CBSE 10 Science Notes
Acids Bases and Salts
SYLLABUS
Their definitions in term of furnishing of H+ and OH- ions, General properties, examples and uses, Concept of pH scale (Definition relating to logarithm not required), Importance of pH in everyday life; Preparation and uses of Sodium hydroxide, Bleaching powder, Washing soda and Plaster of Paris.
Acids - Sour in taste
Turns blue litmus paper into red
Divided into two types on the basis of their occurrence
1.Natural acids - Acids which are obtained from natural sources are called Natural Acids or Organic Acids.
Examples -
Methanoic acid (\(HCOOH\))
Acetic acid (\(CH_3COOH\))
Oxalic acid (\(C_2H_2O_4\)) etc.
2. Mineral acids - Acids that are prepared from minerals are known as Mineral Acids.
Example - Inorganic acids, man-made acids or synthetic acid are also known as Mineral Acids.
Example -
Hydrochloric acid (\(HCl\))
Sulphuric acid (\(H_2SO_4\))
Nitric acid (\(HNO_3\))
Carbonic acid (\(H_2CO_3\))
Phosphoric acid (\(H_3PO_4\)) etc.
Dilution of Acid -
The concentration of hydrogen ion in an acid per unit volume, shows the concentration of acid.
By mixing of acid to water
The concentration of hydrogen ion per unit volume decreases.
This process of addition of acid to water is called Dilution and the acid is called diluted acid.
Note - The dilution of acid is exothermic. If water is added to a concentrated acid, a lot of heat is generated, which may cause splashing out of acid and may cause severe damage as concentrated acids are highly corrosive.
Therefore, acid is always added to water but water is never added to acid.
Strength of Acid-
Strong Acids - Acids in which complete dissociation of hydrogen ion takes place are called Strong Acids.
\(HCl\) (Hydrochloric acid)
\(HNO_3\) (Nitric acid)
\(H_2SO_4\) (Sulfuric acid)
\(HBr\) (Hydrobromic acid)
\(HI\) (Hydroiodic acid)
\(HClO_4\) (Perchloric acid)
\(HClO_3\)(Chloric acid)
Weak Acids - An acid which is partially ionised in water and thus produces a small amount of hydrogen ions (\(H^+\)) is called a Weak Acid.
\(C_2H_2O_4\) (Oxalic acid)
\(H_2SO_3\) (Sulfurous acid)
\(HSO_4\) (Hydrogen sulfate ion)
\(H_3PO_4\) (Phosphoric acid)
\(HNO_2\) (Nitrous acid)
\(HF\) (Hydrofluoric acid)
\(HCO_2H\) (Methanoic acid)
\(C_6H_5COOH\) (Benzoic acid)
\(CH_3COOH\) (Acetic acid)
\(HCOOH\) (Formic acid)
For acidic solutions - pH < 7
Indicators -
Indicators are substances which indicate the acidic or basic nature of the solution by the colour change.
Types of Indicator -
There are many types of indicators. Some common types of indicators are -
1. Natural Indicators - Indicators obtained from natural sources are called Natural Indicators.
Litmus, turmeric, red cabbage, China rose, etc., are some common natural indicators used widely to show the acidic or basic character of substances.
Litmus: Litmus is obtained from lichens. The solution of litmus is purple in colour. Litmus paper comes in two colours- blue and red.
Turmeric - Turmeric is another natural indicator. Turmeric is yellow in colour. Turmeric solution or paper turns reddish brown with base. Turmeric does not change colour with acid.
Red Cabbage - The juice of red cabbage is originally purple in colour. Juice of red cabbage turns reddish with acid and turns greenish with base.
Olfactory Indicator - Substances which change their smell when mixed with acid or base are known as Olfactory Indicators. For example; Onion, vanilla etc.
Onion - Paste or juice of onion loses its smell when added with base. It does not change its smell with acid.
Vanilla - The smell of vanilla vanishes with base, but its smell does not vanish with an acid.
Olfactory Indicators are used to ensure the participation of visually impaired students in the laboratory.
Synthetic Indicator - Indicators that are synthesized in the laboratory are known as Synthetic Indicators. For example; Phenolphthalein, methyl orange, etc.
Phenolphthalein is a colourless liquid. It remains colourless with acid but turns into pink with a base.
Methyl orange is originally orange in colour. It turns into the red with acid and turns into yellow with base.
Universal indicator
Using a litmus paper, phenolphthalein, methyl orange, etc. only the acidic or basic character of a solution can be determined, but the use of these indicators does not give the idea about the strength of acid or base. So, to get the strength as well as acidic and basic nature of a given solution universal indicator is used.
Universal indicator shows different colour over the range of pH value from 1 to 14 for a given solution.
