Introduction to Acid & Base Class 8 science chapter 12 Exercise

Important points to remember :

  1. Acids are soluble in water and they are corrosive in nature. Animals and plants also possess acids in them.
  2. Acids present in food stuffs are called natural acids or organic acids. These acids being weak in nature are called weak acids. 
  3. Some acids are strong in nature. If you taste dilute solution of baking soda, you will find it astringent/bitter. 
  4. Substances having astringent/bitter taste and slippery to touch e.g. Lime water [(CaOH)2], baking soda (NaHCO3) caustic soda (NaOH) and soap are called bases. 
  5. Bases are completely different from acids. The substances which change their colours in presence of acid or base are called ‘Indicators’. Hydrochloric acid is present in our stomach. It helps to ease digestion process. However excessive acid leads to indigestion. 
  6. The acidity of soil increases due to excessive use of chemical fertilizers. Under the guidance of agriculturist lime stone or lime water is mixed in the acidic soil. Such alkaline substances neutralizes excess acid present in the soil.

Question 2: Write chemical names from given formulae. H2SO4,   Ca(OH)2,    HCl,  NaOH,  KOH, NH4OH
Answer :
H2SO4 = Hydrogen sulphate
Ca(OH)2 = Calcium hydroxide
HCl = Hydrogen chloride
NaOH = Sodium hydroxide
KOH = Potassium hydroxide
NH4OH = Ammonium hydroxide

Question 3: Sulphuric acid has highest importance in chemical industry. Why?
Answer :

  1. Sulfuric acid is one of the most important industrial chemicals. More of it is made each year than is made of any other manufactured chemical.
  2. It has widely varied uses and plays some part in the production of nearly all manufactured goods. 
  3. The major use of sulfuric acid is in the production of fertilizers, e.g., superphosphate of lime and ammonium sulfate. 
  4. It is widely used in the manufacture of chemicals, e.g., in making hydrochloric acid, nitric acid, sulfate salts, synthetic detergents, dyes and pigments, explosives, and drugs. 
  5. It is used in petroleum refining to wash impurities out of gasoline and other refinery products. Sulfuric acid is used in processing metals, e.g., in pickling (cleaning) iron and steel before plating them with tin or zinc. 
  6. Rayon is made with sulfuric acid. It serves as the electrolyte in the lead-acid storage battery commonly used in motor vehicles (acid for this use, containing about 33% H 2SO 4 and with specific gravity about 1.25, is often called battery acid).

Question 4: Give answers
a. Which acid is used for getting chloride salt ?
Answer: HCl that is Hydrochloric Acid is used to get chloride salt

b. By squizzing lemon on a piece of rock the gas liberated turned lime water milky. which compound is present in the rock ?
Answer : When lemon is squeezed onto the rock, a gas is liberated. This gas turned the lime water milky. This confirms that Carbon dioxide is present in the rock. This is because insoluble calcium carbonate that is CaCO3 is formed.

c. The label on the bottle of chemical is spoiled. How will you find whether the chemical is acidic or not ?  
Answer : You can find the nature of that chemical by using Acid-Base indicators like litmus, Phenolphthalein, China rose extract , etc. If the stored chemical is acid then the blue litmus will turn into red and if the chemical was basis in nature then the red litmus turn into blue color.

Question 5: Answer the following questions.

a.Explain the difference between acid and alkali.
Answer : Acids and alkalis both contain ions. Acids contain lots of hydrogen ions, which have the symbol H+. Alkalis contain lots of hydroxide ions, symbol OH-. Water is neutral because the number of hydrogen ions is equal to the number of hydroxide ions.

Differences between acids, alkali, and bases:

  1. Acids are chemicals that have a pH of less than 7.0, while bases are substances that have a pH of more than 7.0. Acids can also be thought of as chemicals that donate protons, while bases are chemicals that accept protons. 
  2. An acid can also be thought of as a chemical that can neutralize a base. Similarly, a base can neutralize an acid.
  3. Acids turn litmus paper red, while bases make litmus paper turn blue.
  4. .Some examples of acids are sulfuric acid, hydrochloric acid, nitric acid, and so on. Some examples of bases are sodium hydroxide, potassium hydroxide, and so on.

