CBSE NOTES CLASS 10 SCIENCE CHAPTER 2

ACIDS, BASES AND SALTS

Acids

• Sour in taste

• Turn blue litmus to red

Bases

• Bitter in taste

• Turn red litmus to blue

• They are soapy to touch

Chemical Indicator

A chemical indicator is a substance that undergoes a distinct observable change in the form of a colour change, precipitate formation, bubble formation, temperature change, smell change or some other measurable quality, in the presence of acids or bases.

Natural Indicator

An indicator which is prepared from natural substances is called a natural indicator.

For examples, turmeric, red cabbage juice, litmus, onion, etc.

Litmus as indicator

Litmus solution is a purple dye, which is extracted from lichen, a plant belonging to the division Thallophyta.

When the litmus solution is neither acidic nor basic, its colour is purple.

• An acid turns blue litmus paper to red.

• A base turns red litmus paper to blue.

Turmeric as indicator

Turmeric is yellow in colour. Turmeric solution or paper turns reddish brown with base. Turmeric does not change colour with acid.

Red cabbage juice as indicator

The juice of red cabbage is originally purple in colour. It turns reddish with acid and turns greenish yellow with base.

Synthetic Indicator

Indicators that are synthesized in 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 red with acid and into yellow with base.

Olfactory Indicators

Substances which change their smell when mixed with acid or base are known as olfactory indicators.

For example, onion, vanilla, clove, etc.

Onion as indicator

Paste or juice of onion loses its smell when added with base. It does not change its smell with acid.

Vanilla as indicator

The smell of vanilla vanishes with base, but the smell does not change with an acid.

Summary of Indicators

 Indicator Original Colour Acid Base Red litmus Red No change Blue Blue litmus Blue Red No change Turmeric Yellow No change Reddish brown Red cabbage juice Purple Reddish Greenish yellow Phenolphthalein Colourless Colourless Pink Methyl orange Orange Red Yellow Onion - No change Smell vanishes Vanilla - No change Smell vanishes Clove - No Change Smell vanishes

Universal Indicator

Universal indicator is mixture of several dyes, which show different colours with acids/bases of different strengths. Universal indicator shows different colours over the range of pH value from 1 to 14 for a given solution.

It is normally available in the form of strips.

CHEMICAL PROPERTIES OF ACIDS & BASES

Reaction of Acids and Bases with Metals

Metals react with acids and bases to give salts and hydrogen gas.

(i) Acid + Metal → Salt + Hydrogen gas

H2SO4 + Zn → ZnSO4 + H2

(ii) Base + Metal → Salt + Hydrogen gas

Reaction of Acids with Metal Carbonates and Metal Hydrogencarbonates

All metal carbonates and hydrogencarbonates react with acids to give a corresponding salt, carbon dioxide and water.

Na2CO3 + HCl(aq) → 2NaCl(aq) + H2O(l) + CO2(g)

NaHCO3 + HCl(aq) → NaCl(aq) + H2O(l) + CO2(g)

Check the carbon dioxide gas by passing through lime water, it turns the lime water milky.

On passing excess carbon dioxide the precipitate disappears

Limestone, chalk and marble are different forms of calcium carbonate.

Reaction between an acid and a base

The reaction between an acid and a base to give a salt and water is known as a neutralization reaction.

Base + Acid → Salt + Water

NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

Reaction of Metallic Oxides with Acids

Metal oxide + Acid → Salt + Water

• Since metallic oxides react with acids to give salts and water, similar to the reaction of a base with an acid, metallic oxides are said to be basic oxides.

Reaction of a Non-metallic Oxide with Base

Non metal oxide + Base → Salt + Water

2NaOH + CO2 → Na2CO3 + H2O

• Since non metallic oxides react with bases to give salts and water, similar to the reaction of an acid with a base, non metallic oxides are said to be acidic oxides.

Amphoteric Oxides

Some metal oxides, such as aluminium oxide, zinc oxide, etc., show both acidic as well as basic behaviour.

Metal oxides which react with both acids as well as bases to produce salts and water, are known as amphoteric oxides.