Universal indicator is available both in the form of strips and solution.
Universal indicator is the combination of many indicators, such as water, propanol, phenolphthalein, sodium salt, sodium hydroxide, methyl red, bromothymol blue monosodium salt, and thymol blue monosodium salt.
The pH of a neutral solution is 7.
The pH of an acidic solution is < 7.
The pH of a basic solution is > 7.
Bases
Have a bitter taste.
Soapy to touch.
Turns red litmus blue.
Conducts electricity in solution.
Release OH– ions in Aqueous Solution
Types of bases
Bases can be divided in two types –
Water soluble and
Water-insoluble.
The hydroxide of alkali and alkaline earth metals are soluble in water. These are also known as alkali.
For example; sodium hydroxide, magnesium hydroxide, calcium hydroxide, etc. Alkali is considered a strong base.
All alkalis are bases, but not all bases are alkalis.
Strong Bases - Bases in which complete dissociation of hydroxide ion takes place are called Strong Bases.
\(KOH\) (Potassium hydroxide)
\(NaOH\) (Sodium hydroxide)
\(Ca(OH)_2\) (Calcium hydroxide)
\(RbOH\) (Rubidium hydroxide)
\(Sr(OH)_2\) (Strontium hydroxide)
\(CsOH\) (Cesium hydroxide)
\(Ba(OH)_2\) (Barium hydroxide)
Weak Bases - Bases in which partial dissociation of hydroxide ion takes place are called Strong Bases.
\(NH_3\) (Ammonia)
\(Zn(OH)_2\) (Zinc hydroxide)
\(Fe(OH)_3\) (Ferric hydroxide)
\(Cu(OH)_2\) (Copper hydroxide)
\(Zn(OH)_2\) (Zinc hydroxide)
\(N(CH_3)_3\) (Trimethylamine)
\(CH_3NH_2\) (Methylamine)
Chemical Properties of Acids
1. Reaction of acids with metal
When acids react with metal, salt and hydrogen gas is formed.
Acid + Metal → Salt + Hydrogen gas
When hydrochloric acid reacts with zinc metal, Zinc chloride and Hydrogen gas is formed.
\(HCl+Zn → ZnCl_2+H_2(g)\)
Sulphuric acid reacts with sodium metal and form Sodium sulphate and Hydrogen gas.
\(H_2SO_4+Na →Na_2SO_4+H_2(g)\)
Note - Presence of Hydrogen gas can be tested by bringing a lighted candle near it. If the gas bums with a pop sound, then it confirms the evolution of hydrogen gas. Burning with pop sound is the characteristic test for hydrogen gas.
2. Reaction of acids with metal carbonate/Metal hydrogen carbonate
When acids react with metal carbonate salt and carbon dioxide gas is formed.
Metal carbonate/Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water
When hydrochloric acid reacts with sodium carbonate, carbon dioxide gas, sodium chloride along with water is formed.
\(Na_2CO_3(s)+2HCl(aq)→NaCl(aq)+\)\(2H_2O(l)+CO_2(g)\)
When hydrochloric acid reacts with sodium hydrogen carbonate (bicarbonates) carbon dioxide gas, sodium chloride along with water is formed.
\(NaHCO_3(s)+HCl(aq)→NaCl(aq)+\)\(H_2O(l)+CO_2 (g)\)
When sulphuric acid reacts with sodium bicarbonate and form sodium sulphate, Carbon dioxide gas and water is formed.
\(NaHCO_3(s)+H_2SO_4(aq)→Na_2SO_4(aq)+\)\(H_2O(l)+CO_2 (g)\)
Note - Presence of Carbon dioxide turns lime water milky when passed through it. The gas evolved because of reaction of the acid with metal carbonate or metal hydrogen carbonate turns lime water milky. This shows that the gas is carbon dioxide gas. This happens because of the formation of a white precipitate of calcium carbonate.
But when excess of carbon dioxide is passed through lime water, it makes milky colour of lime water disappear. This happens because of formation of calcium hydrogen carbonate. As calcium hydrogen carbonate is soluble in water, thus, the milky colour of solution mixture disappears.
3. Reaction of Metallic Oxides with Acids
Metal oxides are basic in nature. Thus, when an acid reacts with a metal oxide both neutralize each other. In this reaction, the respective salt and water are formed.
Metal oxide + Acid → Salt + Water
\(2HCl + CuO → CuCl_2 + H_2O\)
4. Reaction of Acids with bases
An acid neutralizes a base when they react with each other and respective salt and water are formed.
Acid + Base → Salt + Water
when HCl (Hydrochloric acid), a strong acid, reacts with NaOH, a strong base, sodium chloride (salt) and water is formed.
