Indicators are some pigments or mixtures of pigments that are used to indicate the presence of acids and bases.
Types of Indicators:
1. Natural Indicators
2. Synthetic indicators
3. Olfactory indicators
Natural indicators: Indicators that are derived from natural sources are called natural indicators. Examples of natural indicators are litmus, turmeric, red cabbage etc.
Litmus: Litmus is a purple pigment derived from a plant called lichen. Litmus paper is available in two colors - blue and red. Blue litmus is used to test acid and red litmus is used to test base.
Turmeric: Another such natural indicator is turmeric. Its color is yellow. Turmeric can only be used for testing bases. Turmeric turns reddish brown on contact with base. This does not change with exposure to acid.
Red Cabbage: The color of the juice of red cabbage leaves is naturally purple. It turns green on contact with acids and green on contact with bases.
Synthetic Indicators : Synthetic Indicators or Artificial Indicators are artificially created in the laboratory. Examples are methyl orange, phenolphthalein etc.
Methyl Orange: This is an orange liquid. It turns red on contact with acid and yellow on contact with base.
Phenolphthalein: This is a colorless liquid. It does not change in acid but it turns pink on contact with base.
Olfactory indicators: Olfactory indicators are those which change their odor in contact with acidic and basic substances. Examples are onions, vanilla, etc.
Onion: It does not change in acid but on contact with base, the smell of onion vanishes.
Vanilla: It does not change in acid but when it comes in contact with base, it becomes odorless.
PROPERTIES OF ACID
1. Reaction between acid and metal
The metal can remove hydrogen from the acid. The metal combine with the remaining part of the acid to form salts. Thus, the reaction of a metal with an acid can be written in the following way.
Acid + metal ⇒ salt + hydrogen
Example: H2SO4 + Zn → ZnSO4 + H2
Test for Hydrogen gas: If the gas from the above reaction is passed through the soap solution, bubbles will form. If a burning candle is held in the path of these gas bubbles , the hydrogen gas will burn with pop sound.
2. Reactions of Acids with Metal Carbonate and Metal Hydrogen Carbonate
Acids react with sodium carbonate (Na2CO3) and sodium hydrogen carbonate (NaHCO3) to produce carbon dioxide gas.
Na2CO3 (s) + 2HCl (aq) → 2NaCl (aq) + H2O (l) + CO2 (g)
NaHCO3 (s) + HCl (aq) → NaCl (aq) + H2O (l) + CO2 (g)
Test for Carbon dioxide: If the resulting gas is allowed to pass through lime water (calcium hydroxide solution), it will turn the lime water milky.
Ca (OH) 2 (aq) + CO2 (g) → CaCO3 (s) + H2O (l)
(Lime water) (white precipitation.)
3. Reaction of acid and base
Acid and base react with each other to form salt and water. This reaction is called neutralization reaction. In general, a neutralization reaction
can be written as –
Acid + Base → Salt + Water.
NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (l)
4. Reaction of metallic oxides with acids
Salt and water are formed by the reaction between a metal oxide and an acid. This reaction can usually be written as:
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.
Copper oxide, for example, reacts with mild hydrochloric acid to produce copper chloride and water.
CuO (s) + HCl (aq) → CuCl2 (aq) + H2O (l)
5. Universal properties of all acids
All acids react with metals to produce hydrogen gas. Therefore, all acids generally contain hydrogen.
In aqueous solution, acids give hydrogen ions, H+ (aq). This is why the acids show the acidic nature.
6. Change of acid in aqueous solution
Acids produce hydrogen ions in the presence of water. In the absence of water, the separation of H+ ions from acid molecules cannot take place.
Hydrogen ions can not exist alone. They always attach themselves with water molecule. This is why hydrogen ions are always shown as H+ (aq) or hydronium ions (H3O+).
Strong Acids and Weak Acids
Acids that break down completely into ions in aqueous solution are strong acids. Sulfuric acid (H2SO4), hydrochloric acid (HCl) etc. are examples of strong acids. On the other hand, acids that are partially broken down into ions in aqueous solution are weak acids. Such as acetic acid (CH3COOH), carbonic acid (H2CO3) etc.
At the same concentration, hydrochloric acid and acetic acid produce different amounts of hydrogen ions. Acids that produce more H+ ions are called strong acids and acids that produce less H+ ions are called mild/weak acids.
PROPERTIES OF BASE
1. Reaction of base with metal
Bases can remove hydrogen from metal by reacting with metal. Thus, the reaction of a metal with an alkali can be written in this way.
Alkali + Metals ⇒ Salt + Hydrogen gas
Eg- 2NaOH + Zn → Na2ZnO2 + H2
2. Reaction of bases with non-metallic oxides
Salt and water are formed when a non-metallic oxide reacts with a base. This reaction can usually be written as:
Non-metallic oxide + acid → salt + water
Calcium hydroxide reacts with carbon dioxide to produce salt and water.
Ca (OH)2 (aq) + CO2 (g) → CaCO3 (aq) + H2O (l)
3. Universal properties of bases
All bases produce hydroxide ions in aqueous solution and this is responsible for the basic property of bases.
