Acids Bases Salts Class 10

[Colette]

Manyacids and bases occur naturally in nature, such as citric acid in fruits like orange, lemon, etc, tartaric acid in tamarind, malic acid in apples, and lactic acid in milk and milk products, hydrochloric acid in gastric juices.

Similarly, many bases are found such as lime water. We use many of these acids in our day-to-day life, such as vinegar or acetic acid in the kitchen, boric acid for laundry, baking soda for the purpose of cooking, washing soda for cleaning, etc.

There are two basic types of acids organic and inorganic acids. Inorganic acids are sometimes referred to as mineral acids. As a group, organic acids are generally not as strong as inorganic acids. The main difference between the two is the presence of carbon in the compound; inorganic acids do not contain carbon.

Earlier, acids, bases, and salts were characterized by the experimental testing of their aqueous solutions. An acid is defined as a substance whose water solution tastes sour, turns blue litmus red and neutralizes bases. A substance is called base if its aqueous solution tastes bitter, turns red litmus blue or neutralizes acids.

Salt is a neutral substance whose aqueous solution does not affect litmus. According to Faraday: acids, bases, and salts are termed as electrolytes. Further, Liebig proposed that acids are compounds which contain hydrogen that can be replaced by metals.

Acidity is a characteristic property of acids. Acidic substances are usually very sour. Apart from hydrochloric acid, there are many other types of acids around us. Citrus fruits like lemons and oranges contain citric and ascorbic acids while tamarind paste contains tartaric acid.

Bases turn red litmus paper blue while the blue litmus paper stays blue. They taste bitter and also feel soapy. Some other common examples of bases include sodium bicarbonate that is used in cooking and household bleach.

Apart from sodium chloride, other common salts are sodium nitrate, barium sulfate etc. Sodium chloride or common salt is a product of the reaction between the hydrochloric acid (acid) and sodium hydroxide (base). Solid sodium chloride is made of a cluster of positively charged sodium ions and negatively charged chloride ions held together by electrostatic forces.

Electrostatic forces between opposite charges are inversely proportional to the dielectric constant of the medium. In other words, we can say that a compound that has acidity in its nature and a compound that has basicity as its nature, may yield salts when combined together.

The universal solvent, water, has a dielectric constant of 80. Therefore, when sodium chloride is dissolved in water, the dielectric constant of water reduces the electrostatic force, allowing the ions to move freely in the solution. They are also well-separated due to hydration with water molecules.

Dissociation is the separation of ions from an ionic crystal when a solid ionic compound dissolves in water. On the other hand, ionization is the process where a neutral molecule breaks into charged ions when dissolved in a solution. The extent of ionization depends on the strength of the bonds between ions and the extent of solvation of ions.

Bronsted and Lowry in 1923 independently proposed a more general definition of acids and bases. According to them, an acid is defined as any hydrogen-containing material (molecule, anion or cation) which can donate a proton to other substance and a Base is any substance(molecule, cation or anion) that can accept a proton from any other substance. Therefore, acids are proton donor whereas bases are proton acceptor.

The advantage of the Lewis theory is that complements the model of oxidation-reduction reactions. Oxidation-reduction reactions take place on a transfer of electrons from one atom to another, with a net change in the oxidation number of one or more atoms.

The Lewis theory further suggested that acids react with bases and share a pair of electrons but there is no change in the oxidation numbers of any atoms. Either an electron is transferred from one atom to another, or the atoms come together to share a pair of electrons.

These reactions are showing clearly: When Aluminium hydroxide accepts protons, it acts as a base. When it accepts electrons, it acts as an acid. This Lewis acid-base theory also explains why non-metal oxides such as carbon dioxide dissolve in H2O to form acids, such as carbonic acid H2CO3.

Note: In actual practice, the acids dissolve in water to form H+ ion or a proton. As H+ ion or proton cannot exist independently in an aqueous solution, it binds itself with water molecule to form hydronium ion.

Commonly used in car batteries, in the manufacture of fertilizers (Ammonium sulphate, super phosphate) detergents etc, in paints, plastics, drugs, in manufacture of artificial silk, in petroleum refining.

(vii) Hydrocyanic acid

VII On the basis of concentration of acid

The measure of amount of water present in a given sample of acid is called concentration.