\(HCl + NaOH → NaCl + H_2O\)
Chemical properties of Bases
1. Reaction of Base with Metals
When alkali (base) reacts with metal, it produces salt and hydrogen gas.
Alkali + Metal → Salt + Hydrogen
When sodium hydroxide reacts with zinc metal gives hydrogen gas and sodium zincate.
\(NaOH(aq)+Zn(S)→Na_2ZnO_2+H_2(g)\)
2. Reaction of Base with Oxides of Non-metals
Non-metal oxides are acidic in nature.
For example; carbon dioxide is a non-metal oxide. When carbon dioxide is dissolved in water it produces carbonic acid.
Therefore, when a base reacts with non-metal oxide, both neutralize each other resulting respective salt and water -
Base + Non-metal oxide → Salt + Water
(Non-metal oxides are acidic in nature)
Sodium hydroxide gives sodium carbonate and water when it reacts with carbon dioxide
Calcium hydroxide gives calcium carbonate and water when it reacts with carbon dioxide.
\(CO_2 + Ca(OH)_2 → CaCO_3 + H_2O\)
3. Reaction of Base with acids (Neutralisation Reaction)
An acid neutralizes a base when they react with each other and respective salt and water are formed.
Acid + Base → Salt + Water
\(6HNO_3(aq)+Fe_2O_3(s)→2Fe(NO_3)_3(aq)+\)\(3H_2O(l)\)
Since, the reaction between acid and base both neutralize each other, hence, it is also known as Neutralization Reaction.
Sodium chloride and water are formed when hydrochloric acid reacts with sodium hydroxide (a strong base).
\(NaOH(aq)+HCl(aq) → NaCl(aq)+H_2O(l)\)
In a similar way, calcium chloride is formed along with water when hydrochloric acid reacts with calcium hydroxide (a base).
Salts
Salt is a combination of an anion of an acid and a cation of a base.
Examples – \(KCl, NaNO_3 , CaSO_4\), etc.
Salts are usually prepared by the neutralisation reaction of an acid and a base.
Most of the salts are crystalline soild.
Salts may be transparent or opaque.
Most of the salts are soluble in water.
Solution of the salts conducts electricity in their molten state also.
The salt may be salty, sour, sweet and bitter.
Neutral salts are odourless.
Salts can be colourless or coloured.
pH of Salts
A salt of a strong acid and a strong base will be neutral in nature. pH = 7 (approx.).
A salt of a weak acid and a strong base will be basic in nature. pH > 7.
A salt of a strong acid and a weak base will be acidic in nature. pH < 7.
Family of Salts
Salts having the same cation or anion belong to the same family. For example - \(NaCl\), \(KCl\), \(LiCl\).
Sodium chloride (\(NaCl\) and Calcium chloride (\(CaCl_2\)) belongs to chloride family.
Calcium chloride (\(CaCl_2\) and Calcium sulphate (\(CaSO_4\) belongs to calcium family.
Zinc chloride (\(ZnCl_2\)) and Zinc sulphate (\(ZnSO_4\)) belongs to the zinc family.
Three types of salts
Neutral, Acidic and Basic Salts
1. Neutral Salt
Salts produced because of reaction between a strong acid and strong base are neutral in nature. The pH value of such salts is equal to 7, i.e. neutral.
Example -
Sodium chloride, Sodium sulphate. Postassium chloride, etc.
Sodium chloride (\(NaCl\)) - It is formed after the reaction between hydrochloric acid (a strong acid) and sodium hydroxide (a strong base).
Sodium Sulphate (\(Na_2SO_4\)) - It is formed after the reaction between sodium hydroxide (a strong base) and sulphuric acid ( a strong acid).
Potassium Chloride (\(KCl\)) - It is formed after the reaction between potassium hydroxide (a strong base) and hydrochloric acid (a strong acid).
2. Acidic Salts
Salts which are formed after the reaction between a strong acid and weak base are called Acidic salts. The pH value of acidic salt is lower than 7.
Example -
Ammonium sulphate, Ammonium chloride, etc.
Ammonium chloride is formed after reaction between hydrochloric acid (a strong acid) and ammonium hydroxide (a weak base).
Ammonium sulphate is formed after reaction between ammonium hydroxide (a weak base) and sulphuric acid (a strong acid).
3. Basic Salts
Salts which are formed after the reaction between a weak acid and strong base are called Basic Salts.
Example -
Sodium carbonate, Sodium acetate, etc.
Sodium carbonate is formed after the reaction between sodium hydroxide (a strong base) and carbonic acid (a weak acid).
Sodium acetate is formed after the reaction between a strong base, sodium hydroxide (a strong base) and acetic acid, (a weak acid).