NaOH (aq) → Na + (aq) + OH- (aq)
4. Change of alkali in aqueous solution
Bases in aqueous solution give OH- ions.
NaOH (s) --- (Water) → Na+ (aq) + OH- (aq)
Base produce hydroxide (OH-) ions in water. Bases that dissolve in water are called alkali.
STRENGTH OF ACIDS AND BASES
Strength of acids and bases depends on the concentration of H+ or OH- ions in their aqueous solution. The higher the concentration of H+ or OH- ions, the higher the strength of acids and bases.
pH scale: It is very important to use Universal indicator to determine the amount of H+ or OH-ions in a solution. This indicator is a combination of different indicators. The different concentrations of hydrogen ions in the solution show different colors in the universal indicator.
A scale has been invented to measure the concentration hydrogen ions in a solution. This scale is called pH scale. A pH scale can measure the pH from 0 (very acidic) to 14 (very alkaline). pH is just a number indicating the acidic or alkaline character of a liquid. The higher the concentration of hydrogen ions, the lower the pH.
The pH value of a neutral liquid is 7. If the pH is less than 7, the liquid is an acidic. 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.
IMPORTANCE OF pH IN EVERYDAY LIFE
Plants and animals are sensitive to pH. Our body operates within the pH range of 7.0 to 7.8. Living organisms can
survive only in a narrow range of 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. The pH of the soil needs to be adjusted within a certain range for healthy and vigorous growth.
Our digestive system: Our digestive system produces hydrochloric acid in our stomach for the digestion of food. It helps in digestion of our food without harming the stomach. When indigestion occurs, the stomach produces too much acid and this causes bloating and pain in the stomach. People use alkali as antacids to get rid of this disorder. Magnesium hydroxide (milk of magnesia) is a mild base. It is often used as an antacid.
Changes in pH and tooth decay: The enamel part of the tooth is made of calcium phosphate is the hardest substance in the body. It does not dissolve in water. However, if the pH inside the mouth drops below 5.5, it will decay. This is called tooth decay. 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.
Self defence by animals and plants through chemical warfare: Bee-sting leaves an acid
which causes pain and irritation. Use of a mild base like baking soda
on the stung area gives relief. Stinging hair of nettle leaves inject
methanoic acid causing burning pain.
SALTS
Salt is formed by the reaction of acids and bases. Salt produced by reaction between strong acids and strong base are neutral. On the other hand, salts produced by reaction of strong acids and mild base are acidic. Salts produced by reaction of strong base and mild acids are alkaline.
SOME IMPORTANT SUBSTANCES
Sodium hydroxide
Aqueous solution of sodium chloride is called brine. When electricity is passed through 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) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g)
Chlorine gas is given off at the anode, and hydrogen gas at the cathode.
Sodium hydroxide solution is formed near the cathode.
Bleaching powder
Bleaching powder is produced
by the action of chlorine on dry slaked lime [Ca(OH)2]. Chemical formula of bleaching powder
is written as CaOCl2, though the actual composition is quite
complex.
Ca(OH)2 + Cl2 → CaOCl2 + H2O
Uses of Bleaching powder:
Bleaching powder is used –
(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 oxidizing agent in many chemical industries; and
(iii) for disinfecting drinking water to make it free of germs.
Baking Soda
The chemical
name of baking soda is sodium hydrogen carbonate (NaHCO3). It is
produced using sodium chloride as one of the raw materials.
NaCl + H2O
+ CO2 + NH3 → NH4Cl + NaHCO3
The following reaction takes place when it is heated
during cooking –
2NaHCO3 → Na2CO3+H2O+C O2
Uses of baking soda
(i) For making baking powder, which is a mixture of baking soda
(sodium hydrogen carbonate) and a mild edible acid such as
tartaric acid. When baking powder is heated or mixed in water,
the following reaction takes place –
NaHCO3 + H+ → CO2 + H2O + Sodium salt of acid
Carbon dioxide produced during the reaction causes bread or cake
to rise making them soft and spongy.
(ii) Sodium hydrogen carbonate 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
Washing soda (Na2CO3.10H2O) can be obtained from sodium chloride. Sodium
carbonate can be obtained by heating baking soda; recrystallisation of
sodium carbonate gives washing soda. It is also a basic salt.
Na2CO3 + 1OH2O → 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.
Plaster of Paris
On heating gypsum at 373 K, it loses water molecules and becomes
calcium sulphate hemihydrate ( CaSO4.1 /2 H2O). This is called Plaster of
Paris. Plaster of Paris is a white powder
and on mixing with water, it changes to gypsum once again giving a
hard solid mass.
CaSO4.1
/2 H2O + 11/2 H20→ CaSO4. 2H2O
Uses of Plaster of Paris:
(i) Plaster of Paris is used as plaster by doctors for supporting fractured bones in the right position.
(ii) Plaster of Paris is used for making toys, materials for decoration
and for making surfaces smooth.
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