Q.1. What is an acid?
Ans. An acid is a hydrogen-containing chemical compound which, when dissolved in water, gives hydrogen ion (H+) or hydrated hydrogen ion (H2O.H+) or hydronium ion (H3O+).
Acids are sour in taste.
HCL ⇌ H+ + Cl-
ion (H+) or hydrated hydrogen ion (H2O.H+) or hydronium ion (H3O+)
Q.2. What are bases and alkalies?
Ans. Bases ionise to give OH– ions in aqueous solution. Bases may be soluble or insoluble in water. The soluble bases are called alkalies. Thus, all alkalies are bases but all bases are not alkalies. Bases have bitter taste and soapy touch.
Examples: NaOH and Cu (OH)2 both are bases, but, since NaOH is soluble in water, it is an alkali. On the other hand, since Cu (OH)2 is insoluble in water, it is not an alkali. Other examples of alkalies are KOH, Ca (OH)2 and NH4OH.
Q.3. Define pH.
Ans. pH is the concept of measuring the concentration of hydrogen ions (H+ (aq)) in a particular solution., The p in pH stands for 'potenz' in German, meaning power. On the pH scale we can measure pH from "0" (very acidic) to 14 (very alkaline).
Q.4. What are the practical applications of neutralisation reactions?
(i) Being alkaline in nature, cold milk is used to neutralise the acidity produced by HCl present in the gastric juice in the stomach.
(ii) Astronauts in space ships use this reaction to neutralise the dangerous levels of CO2.
(iii) Farmers add slaked lime (calcium hydroxide) to reduce acidity of soil.
(iv) Sting of ants and bees contains formic acid. This can be neutralised by rubbing soap, which contains free sodium hydroxide.
(v) Persons suffering from acidity are given antacid tablets, containing magnesium hydroxide which neutralises excess HCl produced, in stomach. Alternately, they are advised to sip cold milk, which neutralises HCl.
Q.5. Why the salts solutions of strong acid and strong alkali are neutral?
Ans. Let us take the example of potassium sulphate, which is a salt of strong acid [sulphuric acid] and strong base [potassium hydroxide solution].
From the above equation, it is clear that water is always feebly ionised and hence solution of potassium sulphate is neutral in nature.
Q.6. What is an universal indicator?
Ans. Universal indicators are usually mixtures of several indicators. Universal indicator is a solution, which undergoes several colour changes over a wide range of pH. The colour is used to ‘indicate’ pH directly.
Q.7. Why common indicators cannot determine pH value of a solution?
Ans. Common indicators, such as litmus, methyl orange and phenolphthalein can easily tell us whether a solution is acidic or alkaline, but they cannot tell us about how much a given acidic solution is stronger than another acidic solution. It means they cannot tell us the pH value of different acidic or alkaline solutions.
In case of litmus, we cannot tell about pH values of 5,6 and 7. Similarly, in case of methyl orange pH values between 3 and 8 will not be very accurate.
Q.8. What are the general characteristics of acids?
Ans. They have a sour taste
They turn blue litmus to red and methyl orange to red.
They react with active metals to give hydrogen
They decompose carbonates to produced carbon dioxide and water.
They turn blue litmus to red and metals to give hydrogen
Zn + H2SO4 → ZnSO4 + H2
They react with bases to form salt and water (Neutralization reaction)
HCL + NaOH → NaCL + H2O
They decompose carbonates to produced carbon dioxide and water
Na2CO3 + 2HCL → 2NaCL + H2O + CO2.
Q.9. What is called deliquescent? Give examples.
Ans. Compounds that take up enough water from the air to dissolve in the water they have taken up are called deliquescent. Calcium chloride (CaCl2) and Sodium hydroxide (NaOH) are the examples of deliquescent.
Q.10. Write the uses of chlorines.
(i) Chlorine is used in the production of bleaching powder.
(ii) Chlorine is used for making solvents for dry cleaning.
(iii) Chlorine is used to sterilise drinking water supply, and the water in swimming pools.
(iv) It is used in the production of hydrochloric acid.