Acids, Bases & Salts

Acids, Bases & Salt

Concept of Acid

1. Definition: An acid is a compound which contains replaceable Hydrogen atom or atoms, a part or whole of which can be replaced by means of a metal or a positive radical.
   
2. Examples: HCl, HNO₃, H₂SO₄ are acid because they contain replaceable Hydrogen atom or atoms. Their replaceable Hydrogen (H) atom or atoms can be replaced by a metal or a positive radical partially or completely. This can be known by following example:
   
   1. Complete replacement by metal:
      

   HCl+NaOH à NaCl+H₂O [Here, Hydrogen (H) is replaced by metal Sodium (Na)].
   

   HNO₃+NaOH àNa NO₃+ H₂O [Here, Hydrogen (H) is replaced by metal Sodium (Na)].
   

   H₂SO₄ +2NaOH à Na₂SO₄+ H₂O [Here, Hydrogen (H) is replaced by metal Sodium (Na)].
   

   1. Complete replacement by Radical (Group of elements):
      

   HCl+NH₄OH à NH₄Cl+H₂O [Here, NH₄ radical replaces H-atom(s) of acid molecules.
   

   1. Partial replacement by metal or ammonium group:
      

      H₂SO₄+ NaOHà NaHSO₄+H₂O [Here, H-atom is partially replaced by metal.
      

      H₂SO₄+ NH₄OHà NH₄HSO₄+H₂O [Here, H-atom is partially replaced by ammonium].
      

3. Characteristic properties (Chemical) of Acids:
   
   1. They turn blue litmus red.
      
   2. They turn Methyl Orange red.
      
   3. They have sour taste.
      
   4. They react with base and Alkali (Metal) to form salt and water.
      

      HCl (Acid) + NaOH (Base) à NaCl (Salt) + H₂O (Water).
      

   5. Strong acids such as HCL, HNO₃, H₂SO₄, HClO₃ etc. are good conductors of electricity in their aqueous solutions.
      
   6. They are not soapy in touch.
      
   7. They liberate H₂ gas from metals.
      
   8. They liberate CO₂ from carbonate, brown fumes of HO₂ from nitrate, SO₂ from sulphite and H₂S from sulphide.
      
4. Olfactory indicator: There are some substances whose ordour changes in acidic or basic media. These are called olfactory indicators.
   

   Questions (Page: 18)
   

Q-1: You are given three test tubes. The three test tubes contain distilled water, acidic solution and the basic solution respectively. There is only red litmus paper available in order to identify what is there in each test tube. How will you find out what is in each of the test tubes?

Ans: The content can be identified easily by using red litmus paper in each of the test tubes. This can be done by noting down the colour change of the red litmus paper.

• If the red litmus paper changes to blue colour the solution is a basic solution.
  
• If the red litmus paper changes to red colour the solution is acidic solution.
  
• If you did not observe any colour change then the solution is distilled water.
  

1. Reaction of Acid with Metals: When acids react with metals, they produce a salt and hydrogen gas. Most metals react with acids, but not all. The general equation that describes the chemical reaction between an acid and metal is:  
   

   Metal + Acid = Salt + Hydrogen gas.
   

   Na + HCl = NaCl + H₂
   

2. 1. Reaction of Metal Carbonate with Acids: For this, consider the reaction between Na₂CO₃ (Sodium Carbonate) with dilute HCl (Hydrocloric Acid) which is formulated below:
      

      Ex: Na₂CO₃+2HCl à 2NaCl+H₂O+CO₂
      

      Here, Sodium Carbonate reacts with dil. yHydrocloric Acid to form Sodium Chloride (Salt), Water & Carbondioxide.
      

   2. Reaction of Metal Hydrogencarbonate with Acids: For this, consider the reaction of
      

      Sodium Hydrogencarbonate with dilute HCl. The reaction is formulated as follows.
      

      NaHCO₃ + dil. HCl à NaCl + H₂O + CO₂.
      

      Here, Hydrogencarbonate (NaHCO₃) reacts with dil. (HCl) Hydrochloric Acid to form Sodium Chloride (NaCL, Salt), Water (H₂O) & Carbondioxide (CO₂₎.
      

