanilla essence has a characteristic pleasant smell. When added to a basic solution such as sodium hydroxide solution, the characteristic smell of vanilla essence vanishes. When added to an acidic solution such as hydrochloric acid, however, the characteristic pleasant smell of vanilla essence remains. This is how it helps such students to identify acids and bases.

Lime (CaO) is the base that reacts with a dilute acid such as HCl to form a salt called calcium chloride. It is an exothermic reaction and releases water but no carbon dioxide. Mable or limestone (CaCO₃) or baking soda (NaHCO₃) react with HCl to release CO_2.

Lime juice is acidic in nature. It has high amount of citric acid present in it.

Human blood is slightly basic (i.e., having ph7.8). Lime water, baking soda and antacid are also basic in nature as they contain more of hydroxide (OH^-) ions.

Bulb will glow as NaOH is a strong base that dissociates completely in water to furnish OH^- and Na⁺ ions. It is thus a strong electrolyte and conducts electricity through it. Strong acids and bases are good conductors of electricity.

NaOH→Na++OH−

Aqua-regia is used for the dissolution of gold. It is a solution of concentrated HNO₃ and concentrated HCl in the ratio of 1:3. Thus this solution is strongly acidic and is used to dissolve gold.

Citric acid is not a mineral acid. It is an organic acid or edible acid that is derived from organic compounds.

Note:

Mineral acids or inorganic acids are generally prepared from inorganic compounds such as minerals present in the earth’s crust. For examples, HCl (hydrochloride acid), H₂SO₄ (sulphuric acid) and HNO₃ (nitric acid) are mineral acids. These acids dissolve in water to produce H⁺ ions.

Edible acids or organic acids are produced by plants or animals, for example, acetic acid, citric acid and lactic acid are edible acids. These acids are corrosive in nature.

C₂H₅OH is not a base. C₂H₅OH or ethyl alcohol has a functional OH^– group. It cannot dissociate ions in a solution. Thus it cannot be a basic compound.

Not all metal oxides are insoluble in water. Some metal oxides such as Na₂O and CaO dissolve in water to form alkalis. When Na₂O reacts with water, NAOH is formed.

Bleaching powder (CaOCl₂) is used bleach clothes and removes colour from clothes. It decomposes on reaction with water and releases chlorine which acts as oxidising, bleaching and disinfecting agent. Baking soda is a natural antacid. It is used to relieve acidity in stomach and heart burn. Washing soda is used in the preparation of glass, soap and paper industries. Sodium chloride is used to produce H₂ and Cl₂ gases. Aqueous sodium chloride on electrolysis yields hydrogen and chlorine.

When equal volumes of hydrochloric acid (strong acid) and sodium hydroxide (strong base) solution of same concentration are mixed, the result is salt and water. This is a neutralisation reaction.

The aqueous solution of NaCl is neutral with pH 7. The colour of this solution obtained is yellowish green.

HCl ionises completely in water to produce H⁺ and Cl^– ions as it is a strong acid. This H⁺ ion combines with water molecules to produce hydronium ions.

Acids turn blue litmus solution to red. They have a sour taste as in case of vinegar and are sticky to touch. On the other hand, bases turn red litmus solution to blue. They are bitter in taste as in case of baking soda and are slippery to touch.

HCl is a strong acid. When HCl is mixed with water, its molecules dissociate completely to furnish H⁺ ions and Cl^– ions. This H⁺ ion combines with water molecules to produce hydronium ions.

Chloralkali is a process used in industries to produce chlorine and sodium hydroxide (caustic soda). This happens by the electrolysis of NaCl (commonly known as brine).

Lactic acid: Sources – Lactic acid is found in products such as cheese, yogurt, soy sauce and pickled vegetables; Acetic acid: Sources - Acetic acid is found in condiments such as ketchup, mayonnaise and mustard; Citric acid: Sources - Citric acid is found in citrus fruits, tomatoes and pineapples; Oxalic acid: Sources - Oxalic acid is found in leafy green vegetables and beets.

(i) Dry HCl gas: No change on litmus paper. This is because litmus paper changes colour only if ions such as hydrogen (H⁺) or hydronium (H3O⁺) ions are present in a solution. HCl can only furnish ions when it reacts with water. Thus, dry HCl gas does not change the colour of litmus paper.

(ii) Moistened NH₃ gas: Red litmus turns blue as it is basic in nature

(iii) Lemon Juice: Blue litmus turns red as it contains citric acid

(iv) Carbonated soft drinks: Blue litmus turns red as it contains carbonic acid

(v) Curd: Blue litmus turns red as it contains lactic acid

(vi) Soap solution: Red litmus turns blue as it is basic in nature.

An acid called formic acid (or methanoic acid) is present in ant stings. The chemical formula of formic acid is HCOOH. To get relief from the discomfort caused by the ant stings, apply baking soda on the affected area. Baking soda is basic in nature and it reacts with acid to produce neutral salt.

Egg shells are rich in calcium carbonate (CaCO₃). When nitric acid reacts with calcium carbonates, calcium nitrate, water and CO₂ gas is formed. Formation of CO₂ gas causes brisk effervescence. The chemical equation for this reaction is

In the absence of litmus, other natural or synthetic substances can be used to test acid and bases. Such substances are called indicators. Indicators such as methyl orange and phenolphthalein can be used to test the nature of a solution. These indicators show change in their colour in acidic, neutral and basic solutions. We can also use natural indicators such as turmeric and grape juice.

A few indicators with characteristic colour change are shown below.

Baking soda (NaHCO₃) on heating produces carbon dioxide (CO₂). Carbon dioxide turns lime water milky. It can also extinguish a burning matchstick.

