Chapter 6 Science CBSE 7th

Science

Class 7

Chapter 5

Acids, Bases and Salts

Important topics covered in NCERT for Class 7 Science Chapter 5 – Acids, Bases and Salts

  • Acids And Bases
  • Natural Indicators Around Us
  • Neutralisation
  • Neutralisations In Everyday Life
Concept Map Chapter 5
Concept Map

Important Terms

  • Acid : Acids are sour in taste.
  • Acidic : An acidic substance contains an acid.
  • Base : Bases are bitter in taste and soapy to touch.
  • Basic : A basic substance contains a base.
  • Indicator : An indicator is a substance that gives different colours in acidic and basic media.
  • Neutral : A neutral substance has neither acidic nor basic character.
  • Neutralization : In a neutralization reaction, an acid reacts with a base to form salt and water.
  • Salt : The product formed (a new substance) by neutralisation reaction between an acid and a base is called salt.
  • Acid Rain : The rain becomes acidic when carbon dioxide, sulphur dioxide and nitrogen dioxide present in the atmosphere, dissolve in rain drops. These gases when dissolved in rain drops form carbonic acid, sulphuric acid and nitric acid respectively. Acid rain may cause damage to historical monuments, plants and animals.
  • Ant sting : The ant sting contains formic acid which when injected into the skin causes pain and irritation. By applying moist baking soda the effect of the sting can be neutralised.
  • Litmus paper : When indicators are found in the form of strips of paper, then it is known as litmus paper. Red litmus paper and blue litmus paper are two types of litmus paper. The acids change blue litmus paper into red colour and the bases change red litmus paper into blue colour.
  • Common acids : Hydrochloric acid, Nitric acid, sulphuric acid etc.
  • Common bases : Sodium hydroxide, potassium hydroxide, ammonium hydroxide.
  • Role of hydrochloric acid in digestion : Our stomach contains hydrochloric acid which helps in digestion.

Revision Notes

  • Acids are sour in taste. Bases are bitter in taste and soapy to touch.
  • Acid turns blue litmus red, while bases turn red litmus blue.
  • Substances which are neither acidic nor basic, are said to be neutral.
  • Solutions of the substances that show different colours in acidic, basic and neutral mediums are called indicators.
  • Litmus and turmeric are natural indicators.
  • An acid and a base neutralize each other to form a salt. This is known as neutralisation.
  • Heat always evolves in neutralisation, salt and water are also formed in neutralisation.
  • We take antacid in the form of ‘Milk of Magnesia’ to remove the acidity in stomach.
  • Since ants have formic acid in their sting, that is why stinging of ants becomes painful.
  • Since sting of an ant contains formic acid, its pain can be relieved by rubbing moist baking soda which neutralises the formic acid.
  • The acidity in the soil can be eliminated by adding quicklime or slaked line.

NCERT Solution

Question 1: State differences between acids and bases.

AcidsBases
Acids are sour in tasteBases are bitter in taste
Acids turn blue litmus paper into red colorThe base does not change the colour of blue litmus paper
Acids does not change the colour of the red litmusBases turn red litmus paper to blue color
Acids do not change the colour of turmericBases turn turmeric to red

Question.2. Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?

Since window cleaner turns red litmus paper to blue, it is basic in nature.

Question 3: Name the source from which litmus solution is obtained. What is the use of this solution?

Litmus solution is extracted from lichens. Litmus solution is used as an indicator to find acidic and basic nature of a solution.

Question 4: Is the distilled water acidic/basic/neutral? How would you verify it?

Distilled water is neutral in character, i.e. it is neither acidic nor basic. Neutral nature of distilled water can be verified by the use of blue and red litmus paper. Distilled water does not change the colour of either blue or red litmus paper.

Question 5: Describe the process of neutralisation with the help of an example.

