Acids And Bases Online Test 8th Science Lesson 19 Questions in English

Explanation

In our daily life we come across different food substances. Some substances like fruits, tamarind, graphs, curd and lemon are sour. They are said to be acidic.

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Some substances like sodium bicarbonate and green tea are bitter in taste. They are said to be basic.

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Acids and bases are one of the important chemical compounds which play a significant role in every field of science. Acids and bases are everywhere right from the soap used for shower to the vinegar present in the kitchen. Acids and bases react with each other and also with water. As a result they are important biologically, industrially and environmentally. For example, among the medicines we use, aspirin is acidic and antacids are basic. Similarly, many biological molecules are also either acids or bases. Dietary fats are acids and the chemical compounds in DNA are bases.

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The term acid is derived from the Latin word ‘acidus’ which means sour. Thus, the chemical compounds which have sour taste are called acids generally.

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All acids contain one or more replaceable hydrogen atoms in their molecules and when dissolved in water they release H+ ions.

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Hydrochloric acid (HCl), Sulphuric acid (H2SO4) and Nitric acid (HNO3) release hydrogen ions (H+) when dissolved in water.

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Swedish chemist Svante Arrhenius proposed a theory on acids. According to him, an acid is a substance which furnishes H+ ions or H3O+ ions in aqueous solution.

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Acids are defined as the chemical substances which release hydrogen ions when dissolved in water. Acids can be classified into organic acids and inorganic acids depending on the sources.

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Some acids occur naturally in fruits and vegetables. These are called organic acids. Examples: Citric acid, tartaric acid etc.

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Organic acids and their sources

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On the other hand, man produces acids artificially in industries. These acids are called mineral acids or inorganic acids. Examples: Hydrochloric acid (HCl), Sulphuric acid (H2SO4), Nitric acid (HNO3) etc., There are many more classifications of acids.

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Physical properties of Acids:  Acids are sour in taste, they are corrosive in nature. Acids are colorless. They are soluble in water.

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Strong acids can spoil substances like human skin, clothes and paper. Solutions of acids conduct electricity.

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Generally acids exist in liquid state but few acids exist in solid state too. E.g. Benzoic acid

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Acids change the color of the indicators. Blue litmus paper turns red and methyl orange turns pink when treated with acids.

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Metals like zinc, magnesium, aluminum, iron etc., react with acids like hydrochloric acid, Sulphuric acid to form metal salts and release hydrogen gas.

Metal + Dilute acids → Metal salt + Hydrogen

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Copper or brass cooking vessels are coated with tin metal (eyam). If it is not coated the organic acids present in the food materials will react with copper and make the food poisonous. The tin isolates the vessel from the action of acids and prevents food poisoning.

Explanation

When carbonates and bicarbonates come into contact with dilute acids carbon dioxide is given out along with water. For example limestone (calcium carbonate) reacts with dilute Sulphuric acid to form calcium sulphate, carbon dioxide and water.

Calcium carbonate + dil Sulphuric acid → Calcium sulphate + Carbon dioxide + Water

CaCO₃ + H₂SO₄ → CaSO₄ + CO₂ + H₂O

Explanation

Oxides of various metals react with dilute acids to form their metallic salts and water.

Metal oxides + dilute Acid → Metal salts + Water

Explanation

Uses of Acids

• Hydrochloric acid present in our stomach helps in the digestion of foodstuff.
• Vinegar (acetic acid) is used to preserve food materials.
• Benzoic acid is also used to preserve food materials like pickles.
• Sodium or potassium salts of higher fatty acids are used to make washing and bathing soaps.

Explanation

Sulphuric acid is called the king of chemicals. It is used in various industries to make detergents, paints, fertilizers and many more chemicals.

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Sulphuric acid is an effective dehydrating agent. It is used in various industries to make detergents, paints, fertilizers and many more chemicals.

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Hydrochloric acid, Nitric acid and Sulphuric acid are important laboratory reagents.

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Cells of all living organisms contain the fundamental nuclear material called nucleic acids. Animals have deoxy ribonucleic acid (DNA) whereas plants contain ribonucleic acid (RNA).

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Pickles remain in good condition for long time because they contain vinegar (acetic acid) or benzoic acid.

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Soaps are slippery due to the presence of ‘base’. Bases are chemical substances that are corrosive and bitter in taste. A lot of bleaches, soaps, detergents, kinds of toothpaste, etc., are bases. In contrast to acids which release hydrogen ions in water, bases release hydroxide ions in water.

Explanation

The chemical substances that release hydroxide ions when dissolved in water are called as bases. Examples: Sodium hydroxide (NaOH) and Potassium hydroxide (KOH).

Sodium hydroxide → Sodium ion + Hydroxide ion

NaOH → Na+ + OH–

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Water soluble bases are called Alkalis. Bases like sodium hydroxide, potassium hydroxide, calcium hydroxide and ammonium hydroxide are highly soluble in water and hence they are called alkalis.

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Certain chemical substances which do not release hydroxide ions when dissolved in water also behave as bases. Examples: Sodium carbonate, Sodium bicarbonate, Calcium carbonate etc.

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Common bases in some products

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Sodium carbonate (Na₂CO₃) is commercially called washing soda. Similarly sodium bicarbonate (NaHCO₃) is commercially called baking soda. Caustic soda is sodium hydroxide (NaOH) and caustic potash is potassium hydroxide (KOH).