Al2O3 + 6HCl → 2AlCl3 + 3H2O

ZnO + 2HCl → ZnCl2 + H2O

Acids in Aqueous Solution

Acids give H+ ions in aqueous solutions

HCl → H+(aq) + Cl-(aq)

The separation of H+ ion from HCl molecules cannot occur in the absence of water. The actual reaction is as follows,

HCl + H2O → H3O+ + Cl(aq)

Hydrogen ions cannot exist alone, but they exist after combining with water molecules. Thus hydrogen ions must always be shown as H+ (aq) or hydronium ion (H3O+).

H+(aq) + H2O → H3O+

Bases in Aqueous Solution

Not all bases are soluble in water. Bases which are soluble in water are called alkalis.

Bases which are soluble in water give OH- ions in aqueous solutions,

NaOH(s) Na+(aq) + OH-(aq)

KOH(s) K+(aq) + OH-(aq)

Mg(OH)2(s) Mg2+(aq) + 2OH-(aq)

Neutralization Reaction

The reaction of an acid and a base to give salt and water is called neutralization reaction.

We can now write the neutralization reaction as

Acid + Base → Salt + Water

HX + MOH → MX + HOH

H+ (aq) + OH (aq) → H2O (l)

Dilution of acids

A process in which the concentration of ions (H3O+/OH) per unit volume, decreases, is called dilution and the acid or the base is said to be diluted.

Mixing of an acid or base with water results in decrease in the concentration of ions (H3O+/OH), per unit volume.

Precautions to be taken during dilution of acids or bases

• Dissolving an acid or a base in water is a highly exothermic reaction. Care must be taken while mixing concentrated nitric acid or sulphuric acid with water.

• The acid must always be added slowly to water with constant stirring.

If water is added to a concentrated acid or base, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating.

Strength of Acid or Base Solution – pH

• Acids or bases which dissociate completely in water are called strong acids or bases, respectivly.

• Mineral acids, such as hydrochloric acid, sulphuric acid, nitric acid, etc. dissociate completely to give hydrogen ions and hence they are considered as strong acid.

• Organic acids do not dissociate completely to give hydrogen ions, so they are weak acid.

• Strength of an acid or a base is decided by the concentration of hydrogen or hydronium ions in the solution.

• The scale for measuring hydrogen ion concentration in a solution is called pH scale.

The p in pH stands for ‘potenz’ in German, meaning ‘power’.

pH = - log10 [H+] = - log10 [H3O+]

pH < 7 – Acidic [lower the pH, stronger is the acid]

pH = 7 - Neutral

pH > 7 – Basic [higher the pH, stronger is the base]

Ph Values of Some Common Solutions Found In Nature

 Solution Colour of pH Paper Approximate pH Value Nature of Substance Saliva (before meal) Green 7.4 Slightly Basic Saliva (after meal) Greenish Yellow 5.8 Slightly Acidic Lemon juice Orange Red 2.2 Acidic Colourless aerated drink Greenish Yellow 6.0 Slightly Acidic Carrot juice Greenish Yellow 6.0 Slightly Acidic Coffee Orange Yellow 5.0 Acidic Tomato juice Orange 4.1 Acidic Tap water Green 7.0 Neutral 1M NaOH Violet 14 Basic 1M HCl Dark Red 0 Acidic Blood Green 7.4 Neutral/ Slightly Basic Milk of Magnesia Navy Blue 10 Basic Gastric Juice Red 1.2 Acidic

IMPORTANCE OF pH IN EVERYDAY LIFE

pH and Life

Living organisms can survive only in a narrow range of pH change.

Our body works within the pH range of 7.0 to 7.8.

pH and Acid Rain

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 endangering the aquatic life.

pH and Productivity of Soil

Plants require a specific pH range for their healthy growth. Plants cannot grow if the soil is too acidic or basic.

pH in Our Digestive System

The medium in our stomach is highly acidic and has pH around 1.2. Our stomach produces hydrochloric acid which helps in digestion of food.

Sometimes the stomach produces too much hydrochloric acid. Excess acid in the stomach causes pain and irritation. It can also cause ulcers in the digestive system. The condition of excess acid in the stomach is referred to as hyperacidity.

In order to treat hyperacidity, a patient is advised to take antacid tablets or antacid suspensions. The antacid preparations contain some mild base such as magnesium hydroxide as the active component. This neutralizes the excess acid in the stomach and thus controls the pH in the stomach.

