Chapter 2: Acids, Bases and Salts
Acids and bases
Acids taste sour, turn blue litmus red and give H+ ions in water (HCl, H2SO4, CH3COOH). Bases taste bitter, feel soapy and turn red litmus blue. A base that dissolves in water is called an alkali, like NaOH and KOH. Litmus, phenolphthalein, methyl orange and turmeric are indicators.
Important reactions
Acid + metal โ salt + hydrogen. Acid + metal carbonate โ salt + CO2 + water. Acid + base โ salt + water (neutralisation). Acid + metal oxide โ salt + water. Base + non-metal oxide โ salt + water.
The pH scale
The pH scale runs from 0 to 14. Below 7 is acidic, 7 is neutral and above 7 is basic. The lower the pH, the stronger the acid. When the pH of the mouth falls below 5.5, tooth enamel starts to dissolve.
Important salts in daily life
Baking soda (NaHCO3): in antacids and baking powder. Washing soda (Na2CO3.10H2O): for washing clothes and softening hard water. Bleaching powder (CaOCl2): for bleaching and disinfecting water. Plaster of Paris (CaSO4.½H2O): for plaster on broken bones and making toys.
Important questions
Q1. Why do acids not show acidic behaviour in the absence of water?
Acids release H+ ions only in water, and acidic properties come from H+ ions. Dry HCl gas does not change the colour of dry blue litmus.
Q2. What is formed when an acid reacts with a metal?
A salt and hydrogen gas are formed. For example, Zn + 2HCl โ ZnCl2 + H2.
Q3. How do we test for hydrogen gas?
When a burning candle is brought near, the gas burns with a pop sound.
Q4. What is a neutralisation reaction? Give an example.
The reaction between an acid and a base that forms a salt and water. For example, NaOH + HCl โ NaCl + H2O.
Q5. What happens when an acid is added to a metal carbonate?
A salt, water and CO2 are formed. For example, Na2CO3 + 2HCl โ 2NaCl + H2O + CO2.
Q6. What does CO2 do to lime water?
Lime water turns milky because CaCO3 forms: Ca(OH)2 + CO2 โ CaCO3 + H2O.
Q7. What is the pH scale?
A scale from 0 to 14 that shows how acidic or basic a solution is, based on its hydrogen ion concentration.
Q8. What kind of solution has a pH of 7?
A neutral solution, like pure water.
Q9. The pH of a solution is 3. Is it acidic or basic?
Acidic, because the pH is less than 7.
Q10. Why does tooth decay start when the pH of the mouth falls below 5.5?
At such a low pH the acid dissolves the tooth enamel (calcium phosphate). A basic toothpaste neutralises this acid.
Q11. Why do we take an antacid for acidity in the stomach?
An antacid (such as milk of magnesia) is basic and neutralises the extra acid in the stomach.
Q12. Give the formula and uses of baking soda.
Formula: NaHCO3. Uses: in antacids and in baking powder, which makes cakes rise.
Q13. How is baking soda prepared?
NaCl + H2O + CO2 + NH3 โ NH4Cl + NaHCO3
Q14. What is washing soda and what are its uses?
Formula: Na2CO3.10H2O. It is made by recrystallising the sodium carbonate obtained by heating baking soda. Uses: washing clothes, glass, soap and paper industries, and removing hardness of water.
Q15. How is bleaching powder made and where is it used?
Formula: CaOCl2. Ca(OH)2 + Cl2 โ CaOCl2 + H2O. Uses: bleaching cloth and paper, and disinfecting drinking water.
Q16. How is Plaster of Paris made?
Gypsum (CaSO4.2H2O) is heated at 373 K to form CaSO4.1/2H2O (Plaster of Paris).
Q17. Why is Plaster of Paris stored away from moisture?
On meeting water it sets into hard gypsum, after which it can no longer be used.
Q18. What is an indicator? Give two examples.
A substance that shows different colours in acidic and basic solutions. For example, litmus and phenolphthalein.
Q19. What colour does phenolphthalein show in a base?
Pink. In an acid it stays colourless.
Q20. Why should curd and sour substances not be kept in brass or copper vessels?
The acids in them can react with the metal and form poisonous salts, which makes the food spoiled and harmful.
Quiz
1. What is the pH of a neutral solution?
pH 7 is neutral, like pure water.
2. Which colour does phenolphthalein show in a base?
It turns pink in a base.
3. Which gas is released when an acid reacts with a metal carbonate?
CO2 is released, which turns lime water milky.
4. Which medicine helps when there is excess acid in the stomach?
An antacid is basic and neutralises the excess acid.
5. What is the formula of baking soda?
Baking soda is sodium hydrogencarbonate (NaHCO3).