Based on the concentration, the acids are classified as follows:

Concentrated acid: A sample of an acid which contains very little or no amount of water is called concentrated acid.

Dilute acid: A sample of an acid which contains far more amount of water than its own weight is called dilute acid.

Note: It must be kept in mind that the concentration of an acid merely tells the measure of water in an acid. It should not be confused with strength of an acid which is determined by the measure of the concentration of H+ (aq) ions in the aqueous solution.

(7) Action with metallic carbonates or metal hydrogen carbonates: All metallic carbonates or metal hydrogen carbonates react with dilute mineral acids to form their respective salts, water and carbon dioxide.

(8) Action with metallic sulphites / metal hydrogen sulphites: All metallic sulphites or metal hydrogen sulphites, react with dilute mineral acids to form their respective salts, water and sulphur dioxide gas.

Triacidic bases: The base whose one molecule reacts with three H+ (aq) ions completely to form salt and water as only products are called triacidic bases.

Examples: Aluminium hydroxide [Al(OH)3], Ferric hydroxide [Fe(OH)3]

Bulb will not glow if glucose (C6H12O6) or ethyl alcohol (C2H6O) solution is kept in the beaker. This means that both of them will not give any ions in solution.

Comparison between properties of Acids and Bases

2.12 Role of water in the ionisation of Acids and Bases

Substances can act as acids and bases only in the presence of water (or) in aqueous solution. In dry state which is also called anhydrous state, these characters cannot be shown. Actually, water helps in the ionization of acids or base by separating the ions. This is also known as dissociation and is explained on the basis of a theory called Arrhenius theory of acids and bases.

In the dry state, hydrochloric acid is known as hydrogen chloride gas i.e. HCl(g). It is not in the position to give any H+ ions. Therefore, the acidic character is not shown. Now, let us pass the gas through water taken in a beaker with the help of glass pipe. H2O molecules are of polar nature which means that they have partial

Applications of Neutralization

(i) People particularly of old age suffer from acidity problems in the stomach which is caused mainly due to release of excessive gastric juices containing HCl. The acidity is neutralized by antacid tablets which contain sodium hydrogen carbonate (baking soda), magnesium hydroxide etc.

III. Methyl Orange

Methyl orange is an orange coloured dye (yellow) and basis in nature. In the acidic medium the colour of indicator becomes red and in the basic or natural medium, it colour remains unchanged.

IV. Red Cabbage Juice

It is purple in colour in natural medium and turns red or pink in the acidic medium. In the basic or alkaline medium, its colour changes to green.

V. Turmeric Juice

It is yellow in colour and remains as such in the neutral and acidic medium. In the basic medium its colour becomes reddish or deep brown.

(b) Properties

(i) It is a white, crystalline substance that forms an alkaline solution with water. The aqueous solution of sodium bicarbonate is neutral to methyl orange but gives pink colour with phenolphthalein orange. (Phenolphthalein and methyl orange are dyes used as acid-base indicators.)

(ii) When heated above 543 K, it is converted into sodium carbonate.

(c) Uses

(i) It is used in the manufacture of baking powder. Baking powder is a mixture of potassium hydrogen tartarate and sodium bicarbonate. During the preparation of bread the evolution of carbon dioxide causes bread the evolution of carbon dioxide causes bread to rise (swell).

(ii) It is largely used in the treatment of acid spillage and in medicine as soda bicarb, which acts as an antacid.

(iii) It is an important chemical in the textile, tanning, paper and ceramic industries.

(iv) It is also used in a particular type of fire extinguishers. The following diagram shows a fire extinguisher that uses NaHCO3 and H2SO4 to produce CO2 gas. The extinguisher consists of a conical metallic container (A) with a nozzle (Z) at one end. A strong solution of NaHCO3 is kept in the container. A glass ampoule (P) containing H2SO4 is attached to a knob (K) and placed inside the NaHCO3 solution. The ampoule can be broken by hitting the knob. As soon as the acid comes in contact with the NaHCO3 solution, CO2 gas is formed. When enough pressure in built up inside the container, CO2 gas rushes out through the nozzle (A). Since CO2 does not support combustion, a small fire can be put out by pointing the nozzle towards the fire. The gas is produced according to the following reaction.

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