Acids, Bases and Salts Class 10 Notes

An acid is a substance that gives hydrogen ions, H+, when dissolved in water. A base is a substance that gives hydroxide ions, OH-, when dissolved in water. This is the definition the board asks for, and it is why water is named in both halves of it: without water there are no free ions to furnish.The two families behave differently on the litmus test and react differently with metals, carbonates and each other. Every practical in this chapter reduces to one of three tests: the litmus test, the pH test, or the gas evolved with a metal carbonate.

• Acids: sour to taste, turn blue litmus red, and give H+ ions in water. Examples: hydrochloric acid, acetic acid, citric acid, sulphuric acid.
• Bases: bitter to taste, slippery to touch, turn red litmus blue, and give OH- ions in water. Examples: sodium hydroxide, calcium hydroxide, magnesium hydroxide.
• Alkali: the common name for a base that is soluble in water, so NaOH and KOH are alkalis while Cu(OH)₂ is a base but not an alkali.
• Indicator: a substance that changes colour in acid or base to show which is present. Litmus, methyl orange and universal indicator are all indicators.
• The reaction to memorise first is neutralisation, acid plus base gives salt plus water, because every later use of an antacid rests on it.

Acids are not one substance but a family with a shared set of reactions. The student who learns the five general properties knows enough to answer a three-mark question on any acid the board names, whether it is the hydrochloric acid in the stomach or the citric acid in a lemon.The two properties that carry the most marks are the reaction with a metal carbonate, which releases carbon dioxide, and the reaction with a metal, which releases hydrogen. Both are worth writing out in full with the gas identified by name.

• Against indicators: acids turn blue litmus red, and turn methyl orange pink or red.
• With active metals: acids react with metals such as magnesium, zinc and iron to give a salt and hydrogen gas. Zn + 2HCl → ZnCl₂ + H₂.
• With metal carbonates: acids give a salt, water and carbon dioxide gas. This is the test that identifies a carbonate. Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂.
• With bases: acids neutralise bases to give a salt and water, and this is neutralisation.
• With water: most acids are soluble in water, and the concentrated acid releases enough heat to be dangerous, so concentrated acid is always added to water slowly and never the other way round.

Bases share the same three structural habits as acids: they change an indicator, they neutralise an acid, and they react with acid salts. What separates them is the feel and the taste, both of which are unsafe to test in the laboratory and must only ever be written about.The practical base of the chapter is sodium hydroxide, which clears blocked drains and is the base used in the preparation of soap. A base that neutralises the excess acid in the stomach is sold as an antacid, and milk of magnesia, magnesium hydroxide, is the standard example.

• Against indicators: bases turn red litmus blue, and turn methyl orange yellow.
• With acid indicators: a basic solution shows blue or green with universal indicator rather than red or orange.
• Feel and taste: bases feel soapy and slippery and taste bitter, and both are written about, never tested by hand or tongue in the laboratory.
• With acids: bases neutralise acids to form a salt and water, which is how an antacid removes the burning of excess acid in the stomach.
• With ammonium salts: a base such as sodium hydroxide gives ammonia with any ammonium salt on gentle warming, which is the standard test for the NH₄⁺ ion.

The pH scale is the single number that the examiner wants attached to every substance in this chapter. It runs from 0 to 14, with 7 exactly in the middle. Below 7 the solution is acidic, at exactly 7 it is neutral, and above 7 it is basic, and the lower the number the more strongly acidic the solution is.Pure water and a neutral solution at ordinary temperature sit at pH 7. The two extreme ends are not arbitrary: pH 0 corresponds to a solution of strong acid and pH 14 to a solution of strong base. A pH test strip carries a colour chart, and the strip is matched against that chart under daylight, not under a lamp.

• Strongly acidic, pH 1 to 3: hydrochloric acid in the stomach, lemon juice, vinegar, and the acid rain that damages buildings and crops.
• Weakly acidic, pH 4 to 6: an antacid being used, mild acid rain, and black tea.
• Neutral, pH exactly 7: pure water at 25 °C, a solution of common salt in water, and rain water that is not affected by pollution.
• Weakly basic, pH 8 to 10: soap solution, baking soda solution, and sea water.
• Strongly basic, pH 11 to 14: sodium hydroxide solution, drain cleaner, quicklime solution, and soap scum that has blocked a pipe.

