Question-2. Into how many classes are the bases classified according to their strength?

Ans- According to the strength, alkali is divided into two classes.

• Strong base:- Sodium Hydroxide (NaOH), Potassium Hydroxide (KOH)

• Weak base:- Calcium hydroxide [Ca(OH)₂], Magnesium hydroxide [Mg(OH)₂]

Question-3. Which are called strong acid and weak acid?

Ans-

Strong acid:- Acids which give more number of hydrogen ions (H⁺) when dissolved in water are called strong acids.

Ex:- Hydrochloric acid, sulfuric acid.

Weak acid:- Acids which give less number of hydrogen ions (H⁺) when dissolved in water are called weak acids.

Ex:- Formic Acid, Citric Acid.

Question-4. What is the difference between acid and base?

Ans- Following is the difference between an acid and a base—

Question-5. What are Indicator?

Ans- Indicator:- Indicators are called substances which, through their color change, give information about whether the substance is acidic, basic or neutral, it is called indicator.

Ex-:- Litmus paper, methyl orange, and phenolphthalein are three common indicators.

Question-6. Writer the names of natural indicator and synthetic indicator?

Ans-

Natural Indicator:- Litmus, turmeric, red cabbage, turnip (beet), hydrangea, petunia.

Synthesis Indicator:- Methyl orange, phenolphthalein.

Question-7. What is an odor indicator?

Ans- There are some substances whose odor changes in acidic or alkaline medium, it is called odor indicator.

Ex:- Onion, Vanilla, Oil of clove, etc.

Question-8. You are given three test tubes, one containing distilled water and the other two containing an acidic solution and the other containing an alkaline solution. If you are given only red litmus paper, how will you identify the substances in each test tube?

Ans- We will put red litmus paper in all the three test tubes in turn, the solution which turns red litmus paper blue will be alkaline vivian and the solution which turns blue litmus paper red will be acidic and the rest of the test tube will be distilled water.

Question-9. Why curd or sour substances should not be kept in brass or copper utensils.

Ans- Curd and sour foods contain acid. Acids react with metals to form salts and hydrogen gas, due to which the material kept in the vessel becomes uneatable and the vessel also becomes corrosive. Therefore curd or sour substances should not be kept in brass and copper utensils.

Question-10. What is called dilution?

Ans- When the concentration of ion (H₂O⁺/OH^-) per unit volume decreases when an acid or a base is added to water, this process is called dilution.

Question-11. What is the pH scale called? When are solutions neutral, acidic and basic in the pH scale?

Ans- The scale used to find the concentration of hydrogen ions present in a solution is called pH scale.

• pH=7 solution (neutral)

• pH<7 solution (acidic)

• pH>7 solution (alkaline)

Question-12. Mention two major uses of washing soda and baking soda?

Ans-

# The use of washing soda or washing soda is as follows—

• • To remove permanent hardness of water.

  • In cleaning the houses.

# Following are the uses of baking soda—

• • In making baking powder.

  • In soda-acid fire extinguishers.

Question-13. Why should plaster of paris be stored in a moisture proof container? Explain it.

Ans- Plaster of Paris absorbs water easily and forms hard gypsum. Therefore plaster of paris should be kept in a moisture proof container.

Question-14. Why does an acid not behave acidic in the absence of water?

Ans- Acids simply dissolve in water to produce hydrogen ions. Therefore, hydrogen ions are not formed in the absence of water. Therefore, the behavior of acids in the absence of water is not acidic.

Question-15. Why is distilled water not a conductor of electricity while rain water is?

Ans- Distilled water does not contain any ionic compounds dissolved in it, due to which it does not dissociate into ions whereas rain water contains different types of acidic gases dissolved in it. That is why it dissociates into ions due to which distilled water does not conduct electricity whereas rain water does.

Question-16. What is chlor-alkali process called?

Ans- On passing electricity through an aqueous solution of sodium chloride, it dissociates to give sodium hydroxide. This process is called chlor-base process.

Question-17. What is bleaching powder? What is its chemical formula and what are its uses?

Ans- When chlorine reacts with dry slaked lime [Ca(OH)₂], bleaching powder is formed.

The chemical formula of bleaching powder is CaOCl₂.

# Bleaching powder has the following uses—

• In industries and for bleaching of clean clothes.

• As a reducing agent in chemical industries.

• To make drinking water free from bacteria.

Question-18. What is washing soda? Give its chemical formula and uses.

Ans- Sodium carbonate is obtained by heating baking soda and re-crystallizing it to obtain washing soda. The chemical formula of washing soda is Na₂CO₃•10H₂O.