      Reaction of Metal Carbonates & Metal Hydrogencarbonates with acids:Metal Carbonates & Metal Hydrogencarbonates react with acids and produce corresponding metal salt, Carbon dioxide and water. Both the reaction produces CO₂ which on passing through lime water makes lime water milky due to formation of Calcium carbonate. 
3. Reaction of Metalic Oxides with Acids: Metallic oxides react with acids to form salts and water. For this, consider the reaction of Copper Oxide (CuO) with dilute hydrochloric acid (HCl) . After taking place of the  reaction¸the colour of the solution becomes blue-green due to the formation of Copper Chloride and the Copper Oxide dissolves. This proves that metallic oxides are basic oxides. The reaction is formulated below:
   

   CuO (Copper Oxide) + 2 HCl (Hydrochloric Acid) à CuCl₂ (Copper Chloride)
   + H₂O (Water).
   

   Here, Copper Oxide reacts with dil Hydrochloric Acid to form Copper Chloride & Water.
   

Concept of Base

1. Definition: Bases are Oxides and Hydroxides of metals.
   

1. Examples: Na₂O, NaOH, KOH, MgO, Mg(OH)₂, CaO, Ca (OH)₂ etc. Oxides & Hydroxides
   

of Alkali & Alkali earth metals are strong bases. Oxides and Hydroxides of other metals  are weak bases.

1. Properties of Bases:
   
   1. They turn red litmus blue.
      
   2. They are bitter in taste.
      
   3. Their solutions are soapy in touch.
      
   4. They turn Methyl Orange yellow.
      
   5. They react with acids to form salt and water.
      
   6. Strong Bases such as Na₂O, NaOH, K₂O & KOH are very good conductors of electricity in aqueous solution and fused state and hence they are called good electrolyte.
      
   7. Oxides and Hydroxides of Alkali metals are soluble in water. Oxides and Hydroxides of Ca, Sr, Ba are partially soluble in water but Oxides and Hydroxides of other metals are insoluble in water.
      
   8. Oxides & Hydroxides of metals are bases and hence they are generally soluble in acids.
      
   9. Oxides & Hydroxides of Zn, Al, Sn, Pb, Cr, As, Sb etc. are amphoteric. They dissolve in acids as well as in hot concentrated NaOH and KOH solutions.
      

   3.1    Reaction of Non-metallic Oxide with Base: Non-metallic oxides are formed by reaction of non-metal with Oxygen (O). Non–metallic oxides react with bases to give salts and water. Here,  metal in base reacts with carbon dioxide to form a salt. Hydrogen (H) adds with Oxygen (O) in Non– metal oxide to form water. This is quite similar to neutralisation reaction. So it can be said that Non-metallic oxides are acidic in nature.
   

   Example: Consider the reaction of Calcium Hydroxide (base) with Carbon dioxide (Non-metallic oxide) to produce salt and water. The reaction is shown as:
   

   Ca(OH)₂ + CO_{2 =} CaCO₃ + H₂O
   

   4.1    Reaction between Acid & Base: When Acid reacts with Base, Salt is formed. The reaction of an acid with a base is called a neutralization reaction. The products of this reaction are a salt and water.
   

   Example: The reaction of Hydrochloric acid (HCl) with Sodium Hydroxide (NaOH) solutions produces a solution of Sodium Chloride (NaCl) and some additional water (H₂O) molecules.
   

   HCl (Acid) + NaOH (Base) = NaCl (Salt) + H₂O (Water)
   

   5.1    Reaction of Acids & Bases with Metals: (a) When acids react with metals, they produce a salt and Hydrogen (H) gas. Most metals react with acids, but not all. The general equation that easily describes the chemical reaction between an acid and metal is: Metal + Acid = Salt + Hydrogen (H) gas. Metals that are more active than acids can undergo a single displacement reaction.
   

   Example: When Zinc (Zn) metal reacts with Hydrochloric acid (HCl) producs Zinc Chloride (ZnCl₂₎ and Hydrogen (H) gas.
   

Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

(b) When Alkali (base) reacts with metal, it produces salt and Hydrogen (H) gas.

Example: (a) Sodium hydroxide (NaOH) produces Hydrogen (H) gas and Sodium zincate when reacts with Zinc (Zn) metal.

Zn(s) + 2NaOH(aq) + 2H₂O(l) = Na₂Zn(OH)₄(aq) +H₂(g).

(b) Sodium aluminate and Hydrogen (H) gas are formed when Sodium hydroxide reacts with Aluminium metal.