b. Why indicator does not get affected by salt?
Answer : Indicators are substances which gives a prominent sign to identify the presence of acids and alkali in the solution. Salt is produced from the reaction of acids and bases. So, salts are considered as neutral. For this reason, Indicators are not affected by salt as salt is the product of acid and base which is neutral.

c. Which substances are produced by neutralization process?
Answer : Salt and acid are produced.
When an acid and a base react, the reaction is called a neutralization reaction. In a neutralization reaction, the acid will produce H+ ions that react to neutralize the OH- ions produced by the base, forming neutral water. Water is always one product, and a salt is also produced.

d.Which are the industrial uses of adids ?
Answer : Use of acids
1. Acids are used in the production of chemical fertilizers.
2. Acids are used in the production of explosives, oil purification, medicines, dyes and paints.
3. Hydrochloric acid is used for the preparation of different types of chloride salts.
4. Dil. H2SO4 acid is used in the batteries.
(electric cell)
5. Dil. HCl is used for sterilization of water.
6. Acid is used for making of white paper from wood pulp

Question 6: Select proper word given in bracket and fill in the blanks.
a.Main constituent of acid is H+ ion.

b.Main constituent of alkali is OH- ion.

c.Tartaric acid is a weak acid.

Question 7: Match the pairs. 
  Group A Group B
1. Tamarind a. Acetic acid
2. Curd b. Citric acid
3. Lemon c. Tartaric acid
4.Vinegar d. Lactic acid
Answer :
1. Tamarind – a. Tartaric acid
2. Curd – b. Lactic acid
3. Lemon – c. Citric acid
4. Vinegar –  d. Acetic acid

Question 8: State true of false

a. Oxides of metals are alkaline in nature = True

b. Salt is acidic = False

c. Metal corrodes due to salts = True

d. Salts are neutral = True

Question 9: Classify following substances into acidicm basic and neutral group-
HCl,   NaCl,  MgO,  KCl,  H2SO4,  HNO3,  H2O,  Na2CO3
Answer :
HCl = Acidic
NaCl = Neutral
MgO = Basic
KCl = Neutral
H2SO4 = Acidic
HNO3 = Acidic
H2O = Neutral
Na2CO3 = Basic

#Can you Tell
1. We consume many food items in our daily life e.g. lemon, tamarind, 
tomato, sugar, vinegar, salt etc. Do all items taste similar?
Answer : No

2. Write the taste of lemon, sugar, curd, lime, baking soda, amla, 
tamarind, raw mango, pomegranate, water (sour, astringent, sweet, 
bitter, tasteless)
Answer : The taste of lemon is sour taste because lemon contains citric acid so it gives a sour taste to lemons.

• Sugar gives sweetness taste because sugar contains glucose and fructose.

• Baking soda gives a little bit of salt and bitter taste because basically, it contains sodium hydrogen carbonate.

• Curd, Amla, Tamarind, Lime, Raw mango these are all given a sour taste.

• when milk converted to curd the lactobacillus bacteria converts the lactose into lactic acid that’s why curd contains sour taste.

• Pomegranate also gives sour taste but it will depend on the ripeness of the seeds.

• The water itself has no taste. Water is tasteless, colorless and odorless.

3. What happens when sour substances like lime juice, tamarind water falls on 
shahabad stones or kitchen platform? 
Answer : tamarind & lime juice contains acid which when dropped on kitchen platforms [which are mostly marble] & shahabad stone will react with it. it can leave white spot which is because the marble or stone get decomposed and it leaves salt as result.

4. Collect soil samples from your 
surroundings and find out whether it is 
acidic, alkaline or neutral? 
Answer : The samples of soil are usually collected from the surroundings. Now, the soil is checked whether it is neutral, alkaline or acidic in nature. After analyzing the soil, we can see that the soil is slightly acidic in nature. So, the average pH of this particular soil is normally 5/6 so it is acidic.

5. Which substances are used to clean 
greenish stains on copper vessels and to 
shine blackish silver utensils?
Answer : Lemon juice, salt, vinegar and tamarind are used to clean green stains on copper vessels and to shine black is silver utensils.

4. Why tooth-paste is used for brushing 
Answer : Toothpaste is used to promote oral hygiene: it is an abrasive that aids in removing dental plaque and food from the teeth, assists in suppressing halitosis, and delivers active ingredients (most commonly fluoride) to help prevent tooth decay (dental caries) and gum disease (gingivitis).

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