# It has the following usage—

• • In cleaning the houses.

  • To remove temporary hardness of water.

  • In the soap, glass and paper industries.

Question-19. A cowherd mixes some baking soda in fresh milk.

(a). Why does raising the pH of fresh milk from 6 make it slightly alkaline?

Ans- Milk is acidic in nature, so by adding a little baking soda to milk, it is made alkaline so that the milk can last longer, that is, the milk does not crack.

(b). Why does this milk take longer to turn into curd?

Ans- It takes longer time for alkaline milk to form curd because alkaline milk is acidic first and then curd is formed.

Question-20. What is the water of crystallization and what is the chemical formula of aqueous copper sulphate?

Ans- The fixed number of water molecules in a formula unit of a salt is called water of crystallization. Gypsum is a salt that contains water of crystallization. The chemical formula of aqueous copper sulfate is CuSO₄•H₂O.

Question-21. What is plaster of paris, and what are its uses?

Ans- When gypsum is heated above 373K it gives up water molecules to form calcium sulphate semi hydrate hemihydrate, which is called plaster of paris. The chemical formula of plaster of paris is CaSo₄•½H₂O.

Plaster of Paris has the following uses—

• • in making toys.

  • In making decorations.

  • To make the surface smooth.

  • To fix broken bones in their right place.

Question-22. Why does an aqueous solution of an acid conduct electricity?

Ans- Acids dissolve in water to form ions. Therefore an aqueous solution of an acid conducts electricity.

HCl → H⁺ + Cl^-

Question-23. Why does dry hydrochloric gas not change the color of dry litmus paper?

Ans- H⁺ ions are not produced in dry hydrochloric gas, due to which dry hydrochloric gas does not change the color of dry litmus paper.

Question-24. While diluting an acid, why is it recommended that acid should be added to water and not water to acid?

Ans- The acid should be mixed slowly by adding it to the water. By doing this the amount of heat generated in it is very less. Conversely, if water is mixed with an acid, then the amount of heat generated in it will be very high, due to which it can boil. Which can burn our face and clothes and also break glass utensils. So we should mix acid with water and not water in acid.

Question-25. How is the concentration of hydronium ion (H₃O⁺) affected when an acid is diluted?

उत्तर- When the acid is diluted, the concentration of hydronium (H₃O⁺) ion decreases.

Question-26. How is the concentration of hydroxide ion (OH‾) affected when excess bases are added to sodium hydroxide solution?

Ans- When more bases are added to sodium hydroxide solution, the concentration of hydroxide ion (OH‾) increases.

Question-27. How does the concentration of H⁺ (hydrogen ion) affect the nature of the solution?

Ans- The solution becomes more acidic as the concentration of H⁺ (hydrogen ion) increases.

Question-28. Do basic solutions contain H⁺ (hydrogen ions)? If yes, why are they basic?

Ans- Yes, basic solutions contain H⁺ (hydrogen ions). In this solution, the amount of hydrogen ion is less and the amount of OH‾ (hydroxide ion) is high. Hence they are basic.

Question-29. Under what conditions would a farmer use slaked lime (calcium oxide), quicklime (calcium hydroxide) or chalk (calcium carbonate) in the soil of the field?

Ans- Quicklime, slaked lime and chalk are all three bases. Therefore, when the farmer comes to know that the soil of the field has become acidic, then he puts these three things in the field so that the acidity of the soil of the field is reduced.

Question-30. What is the common name of CaOCl₂ compound?

Ans- The common name of CaOCl₂ compound is bleaching powder.

Question-31. Name the substance which reacts with chlorine to form bleaching powder?

Ans- slaked lime [Ca(OH)₂]

Question-32. Which sodium compound is used to soften hard water?

Ans- Sodium carbonate decahydrate [Na₂Co₃•10H₂O]

Question-33. What will happen on heating a solution of sodium hydrogen carbonate? Write the equation for this reaction?

Ans- On heating a solution of sodium hydrogen carbonate, sodium carbonate, water and carbon-di-oxide gas are formed.

2NaHCO₃ → Na₂Co₃ + H₂O + Co₂

Question-34. Write the equation for the reaction of plaster of paris with water.

Ans- CaSo₄▪½H₂O + ³⁄₂H₂O → CaSo₄•2H₂O

Question-35. The pH value of fresh milk is 6. What will be the change in the value of its pH when curd is formed? Explain your answer.

Ans- Yogurt contains lactic acid i.e. when curd is made from milk, it becomes more acidic. Therefore the value of its pH will be less than 6.