4NaOH + 4Al = 2Na₂Al₂O₄ + 2H₂

Questions (Page 22)

Q-1:Why should curd and sour substances not be kept in brass and copper vessels?

Ans: Curd and sour food substances contain acids, these acidic substances combine with metal. This reaction turns food to poison which damage people’s health.

Q-2: Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?

Ans: When an acid reacts with any metal, salt and hydrogen gas are formed.

Metal + Acid → Salt + Hydrogen gas

Q-3: Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.

Ans: As metal compound released is Calcium Chloride the gas evolved here is CO2. Hence metal A should be Calcium Carbonate. Hence the reaction between Calcium Carbonate and HCL is

CaCO₃ (s)+2Hcl(Aq) → CaCl₂( Aq)+ Co₂ (g)+H₂O (l)

6.1    Common between Acid & Base: The common properties between Acid & Bases are as follows:

• When and Acid or Base is dissolved in water, it makes ions. This makes the water conduct electricity better.
  
• The stronger the Acid or Base, the more ions are produced. Hence, the conductivity of the solution increases.
  
• Acid & Base can both conduct electricity.
  
• When an acid reacts with a base, a salt is formed.
  
• Acid & Base both can carry and share ions.
  

7.1    Effect on Acid & Base in a water solution: When dissolved in water, Acids donate Hydrogen ions (H⁺).  Bases on the other hand, mixed with water  to yield Hydroxide ions (OH^–). If a solution has a high concentration of H⁺ ions, then it is acidic & If a solution has a high concentration of OH^– ions, then it is basic.

   7.2 Example: (a) When Hydrochloric Acid is dissolved with water then Hydrogen ions ydrogen ionin HCl are produced in the presence of water. The separation of H⁺ ion from HCl molecules cannot occur in the absence of water.

HCl + H₂O = H₃O⁺ + Cl^–

            Hydrogen ions (H⁺) cannot exist alone, but they exist after combining with water molecules. As a result Hydrogen must always be shown as H⁺ (aq) or Hydronium ion (H₃O⁺ ). Acids give H₃O⁺ or H⁺(aq) ion in water.

H⁺ + H₂O = H₃O⁺

(b) When Base is dissolved in water it generates Hydroxide (OH^–) ions in water. Bases which are soluble in water are called Alkalis.

NaOH (s) + H₂O = Na⁺(aq) + OH^– (aq)

KOH (s) + H₂O = K⁺ (aq) + OH^– (aq)

Mg(OH)₂ + H₂O = Mg²⁺ (aq) + 2OH^– (aq)

Please note that all bases do not dissolve in water. An Alkali is a base that dissolves in water. They are soapy to touch, bitter and corrosive. The process of dissolving an Acid or Base in water is a highly exothermic reaction. Hence, Acid must always be added slowly to water with constant stirring. If water is added to a concentrated acid, the heat generated may cause the mixture of splash out and cause burns. The glass container may also brake due to excessive local heating.

Questions (Page 25)

Q-1: Why do HCl, HNO₃, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

Ans: Release of H+ ion in water will make a compound acidic or non- acidic. Acids are the substance which upon dissociating with water results in production of Hydrogen ions. Some compounds show acidic character as they dissociate in the aqueous solution which results in the production of hydrogen ions (acids like HCl, HNO₃). Compounds similar to glucose or alcohol do contain hydrogen element but they do not show signs of acidic nature. The fact that the hydrogen in them will not separate as like the hydrogen in the acids. They will not separate to become hydrogen ions, on dissolving in the water.

Hence dissociation of hydrogen gas will decide the acidic or non-acidic nature of a compound.

Q-2: Why does an aqueous solution of an acid conduct electricity?

Ans: Charged particles are responsible for the conductance of electricity in an acid. These charged particles called as ions are the reason behind conductance of electricity in acid.

Q-3: Why does dry HCl gas not change the colour of the dry litmus paper?

Ans: HCL does not give out Hydrogen ions, therefore HCL does not show any acidic behaviour and colour of the litmus paper remain the same on reacting with HCl gas.

Q-4: While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Ans: While diluting an acid, it is recommended that the acid should be added to water and not water to the acid because if water is added to concentrated acid, it release huge amount of heat which may result in explosion and can cause acid burns o face, clothes and body parts. Hence it is safe to add acid to water but not water to acid.

Q-5: How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?