Explanation

Properties of Bases: Bases generally exist in solid state but some bases exist in liquid state also. E.g. Ammonium hydroxide, calcium hydroxide

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Physical properties of Base: Bases give soapy touch only in aqueous media not in dry nature. Bases are bitter in taste. Bases are corrosive in nature. When come in contact with the skin frequently they form painful blisters. Bases are generally colorless. Bases also conduct electricity in aqueous solution.

Explanation

Bases also change the color of the indicators. Red litmus paper turns blue when treated with bases. Similarly, they turn methyl orange yellow and phenolphthalein pink.

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Reaction with metals: Generally metals do not react with bases. All bases react with non- metallic oxides to form salt and water.

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Reaction with metals: Generally metals do not react with bases. Metals like Aluminium and Zinc react with bases like sodium hydroxide forming aluminates and release hydrogen.

Aluminum + Sodium hydroxide + Water → Sodium aluminate + Hydrogen

2Al + 2NaOH + 2H₂O → 2NaAlO₂+ 3H₂

Explanation

Reaction with metal oxides: All bases react with non-metallic oxides to form salt and water. For example sodium hydroxide reacts with carbon dioxide to form sodium carbonate.

Sodium hydroxide + Carbon dioxide → Sodium carbonate + Water

2NaOH + CO₂ → Na₂CO₃ + H₂O

Explanation

Bases react with ammonium salts to form metal salts, ammonia gas and water.

Sodium hydroxide + Ammonium chloride → Sodium chloride + Ammonia + Water

NH₄Cl + NaOH → NaCl + NH₃ + H₂O

Explanation

Though acids and bases have some unique properties there are certain similarities between them. Some of them are given below.

• They are corrosive in nature.
• They undergo ionization in aqueous solution.
• They conduct electricity in aqueous solution.
• They undergo neutralization reaction

Explanation

Difference between acids and bases

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Uses of Bases: Potassium hydroxide is used to make bathing soaps. Sodium hydroxide is used to make washing soaps. Sodium hydroxide is also used in paper industries, textile industries and in the preparation of medicines. Calcium hydroxide is used for white washing.

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Aluminum hydroxide and magnesium hydroxides are used in antacids to cure acidity problems.

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Ammonium hydroxide is used to manufacture fertilizers, nylon, plastics and rubber.

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When neutrality is achieved between two different chemical substances with different chemical properties through a reaction then it is called neutralization in chemistry. Thus neutralization is a chemical reaction in which an acid and a base react with each other to form salt and water. Neutralization reaction between an acid and a base can be written as: Acid + Base → Salt + Water

Explanation

Salts produced by neutralization

Explanation

Ant bite: Whenever bees or red ants bite they inject an acid called formic acid. These acids cause burning sensation and pain. To suppress the pain a suitable base in the form of calcium hydroxide (readily available at home) is applied so as to neutralize the formic acid.

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When we are bitten by wasp, we feel the burning sensation and pain. It is due to an alkaline substance injected by the insect. To neutralize the alkalinity we use vinegar which is an acid.

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Generally it is advised by the doctors that we should brush our teeth twice a day. This is because the bacteria present in our mouth decompose the food particles stuck in the gaps between our teeth thereby causing acid formation which leads to tooth decay. To prevent this we have to neutralize the acid. When we brush with tooth powder or tooth paste containing weak bases the acid gets neutralized. So our teeth will be strong and healthy.

Explanation

As we know, hydrochloric acid present in our stomach helps the digestion of food material along with the enzymes secreted by liver, gallbladder and pancreases. Sometimes due to excessive production of hydrochloric acid in our stomach we feel burning sensation in food pipe and in chest area. If this happens again and again ulcer will be formed in stomach and food pipe, which further aggravates the conditions. In order to neutralize, antacids which are nothing but weak bases like aluminum and magnesium hydroxides are used. As a result the acidity is removed.

Explanation

Effluents from the industries contain acids such as Sulphuric acid. It is treated by adding lime to neutralize it before it is discharged into rivers and streams. Power stations treat this acidic gas using powdered lime (CaO) or limestone (CaCO3) to neutralize it so that air pollutant can be prevented.

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In power stations fossil fuels such as coal are burnt to produce electricity. Burning fossil fuels will liberate sulphur dioxide gas as an acidic pollutant in the air.

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An indicator or acid–base indicator is a chemical substance which indicates the acidic or basic nature of a solution by suitable color change. These may be natural or synthetic.

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An indicator or acid–base indicator is a chemical substance which indicates the acidic or basic nature of a solution by suitable color change. These may be natural or synthetic.

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Natural indicators are chemical substances which are obtained from the natural resources. Litmus, turmeric juice, China rose petals, red cabbage, grape juice and beetroot juice are the indicators obtained from natural resources.

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Turmeric indicator: By adding small amount of water to turmeric powder a paste is prepared. This is applied on a blotting paper or filter paper and dried. These strips are used as indicators to find the nature of the solution. In acidic solution turmeric indicator paper has no change in color. That means it remains yellow. In basic solution the color changes from yellow to red.

Explanation

Some hibiscus flowers soaked in warm water for about 5 to 10 minutes forms a solution. This solution can be used as indicator. In acidic solution, the color will be changed to deep pink or deep red. In basic solution the color will be changed into a green.

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An indicator prepared from artificial substances is known as synthetic indicators. Phenolphthalein and methyl orange are the examples for synthetic indicators.

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Phenolphthalein is a colorless compound. Its alcoholic solution is used as an indicator. It is colorless in acidic solution but turns pink in basic solution.

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Solid methyl orange is dissolved in hot water and its filtrate is used as an indicator. It turns red in acidic solution and yellow in basic solution.

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Color Changing Indicators