Mg(OH)2 + 2HCl → MgCl2 + 2H2O

pH and Tooth Decay

Tooth enamel is the hardest substance in our body. It is made up of calcium phosphate. It does not dissolve in water or mildly acidic solutions. However, it gets corroded slowly when pH in the mouth is below 5.5.

When we eat food, some food particles remain stuck to the teeth. Bacteria present in the mouth produce acid by the degradation of sugar and food particles present in the mouth. This results in lowering of pH in the mouth which promotes tooth decay.

In order to prevent tooth decay avoid eating sugary foods. The mouth should be cleaned after eating food. Cleaning the teeth using toothpastes also helps in preventing tooth decay.

Toothpastes are generally basic. Therefore, they neutralize the excess acid in the mouth and thus prevent tooth decay.

pH and self defence of animals and plants

Some plants (nettle) and animals (ants) use acids for self defence. For example a bee-sting leaves methanoic acid which causes pain and irritation.

Similarly, stinging hair of nettle leaves inject methanoic acid causing burning pain and rashes.

• Use of a mild base like baking soda on the stung area gives relief.

• Dock leaves also provide relief on the nettle rash.

Other Uses of pH

The knowledge of pH also helps us in maintaining the proper acidity in many products such as shampoos, medicines, etc. pH of rain water can be used to detect the extent of certain pollutant gases in the atmosphere.

Remedy for Nettle

Nettle is a herbaceous plant which grows in the wild. Its leaves have stinging hair, which cause painful stings when touched accidentally, due to the methanoic acid secreted by them.

Rubbing by leaf of the dock plant, which grows by the side of the nettle, relieves the effect. Nature of the dock plant is basic.

Some Natural Substances and Acids In Them

 Natural source Acid Vinegar Acetic Acid Sour Milk Lactic Acid Orange Juice Citric Acid & Ascorbic Acid (Vitamin C) Lemon Citric Acid Tamrind Tartaric Acid Ant sting Methanoic Acid or Formic Acid Tomato Oxalic acid Nettle sting Methanoic Acid or Formic Acid

SALTS

Salts are the ionic compounds which are produced after the neutralization reaction between an acid and a base. Salts are electrically neutral.

Family of Salt

Salts having common acidic or basic radicals are said to belong to same family.

Example - Sodium chloride (NaCl) and Calcium chloride (CaCl2) belong to chloride family.

Calcium chloride (CaCl2) and calcium sulphate (CaSO4) belong to calcium family.

Zinc chloride (ZnCl2) and Zinc sulphate (ZnSO4) belong to zinc family.

Acidic, Basic and Neutral Salts

Depending upon the type of acid and base involved in the neutralization reaction, the salts can be acidic, basic or neutral.

pH value of salts

• Neutral salt, pH ≈ 7

• Acidic salt, pH < 7

• Basic salt, pH > 7

List of Acids and Bases

 Acids Strong Weak Hydrochloric Acid (HCl) Acetic Acid or Ethanoic Acid (CH3COOH) Sulphuric Acid (H2SO4) Formic Acid or Methanoic Acid (HCOOH) Nitric Acid (HNO3) Carbonic Acid (H2CO3) Bases Strong Weak Sodium Hydroxide (NaOH) Ammonium Hydroxide (NH4OH) Potassium Hydroxide (KOH) Magnesium Hydroxide [Mg(OH)2] Ca(OH)2 Sodium Hydrogencarbonate (NaHCO3)

Nature of Salts

 Acid Base Nature of Salt Example Strong Strong Neutral NaCl, KCl Weak Weak Neutral CH3COONH4 (NH4)2CO3 Strong Weak Acidic (NH4)2SO4 NH4 Cl Weak Strong Basic CH3COONa Na2CO3

Common Salt (Sodium Chloride)

Sodium chloride (NaCl) is also known as common or table salt.

It is formed after the reaction between sodium hydroxide and hydrochloric acid.

NaOH + HCl → NaCl + H2O

It is a neutral salt. The pH value of sodium chloride is about 7.

It is prepared by evaporating the water from sea water. The sea water contains many salts dissolved in it. Sodium chloride is separated from these salts by process of crystallization.

Deposits of solid salt are also found in the forms of minerals. These large crystals are often brown due to impurities. This is called rock salt.