Ans: When acid is added to water there will be a fixed amount of hydronium present in the fixed volume of solution. If we dilute the solution hydronium ion per volume of solution decrease, this in-turn decreases Hydronium concentration in the solution.

Q-6: How is the concentration of hydroxide ions (OH– ) affected when excess base is dissolved in a solution of sodium hydroxide?

Ans: When base is dissolved in sodium hydroxide solution its hydroxide ions increase but it will reach saturation at some point. After saturation point hydroxide ion concentration is not affected even after adding base further.

8.1    Strength of Acid or Base solutions: It is learnt that Acids yield H⁺ ions & Bases yield OH^– ions. In the same way the strength of an acid and base can be measured that how an acid or base is strong or weak. Hence, a scale for measuring Hydrogen iion concentration in a solution called p^H scale was developed. Here, the ‘p’ in pH stands for ‘potenz’ in German means power. On the pH scale, it can be measured pH generally from 0 (very acidic) to 14 (very Alkaline means basic). pH should be thought of simply as a number which indicates the acidit or basic nature of a solution. Higher the Hydronium ion (H₃O⁺) concentration, lower is the pH value.

8.2    The pH of a neutral solution is 7. Value less than 7 on he pH cale represents an acidit solution. As the pH value increases from 7 to 14, it represents an increase in OH^– ion concentration in the solution, that is increase in the strength of alkali. Generally paper impregnated with the universal indicator is used for measuring of pH.

pH value of common substances shown in a pH paper (colours are only a rough guide)

8.3    The strength of acids and bases depends on the number of H⁺ ions and OH^– ions produced respectively. If Hydrochloric acid & Acetic acid are taken in equal concentration, then these produce different amounts of Hydrogen ions. Acids that give rise to more H⁺ ions are said to be strong acids and acids that give less H⁺ are said to be weak acids. Similarly, weak & strong base can be known as per concentrations of OH^– ions. Bases that give rise to OH^– ions are said to be strong bases and bases that give less OH^– ions are said to be weak bases.

9.1    P^H Scale & it’s importance in everyday life: The strength of pH scale have huge effect in everyday life of living organism (plants and animals). All living organism are pH sensitive. The human body works between pH range of 7.0 to 7.8. Living organism can survive only in a narrow range fo pH change. When pH of rain water is less than 5.6, it is called acid rain. When acid rain flows into the rivers, it lowers the pH of the river water. The survival of aquatic life in such rivers becomes difficult.

9.2    The atmosphere of Venus (Planet) is made up of thick white and yellowish clouds of sulphuric acid. The life on Venus can be imagined accordingly.

9.4    pH of the soil in backyard: Plants requires a specific range of pH range for their healthy growth.

9.5    pH in human digestive system: It is very interesting to note that human stomach produces Hydrochloric acid (HCl). It helps in the digestion of food without harming the stomach. During the course of indigestion, the stomach produces too much acid and this causes pain and irritation. To get rid of this pain, people use bases called antacids. These antacids neutralise the excess acid. Magnesium hydroxide (Milk magnesia), a mild base, is often used for this purpose.

9.6    pH chance as the case of tooth decay: Tooth decay starts when the pH of the mouth is lower than 5.5. Tooth enamel is made up of Calcium hydroxyapatite ( A crystalline form of Calcium phosphate) is the hardest substance in the body. It does not dissolve in water, but is corroded when the pH in the mouth is below 5.5. Bacteria present in the mouth produce acids by degradation of sugar and food particles remaining in the mouth after eating.. The best way to prevent this is to clean the mouth after eating food. Using toothpastes, which are generally basic, for cleaning the teeth can neutralise the excess acid and prevent tooth decay.

9.7    Self defence by animal & plants through chemical warfare: When anyone is stung by honey-bee, bee-sting leaves an acid which causes pain and irritation… the use of mild base like baking soda on the stung area gives relief. Stinging hair of nettle leaves inject methanoic acid causing burning pain.

9.8    Sources of naturally occurring acids:

Questions (Page 28)

Q-1: You have two solutions, A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic?

Ans: In order to find the hydrogen ion concentration, we can use the rule that states, “The pH of any solution is inversely proportional to the hydrogen ion concentration”. Therefore, it means that the solution that has a lower pH number will have a higher hydrogen ion concentration. Hence, solution A will have a higher hydrogen ion concentration. In addition, solution B will be basic and A will be acidic.

Q-2: What effect does the concentration of H+(aq) ions have on the nature of the solution?