The syllabus asks for the importance of pH in daily life, which is a memory question built on a handful of situations the student has already met: the stomach, the soil, the toothpaste, the rain and the litmus-free pH strip in the science laboratory.The two linked ideas that carry the most weight are acid rain and the treatment of soil. Acid rain is rain whose pH is below 5.6, and the standard fix offered for it is a neutralisation, the chalk or lime that is spread on acid soil or mixed into the acid water.

• Digestion: the stomach secretes hydrochloric acid, and pepsin works best in an acidic medium. An antacid such as milk of magnesia neutralises the excess and raises the pH back towards 7.
• Acid rain: rain with a pH below 5.6 dissolves marble, limestone and other building stones, and it damages crops and aquatic life.
• Soil: most plants grow best in soil that is close to neutral, and strongly acidic soil is improved with quicklime or chalk, both of which neutralise it.
• Toothpaste and mouthwash: mildly basic products neutralise the acid produced by bacteria in the mouth and protect the enamel.
• Self-defence in nature: a bee sting and a nettle sting inject an acid, and rubbing with a mild base such as baking soda relieves the sting.

Neutralisation is the reaction of an acid with a base to give a salt and water. It is the joining idea of the chapter, because it links the acid-alkali practical, the antacid, the treatment of acid soil, the factory neutralisation of acidic effluent, and the neutralisation of the acid in the stomach all at once.Strength is the term students get wrong most often. The strength of an acid is the concentration of H+ ions it furnishes, and the strength of a base is the concentration of OH- ions it furnishes. It has nothing to do with how much acid is present, and it is a measure of how completely the acid ionises in water, not of how strong the solution feels.

• The definition: neutralisation is acid plus base, giving salt plus water.
• The equation: NaOH + HCl → NaCl + H₂O, a typical base neutralising a typical acid to give common salt and water.
• Antacids: milk of magnesia, magnesium hydroxide, and aluminium hydroxide gel neutralise the excess hydrochloric acid in the stomach and are therefore themselves bases.
• Strength: the concentration of H+ ions in an acid and of OH- ions in a base, not the amount of the substance.
• Dilution: diluting an acid lowers the H+ concentration so the pH rises, and diluting a base lowers the OH- concentration so the pH falls.

Sodium hydroxide and bleaching powder are the first two of the five compounds the syllabus names, and each is examined for its formula, its one-line preparation equation and its uses. Learn them as a set of three lines rather than as paragraphs, because the marks are awarded line by line.Sodium hydroxide, also called caustic soda, is the base of the chapter and the raw material of soap. Bleaching powder is the oxidising agent of the chapter, and its use in making drinking water safe is the single most quoted application in the board papers.

• Sodium hydroxide, NaOH, a white solid that dissolves readily in water to give a strongly basic solution that turns red litmus blue.
• Its preparation: sodium metal reacts vigorously with water, 2Na + 2H₂O → 2NaOH + H₂↑, and the hydrogen that rises can be lit with a pop.
• Its uses: manufacture of soap, paper and textiles, clearing blocked drains, and as a laboratory base.
• Bleaching powder, written as CaOCl₂, has a pungent smell and strong oxidising and bleaching properties.
• Its preparation: dry chlorine gas is passed over dry slaked lime, Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O, and its uses are sterilising drinking water, bleaching cotton, wool and paper, and working as an oxidising agent.

The last three compounds are asked together in the practical file, where a student prepares a salt, a base and an efflorescent substance and then tests each of them. Baking soda is sodium hydrogen carbonate, washing soda is the decahydrate of sodium carbonate, and plaster of Paris is the half hydrate of calcium sulphate.Each has one thermal or chemical behaviour that carries its own mark. Baking soda decomposes on heating, washing soda loses water of crystallisation in dry air, and plaster of Paris sets hard when mixed with water. These three facts are the reason the compounds matter beyond their formulas.

• Baking soda, NaHCO₃, is a mild white crystalline solid that is non-corrosive, so it is safe to use as an antacid and in the kitchen.
• On heating baking soda decomposes: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂↑, and the carbon dioxide released is what raises a cake.
• Washing soda, Na₂CO₃·10H₂O, is a colourless crystalline solid that dissolves in water and makes the solution alkaline, which is why it is an excellent cleaning agent.
• Washing soda effloresces in dry air, losing water of crystallisation and crumbling to a powder, and it softens hard water for washing.
• Plaster of Paris, CaSO₄·½H₂O, is made by heating gypsum, CaSO₄·2H₂O, at about 373 K, and it sets into a hard solid when mixed with water.