• Sodium chloride is used to enhance the taste of food.

• Sodium chloride is used in manufacturing of many chemicals.

Important chemicals from sodium chloride

Sodium Hydroxide (NaOH) - Sodium hydroxide is a strong base. It is also known as caustic soda or lye.

Chlor-Alkali Process

In the process of electrolytic decomposition of brine (aqueous solution of sodium chloride), brine decomposes to form sodium hydroxide.

Chlorine is obtained at anode and hydrogen gas is obtained at cathode as byproducts. This whole process is known as Chlor-Alkali process.

2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g)

Use of products after the electrolysis of brine

• Hydrogen gas is used as fuel, in margarine, in making of ammonia for fertilizer, etc.

• Chlorine gas is used in water treatment, manufacturing of PVC, disinfectants, CFC, pesticides. It is also used in manufacturing of bleaching powder and hydrochloric acid.

• Sodium hydroxide is used for de-greasing of metals, manufacturing of paper, soap, detergents, artificial fibres, bleach, etc.

Bleaching Powder (CaOCl2)

Bleaching powder is also known as chloride of lime. It is a solid and yellowish white in colour. Bleaching powder can be easily identified by the strong smell of chlorine.

Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)2].

Ca(OH)2 + Cl2 → CaOCl2 + H2O

Use of bleaching powder

1. For bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in laundry;

2. As an oxidising agent in many chemical industries; and

3. For disinfecting drinking water to make it free of germs.

Baking soda

The chemical name of the baking soda is sodium hydrogencarbonate (NaHCO3).

It is produced using sodium chloride as one of the raw materials, in Solve’s process.

Sodium hydrogencarbonate is mildly basic in nature and can neutralize an acid.

When it is heated, CO2 is evolved,

Uses of sodium hydrogencarbonate (NaHCO3)

(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 –

NaHCO3 + $\underset{\mathrm{From any acid}}{\underset{⏟}{{\mathrm{H}}^{+}}}$→ CO2 + H2O + Sodium salt of acid

For example if the acid is H2SO4, then,

2NaHCO3 + H2SO4 → 2CO2 + 2H2O + Na2SO4

Carbon dioxide produced during the reaction causes bread or cake to rise making them soft and spongy.

(ii) Sodium hydrogencarbonate is also an ingredient in antacids.

Being alkaline, it neutralizes excess acid in the stomach and provides relief.

(iii) It is also used in soda-acid fire extinguishers.

Washing soda (Na2CO3.10H2O)

Sodium carbonate can be obtained by heating of baking soda and re-crystallization. It is also a basic salt.

Na2CO3 + 10H2O → Na2CO3.10H2O

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

Water of crystallization

Many salts contain water molecules and are known as hydrated salts. When heated, they lose the water molecules to become anhydrous salts.

Water of crystallization is the fixed number of water molecules present in one formula unit of a salt.

Examples:

Copper sulphate pentahydrate (CuSO4.5H2O)

Blue colour of copper sulphate is due to presence of 5 molecules of water of crystallisation. When copper sulphate is heated, it loses water molecules and turns white, which is known as anhydrous copper sulphate.

CuSO4.5H2O CuSO4 + 5H2O

CuSO4 + 5H2O → CuSO4.5H2O

Ferrous sulphate heptahydrate (FeSO4.7H2O)

The green colour of ferrous sulphate heptahydrate; commonly known as ferrous sulphate; is due to the presence of 7 molecules of water in it.

Plaster of Paris (CaSO4.$\mathbit{}\frac{1}{2}\mathbit{}$H2O)

Plaster of Paris is obtained by heating of gypsum, a hydrated salt of calcium.

CaSO4.2H2O (Gypsum) CaSO4 .H2O (PoP) + $\frac{3}{2}$ H2O

Plaster of Paris is a white powder and on mixing with water, it changes to gypsum once again giving a hard solid mass.

CaSO4.H2O (PoP) + $\frac{3}{2}$ H2O → CaSO4.2H2O (Gypsum)

In PoP only half a water molecule is shown to be attached as water of crystallisation. It is written in this form because two formula units of CaSO4 share one molecule of water.

Plaster of Paris is used for making toys, materials for decoration and for making surfaces smooth. Doctors use Plaster of Paris to set the fractured bone.