Ans: Hydrogen ion concentration decides the nature of the solution. If Hydrogen ion concentration increase then solution turn acidic and similarly if Hydrogen ion concentration decreases then solution turn basic.

Q-3: Do basic solutions also have H+(aq) ions? If yes, then why are these basic?

Ans: Basic solutions has H+ ions, but hydroxide ions present in basic solution are more in basic solution . Hence Hydroxide ions turn solution to basic.

Q-4:  Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?

Ans: If the soil is acidic in nature ( P^H below 7) then such field should be treated with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate).

Concept of Salt

10.1    Definiton: A salt is a compound formed by partial or complete replacement of the replaceable Hydrogen atom or atoms present in an acid molecule by means of a metal or a radical which acts like a metal.

10.2    Example:
(a) HCl + NaOH = NaCl + H₂O

         (b) HNO₃ + KOH = KNO₃ + H₂O

(c) H₂SO₄ + NaOH = NaHSO₄ + H₂O

Acid + Base = Salt + Water

Type of Salts

10.3    Normal Salt: A normal salt is a salt formed by complete replacement of replaceable H-atoms from an acid molecule by means of a metal or a group of elements acting like a metal. A normal salt does not contain replaceable H-atom in its molecule. Hence, NaCl, KCl, K₂SO₄, Na₂SO₄, FeSO_4, FeCl_2, SO₄ etc. are normal salts.

10.4    Acid Salt: An acid salt is that which contains one or more replaceable H-atoms in its molecule and is formed by partial replacement of H-atoms present in an acid molecule by means of metal or positive radical. Here are some examples of acid salts: NaHSO₄, NaHCO₃, KHCO₃, Na₂HPO₄, NaH₂PO_4, NH₄HCO₃ are acid salts.

10.5    Basic salt: A salt which contains O^2- or OH^– group in its molecule is called basic salt. Example: [Mg(OH)₂. MgCO₃], [Cu(OH)₂. CuCO₃] & BiOCl are basic salts.

10.6    Acidic salt: A normal salt which is formed by the neutralisation of a strong acid and weak base is called acidic salt because its aqueous solution turns blue litmus red. Example: FeCl₃, ZnCl₂, HgCl₂ etc.

10.7    Alkaline salt: Normal salts which are formed by the neutralisation of strong acids and strong bases are called neutral salts because their aqueous solutions are neutral to litmus. Example:

10.8    Neutral salt: Normal salts which are formed by the neutralisation of strong acids and strong bases are called neutral salts because their aqueous solutions are neutral to litmus. Example: NaCl, KCl, Na₂SO₄, K₂SO₄ etc.

10.9    Double salt: A double salt is a mixture of two salts which on dissolution in water gives two types of metal ions. Example: (a) Potash Alum: K₂SO₄.Al₂(SO₄)₃, 24H₂O (b) Mohr’s salt: FeSO₄. (NH₄)2SO₄. 6H₂O (c) Chrome Alum: K₂SO₄.Cr₂(SO4)₃.24H₂O (d) Ferric Alum: K₂SO₄.Fe₂(SO4)₃. 24H₂O.

10.10    Complex salt: A complex salt is a salt which contains a complex ion or complex neutral molecule in which there is a central metal ion surrounded by a number of neutral molecules or negative ions called ligands. Example: (a) Potassium ferricyanide: K₃[Fe(CN)₆] (b) Potassium argento cyanide: K[Ag(CN)₂] etc.

11.1    Bleaching powder: Calcium hypochlorite is an inorganic compound with formula Ca(ClO)₂. It is the main active ingredient of commercial products called bleaching powder.
Bletching powder is produced by the action of Chlorine (Cl) on dry slaked lime [Ca(OH)₂]. Bletching powder is represented as CaOCl₂.

Ca(OH)₂ + Cl₂ = CaOCl₂ + H₂O

11.2    Uses of Bleaching powder: (a) For bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleching washed cloths in laundry. (b) It is used as an oxidising agent in many chemical industries. (c) It is used to make drinking water free from germ.

12.1    Baking soda: Backing soda is commonly used in the kitchen for preparation of tasty and crispy dishes. Sometimes it is added for faster cooking. The chemical name of the compound is Sodium Hydrogencarbonate (NaHCO₃). It is produced by using Sodium Chloride as one of the raw materials. It is a mild non-corrosive basic salt.