When the solution of acid is mixed with the solution of base in proper ratio, both of them neutralize the effect of each other and a new substance (called salt) is formed; along with water. This is called neutralization or neutralization reaction.
Example: When the solution of sodium hydroxide (a base) is mixed with the solution of hydrochloric acid (an acid) in proper ratio, both neutralize each other and the reaction mixture so obtained is neutral in character. In this reaction, a new substance sodium chloride (common salt) is formed. The reaction involved in this can be written as follows:
HCl + NaOH → NaCl + H2O

Question 6: Mark ‘T’ if the statement is true and ‘F’ if it is false:
(i) Nitric acid turn red litmus blue. (T/F)

False

(ii) Sodium hydroxide turns blue litmus red. (T/F)

False

(iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. (T/F)

True

(iv) Indicator is a substance which shows different colours in acidic and basic solutions. (T/F)

True

(v) Tooth decay is caused by the presence of a base. (T/F)

False

Question 7: Dorji has a few bottles of soft drink in his restaurant. But, unfortunately, these are not labelled. He has to serve the drinks on the demand of customers. One customer wants acidic drink, another wants basic and third one wants neutral drink. How will Dorji decide which drink is to be served to whom?

Dorji can taste a few drops out of soft drinks bottles, acidic solution is sour in taste, basic solution is bitter in taste, and the neutral solution has no taste. Along with tasting, Dorji can use litmus paper to test the nature of soft drink. Dorji would dip blue and red litmus paper in the sample of each of the bottles.

  • Sample of solution of bottle which turns blue litmus paper red is acidic.
  • Sample of solution of bottle which turns red litmus paper blue is basic.
  • Sample of solution of bottle which does not change the colour of either blue or red litmus paper is neutral in nature.

After detecting the acidic, basic and neutral nature of soft drink, Dorji would serve the drink to the customers according to their requirement.

Question.8.  Explain why:
(a) An antacid tablet is taken when you suffer from acidity

The antacid tablet contains base like milk of magnesia which neutralises the acid produced in the stomach. Antacid tablet is taken in the case of acidity to neutralize the excess acid produced in the stomach. Antacid tablets contain basic substances and hence neutralize the acid produced in the stomach and give relief from acidity.

(b) Calamine solution is applied on the skin when an ant bites.

Calamine solution, which is zinc carbonate, is a base. In the case of ant bites, ant injects an acid (called formic acid) in the skin which causes pain and irritation. By applying calamine solution, it neutralizes the effect of acid inject in the course of ant bite and gives relief from pain.

(c) Factory waste is neutralised before disposing it into the water bodies.

Most of the factory wastes contain acid. It they are flushed into water as they are, then acid present in them would kill the aquatic organisms, along with creating pollution. Thus, it is necessary to neutralize the factory waste before disposing it into the water bodies.

Question.9. Three liquids are given to you. One is hydrochloric acid, another is sodium hydroxide and third is a sugar solution. How will you identify them? You have only turmeric indicator.

The following steps are taken to test the given liquids:

  • Put a drop of provided liquid on the turmeric indicator. The solution that changes the colour of the indicator to red, is sodium hydroxide, which is basic in nature.
  • Now, to make two mixtures, add a drop of sodium hydroxide on the other two liquids individually.
  • The drop of each combination added to the turmeric indicator one after another.
  • The mixture that changes the indicator to red colour includes a neutral solution of sugar.
  • While the mixture contains hydrochloric acid that has been neutralized by the addition of sodium hydroxide, which does not show any colour change in the indicator.

Question.10. Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.

If blue litmus paper is dipped in a solution and it would remain blue then the solution may be basic or neutral in character.

Question.11. Consider the following statements:
(a) Both acids and bases change colour of all indicators.
(b) If an indicator gives a colour change with an acid, it does not give a change with a base.
(c) If an indicator changes colour with a base, it does not change colour with an acid.
(d) Change of colour in an acid and a base depends on the type of the indicator.
Which of these statements are correct?

(i) All four
(ii) a and d
(iii) b, c and d
(iv) only d

(iv) Only d is correct

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