Cause of formation of acidic, basic and neutral salts -
When a strong acid reacts with a weak base, the base is unable to fully neutralize the acid. Due to this, an acidic salt is formed.
When a strong base reacts with a weak acid, the acid is unable to fully neutralize the base. Due to this, a basic salt is formed.
When equally strong acid and a base react, they fully neutralize each other. Due to this, a neutral salt is formed.
pH value of salt:
Neutral salt: The pH value of a neutral salt is almost equal to 7.
Acidic salt: The pH value of an acidic salt is less than 7.
Basic salt: The pH value of a basic salt is more than 7.
Importance of pH in everyday life
👉Our body works well within a narrow pH range of 7.0 to 7.8.
👉pH of acid rain water is less than 5.6 When this acid rain flows into rivers, it lowers the pH of the river water making the survival of
aquatic life difficult.
👉Our stomach produces hydrochloric acid which helps in the digestion of food without harming the stomach. Sometimes excess acid is produced in the stomach which causes indigestion. To get rid of this pain, bases called antacids are used.
👉Antacids are a group of mild bases which react with the excess acid and neutralise it. Commonly used antacids are magnesium hydroxide [\(Mg(OH)_2\)] & sodium bicarbonate [\(NaHCO_3\)]
👉The bacteria present in the mouth produce acids due to the degradation of sugar and food particles after eating.
Tooth decay starts when the pH in the mouth falls below 5.5.
Tooth enamel is made up of calcium phosphate which is the hardest substance in the body.
It is insoluble in water but gets corroded when the pH in the mouth falls below 5.5.
Sodium hydroxide
When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide. The process is called the chlor-alkali process because of the products formed– chlor for chlorine and alkali for sodium hydroxide.
\(2NaCl(aq) + 2H_2O(l) → 2NaOH(aq) +\)\(Cl_2(g) + H_2(g)\)
Sodium hydroxide is used for degreasing of metals, manufacturing of paper, soap, detergents, artificial fibres, bleach, etc.
Bleaching powder
Bleaching powder is produced by the action of chlorine on dry slaked lime \(Ca(OH)_2\).
Bleaching powder is represented as \(CaOCl_2\), though the actual composition is quite complex.
\(Ca(OH)_2 + Cl_2 → CaOCl_2 + H_2O\)
Uses of Bleaching powder
(i) for bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in laundry;
(ii) as an oxidising agent in many chemical industries; and
(iii) to make drinking water free from germs.
Baking soda
The chemical name of the compound is sodium hydrogencarbonate \(NaHC0_3\). It is produced using sodium chloride as one of the raw materials.
\(NaCl+H_2O + CO_2+NH_3→NH_4Cl\)(Ammonium chloride)+ \(NaHCO_3\) (Sodium hydrogencarbonate)
Uses of Baking soda
(i) For making baking powder, which is a mixture of baking soda (sodium hydrogencarbonate) and a mild edible acid such as tartaric acid.
When baking powder is heated or mixed in water, the following reaction takes place –
\(NaHCO_3+Heat→Na_2CO_3+CO_2+H_2O\)
Carbon dioxide produced during the reaction can cause bread or cake to rise making them soft and spongy.
(ii) Sodium hydrogencarbonate is also an ingredient in antacids. Being alkaline, it neutralises excess acid in the stomach and provides relief.
(iii) It is also used in soda-acid fire extinguishers.
Washing soda
Sodium carbonate can be obtained by heating baking soda; recrystallisation of sodium carbonate gives washing soda. It is also a basic salt.
\(Na_2CO_3+10H_2O→Na_2CO_3.10H_2O\) (Sodium carbonate)
Uses of washing soda
(i) Sodium carbonate (washing soda) is used in glass, soap and paper industries.
(ii) It is used in the manufacture of sodium compounds such as borax.
(iii) Sodium carbonate can be used as a cleaning agent for domestic purposes.
(iv) It is used for removing permanent hardness of water.
Plaster of Paris
On heating gypsum at 373 K, it loses water molecules and becomes
calcium sulphate hemihydrate (\(CaSO_4\).\(\frac{1}{2}H_2O\)). This is called Plaster of Paris, the substance which doctors use as plaster for supporting fractured bones in the right position. Plaster of Paris is a white powder and on mixing with water, it changes to gypsum once again giving a hard solid mass.
\(CaSO_4\).\(\frac{1}{2}H_2O+\frac{11}{2} H_2O→\)\(CaSO_4+2H_2O\)
(Plaster of Paris) (Gypsum)
(i) Plaster of Paris is used for making toys.
(ii) Materials for decoration and for making surfaces smooth.
(iii) Doctors cast for healing fractures
(iv) Used as a fire-proofing material.
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