NaCl + H₂O + CO₂ + NH₃ = NH₄Cl + NaHCO₃

12.2    Uses of Baking Soda: (a) It is used for making baking powder, which is mixture of baking soda and mild edible acid such as tartaric acid. (b) Sodium Hydrogencarbonate is also an ingredient in antacids. Being alkaline, it neutralises excess acid in the stomach and provides relief. (c) It is used in soda-acid fire extinguishers.

13.1    Washing soda: Sodium Carbonate, commonly called Washing Soda or Soda Ash, is a water-soluble sodium salt of carbonic acid. Its proportions of sodium, carbon and hydrogen make it effective as a natural cleaner and cleaning booster. Sodium carbonate is obtained by heating baking soda; recrystallization of Sodium carbonate gives washing soda. It is also a basic salt.

Na₂CO3 + 10 H₂O = Na₂CO₃. 10H₂O

13.2     Uses of Washing Soda: (a) Sodium Carbonate (Washing Soda) is used in glass, soap and paper industries. (b) It is used in the manufacture of Sodium compounds such a borax. (c) Sodium carbonate can be used as a cleaning agent for domestic purposes. (d) it is used for removing permanent hardness of water.

14.1     Plaster of Paris: On heating Gypsum at 373K, it loses moles and becomes Calsium sulphate hemihydratre [(CaSO₄)₂. H₂O]. This is called Plaster of Paris. It is a white powder and on mixing with water, it changes to Gypsum once again giving a hard solid mass.

(CaSO₄)₂. H₂O on heating = CaSO₄.2H₂O

14.2    Uses of Plaster of Paris: (a) Plaster of Paris is a building material that is used as a protective coating on walls and ceilings. (b) It is also used as a moulding and casting agent for decorative elements. (c) It is used to give aesthetic finishing touches to the buildings. (d) It is used by doctors for plastering the fractured limbs etc.

Questions (Page 33)

Q-1: What is the common name of the compound CaOCl₂?

Ans: Common name of CaOCl₂ is Bleaching powder.

Q-2: Name the substance which on treatment with chlorine yields bleaching powder.

Ans: The substance which on treatment with chlorine yields bleaching powder is Calcium hydroxide.

Q-3: Name the sodium compound which is used for softening hard water.

Ans: Sodium carbonate is the compound which is used for softening hard water.

Q-4: What will happen if a solution of Sodium hydrocarbonate is heated? Give the equation of the reaction involved.

Ans: Heating Sodium hydrocarbonate yields sodium carbonate and carbon dioxide gas is liberated in the process.

2NaHCO₃ → heat → Na₂CO₃ + H₂O + CO₂

Q-5: Write an equation to show the reaction between Plaster of Paris and water.

Ans: The chemical equation for the reaction of Plaster of Pans and water is

CaSO₄.1/2H₂O+3/2H₂O → CaSO₄.2H₂O

Exercise (Page 34 – 35)

Q-1: A solution turns red litmus blue, its pH is likely to be
(a)1 (b) 4 (c) 5 (d) 10
Ans: The correct answer is 10 because litmus paper turns blue when reacts with basic solution ( pH more than 7). Hence, 10 is the correct answer.

Q-2: A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains
(a) NaCl (b) HCl (c) LiCl (d) KCl

Ans: The correct answer is HCl as Egg shells contains calcium carbonate, which on reaction with HCl liberates CO₂ gas which turn lime water to milky.

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

Q-3: 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount HCl solution (the same solution as before) required to neutralise it will be

(a) 4 ml (b) 8 ml(c) 12 ml (d) 16 ml

Ans: Since, 10 ml of NaOH requires 8 ml of HCl, 20 ml of NaOH require 8 x 2 = 16ml of HCl Hence the answer is option (d) 16 ml.

Q-4: Which one of the following types of medicines is used for treating indigestion?

(a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic

Ans: Indigestion is due to excess production of acid in the stomach. It is required to neutralise the excess produced acid. To neutralise acid, base is required. Hence, medicines which is basic in nature is used to treat indigestion is called as Antacid.

Q-5: Write word equations and then balanced equations for the reaction taking place when

(a) Dilute sulphuric acid reacts with zinc granules.

(b) Dilute hydrochloric acid reacts with magnesium ribbon.

(c) Dilute sulphuric acid reacts with aluminium powder.

(d) Dilute hydrochloric acid reacts with iron filings.

