ACIDS AND BASES | SPEED NOTES | SCIENCE | CBSE | CLASS 7

Chapter – 5 Acids And Bases There are three typesof Substances: Acids, Bases and Salts Acids: Acids aresour in taste. They are corrosive in nature. A concentrated acidcuts through clothes and eats awaythe wool. Ifit falls onthe skin, itcan cause burns. They are goodconductors of electricity, as they allowthe passage of electric current through them.

Types ofAcids: (i) Mineral Acids: These are acidsprepared from minerals present in theearth’s crust. (ii) Organic Acids: These are acids produced by plants and animals (except hydrochloric acid). (iii) Weak Acids: These do not dissociate completely in solution. Example: tartaric acid, lactic acid. (iv) Strong Acids: These dissociate completely in solution. Example: nitric acid, sulphuric acid. Neutralization: It is the reaction between an acidand a basewhich results in formation of saltand water. Acid+ Base Salt+ Water Example: HCl + NaOH NaCl + H2O Neutralisation inEveryday Life: (i) Indigestion: Too muchacid in stomach causes indigestion. Itis neutralized bytaking an antacid like milk ofmagnesia. (ii) Ant sting: Whenan ant bites, it injects formic acid into theskin. The effect is neutralized by rubbing moistbaking soda (sodium hydrogen carbonate) or calamine (containing zinc carbonate).

(iii) Soil treatment: When the soilis too acidic, it is neutralized by treating with quicklime (calcium oxide) or slaked lime(calcium hydroxide). Bases: Bases are bitter in taste andsoapy to touch. Types ofBases: (i) Weak Bases: These naturally produce lesshydroxide ions insolution. Example: magnesium hydroxide, ammonium hydroxide. (ii) Strong Bases: These produce more number of hydroxide ionson dissolving inwater. Example: Sodium hydroxide(NaOH), Potassium hydroxide (KOH) Substances which areneither acidic norbasic are called neutral. An acid and a base neutralise each other and form a salt. A salt may be acidic, basic or neutral innature. Solutions ofsubstances that showdifferent colour in acidic, basicand neutral solutions are called indicators. Indicators: It is special chemical that changes its colour to indicate the presence of a chemical substance. It is usedto confirm thepresence of anacid, a baseor a neutral solution. Classification ofIndicators: Natural Indicators: (i) Litmus: It is extracted fromlichens. It is available in the formof strips of paper orin the formof a solution.· Acid turns bluelitmus red. Bases turn red litmus blue. (ii) Turmeric: It remains yellow in neutral and acidic solutions but turns redin alkaline solutions. (iii) China rose:It turns acidicsolutions to darkpink (magenta) and basic solution to green. (iv)Red cabbage: It turns acidic solutions tored and basic solutions to blue. Other Indicators: (i) Methyl Orange: It gives pinkish red colour withacidic solutions and yellow colourwith bases. (ii) Phenolphthalein: Itis an acid-base indicator. It iscolourless in acidic solutions but turns pinkin alkali solutions.