Whereas heating washing soda (Na₂CO₃.10H₂O) does not produce any such gas and hence there is no effect on lime water or a burning matchstick. The water of crystallisation in washing soda is released due to heating and the salt becomes anhydrous white powder called soda ash.

Salt A is sodium bicarbonate (NaHCO₃) as it is commonly used in bakery products and on heating produces Na₂CO₃. Salt B is sodium carbonate Na₂CO₃ as it is used for removal of hardness of water. Gas C is carbon dioxide (CO₂ ) as it turns lime water milky.

This can be shown as a chemical equation:

During the manufacture of sodium hydroxide (NaOH), hydrogen gas (H₂) and chlorine gas Cl₂ (X) are formed as by-products. Chlorine gas reacts with lime water [Ca(OH) ₂ ]to produce bleaching powder (CaOCl₂), a bleaching agent.

Thus X is chlorine gas (Cl₂ gas). Y is calcium oxychloride or bleaching powder (CaOCl₂).

The chemical equation for the preparation of sodium hydroxide is as follows:

(i) Acid : HCl

(ii) Formula : CuSO₄

Base : Cu(OH)₂

(iii) Acid : HCl

(iv) Base : Mg(OH)₂

(v) Base : KOH

Acid : H₂SO₄

(vi) Acid : HNO₃

Strong acids Acids that dissociates into hydrogen ions completely in an aqueous solution are strong acids. These hydrogen ions attach themselves to the water molecules producing a high concentration of H₃O⁺ ions. Acids such as HCl, H₂SO₄ HNO₃ are strong acids.

Weak acids Acids that do not dissociate completely into hydrogen ions in an aqueous solution are weak acids. Acids such as acetic acid and carbonic acid are weak acids.

When zinc metal reacts with a dilute solution of strong acid [such as hydrochloric acid HCl], a salt (ZnCl₂) is formed and hydrogen gas is evolved which is used in the hydrogenation of oil.

To test the presence of H₂ gas, bring a burning splinter or a matchstick near the mouth of the test tube, you will see that the gas burns with a pop sound.

When same amount of zinc dust is taken in the test tube in place of zinc granules, the reaction rate is comparatively faster because zinc dust has larger surface rea than zinc granules. Also the hydrogen gas will evolve with greater speed when zinc dust is used in place of zinc granules.

(b) With dilute hydrochloric acid, zinc chloride is formed instead of zinc sulphate. Almost same amount of hydrogen gas is evolved in both the cases.

Zn + 2HCl⇨ZnCl₂ + H2O

(c) If copper turnings are taken instead of zinc, no reaction will take place. Hydrogen gas will not evolve. This is because copper does not react with dilute acids of H2SO4 or HCl under normal conditions because copper lies at lower position in the reactivity series.

(d) If sodium hydroxide is taken in place of dilute sulphuric acid and the tube is heated, sodium zincate is formed along with hydrogen gas. Heating of test tube will just increase the rate of reaction thus increasing the formation of hydrogen gas.

While making cake if baking soda is used instead of baking powder the cake will taste bitter. Baking soda (NaHCO₃) is a base and we know that bases are bitter. In case of baking powder, tartaric acid neutralizes the bitter taste of baking soda. On heating baking powder produces sodium carbonate, carbon dioxide and water. Sodium carbonate makes the cake taste bitter.

2NaHCO₃ + Heat ⇨ Na₂CO₃ + CO₂ + H₂O

(b) Baking soda can be converted into baking powder by adding tartaric acid and starch.

(c) Tartaric acid produces hydrogen ions when it reacts with water. Hydrogen ions produced by tartaric acid reacts with sodium bicarbonate and gives carbon dioxide which makes the dough soft and fluffy. Tartaric acid also neutralizes sodium carbonate (formed as a result of heating baking soda) to form sodium tartarate that has pleasant smell and good taste.

2NaHCO₃ + C₄H₆O₆⇨ 2CO₂ + 2H₂O + Na₂C₄H₄O₆

X is calcium carbonate. When calcium carbonate reacts with acid such as hydrochloric acid, carbon dioxide is released.

Solution Y is lime water [Ca(OH)₂ ]. When CO₂ is passing through lime water, it turns milky due to the formation of calcium carbonate.

During electrolysis of brine, chlorine (G) gas is deposited at anode.

When chlorine gas is passed through dry slaked lime Ca(OH)₂ (Y), it produces bleaching powder (Z). Bleaching powder is used for disinfecting drinking water.

Ca(OH) + Cl₂ CaOCl₂ + H₂O

Hence, Z is calcium oxychloride (CaOCl₂) or bleaching powder.

To sum up,

1. Metal carbonate (X) is calcium carbonate.

2. Solution (Y) is lime water (calcium hydroxide).

3. Gas (G) is chlorine gas.

4. Dry (Y) is dry calcium hydroxide (dry slaked lime).

5. Compound (Z) is bleaching powder (calcium oxychloride).

Base B is sodium hydroxide (NaOH). Dry pellets of sodium hydroxide (NaOH) absorb moisture from the atmosphere and becomes sticky. It is also a by-product of chloralkali process. The acidic oxide such as CO₂ reacts with a base such as NaOH to give salt (sodium carbonate) and water. Such reaction is called neutralisation reaction.

Thus, ‘B’ is NaOH and reaction between ‘B’ and acidic oxide is called a neutralization reaction.

The group 2 element is calcium. The sulphate salt of calcium is calcium sulphate that is a soft substance. It is also known as Plaster of Paris. It can be moulded into various shapes because of presence half water of crystallization. When left in open for sometime, this soft mass hardens and cannot be further remoulded. This is called gypsum.

Compound X is NaOH (sodium hydroxide).