Ans:

(a) dilute sulphuric acid reacts with zinc granules:

⇒ dilute sulphuric acid + zinc → Zink Sulphate + Hydrogen Gas

⇒ H₂SO₄(aq) + Zn → ZnSO₄(aq) + H₂(g)

(b) dilute hydrochloric acid reacts with magnesium ribbon.

⇒ dilute Hydrochloric + Magnesium → Magnesium Chloride + Hydrogen Gas

⇒ 2HCl(aq) + Mg → MgCl₂(aq) + H₂(g)

(c) dilute sulphuric acid reacts with aluminium powder.

⇒ dilute Sulphuric Acid + Aluminium → Aluminium Sulphate + Hydrogen Gas

⇒ 3H₂SO₄(aq) + 2Al(s) → Al₂(SO₄)₃(aq) + 3H₂(g)

(d) dilute hydrochloric acid reacts with iron filings.

⇒ dilute Hydrochloric Acid + Iron → Ferrous Chloride + Hydrogen Gas

⇒ 6HCl(aq) + 3Fe(s) → 3FeCl₂(aq) + 3H₂(g)

Q-6: Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it

Ans: Insert two nails on the wooden or rubber cork and place them on a beaker as shown in figure. Connect iron nail to a bulb, 6 volt battery and a wire connected to switch. Pour some alcohol or glucose so as to dip the nails in glucose or alcohol. Turn the switch on and you the see the bulb not glowing despite of connection to switch. Now empty the beaker and add HCL solution. This time bulb glows. This proves acid can conduct electricity but alcohol and glucose does not conduct electricity.

Q-7: Why does distilled water not conduct electricity, whereas rain water does?

Ans:

• Distilled water does not contain any ionic compounds in it.
  
• Whereas rainwater has a lot, more compounds.
  
• Rainwater has dissolved acidic gas such as carbon dioxide from the air and that forms carbonic acid. This means that it has hydrogen ions and carbonate ions. Therefore, with the presence of acids, rainwater can conduct electricity.
  

Q-8: Why do acids not show acidic behaviour in the absence of water?

Ans: The acidic behaviour from acids is because of the presence of hydrogen ions. Hydrogen ions can only be produced in the presence of water and therefore water is definitely needed if acids are to show their acidic behaviour.

Q-9: Five solutions A,B,C,D and E when tested with universal indicator showed pH as 4,1,11,7 and 9, respectively. Which solution is

(a) neutral?

(b) Strongly alkaline?

(c) Strongly acidic?

(d) Weakly acidic?

(e) Weakly alkaline?

Ans: In increasing order of hydrogen ion concentration:

pH 11(B) → pH 9(E) → pH 7(A) → pH 4(D) → pH 1 (B)

PH11-Strongly alkaline

pH9- weakly alkaline

PH7-Neutral

pH-4- Weakly acidic

pH-1- Strongly acidic

Q-10: Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3COOH) is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?

Ans: HCl is a strong acid whereas acetic is a weaker acid. Fizzing occurs because of the production of the hydrogen gas obtained due to reaction of the acid on the magnesium ribbon. Since Hcl is a very strong acid there is a lot of liberation of hydrogen gas from test tube A. therefore, more fizzing take place in test tube A.

Q-11: Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

Ans: Fresh milk is turned to curd due to production of lactic acid. Lactic acid reduces the pH of the milk.

Q-12: A milkman adds a very small amount of baking soda to fresh milk.

(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?

(b) Why does this milk take a long time to set as curd?

Ans: (a)He shifted the pH of the fresh milk from 6 to slightly alkaline to prevent milk from getting sour due to production of lactic acid.

(b) This milk takes long time to set into curd because the lactic acid produced here first neutralises the pH then the pH is reduced to turn milk to curd.

Q-13: Plaster of Paris should be stored in a moisture-proof container. Explain why?

Ans: Plaster of Paris should be stored in moisture-proof container because moisture can affect plaster of Paris by slowing down the setting of the plaster because of hydration. This will turn plaster useless.

Q-14: What is a neutralisation reaction? Give two examples.

Ans: The reaction of the acid + base gives a product of salt + water, which is considered as neutralization reaction.

Examples:

NaOH + HCl → NaCl + H₂O

Mg(OH)₂ + H₂CO₃ → MgCO₃ + 2H₂O

15. Give two important uses of washing soda and baking soda.

Ans: