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NCERT Class 7 Science Chapter 5 Acids, Bases and Salts
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Acids, Bases and Salts
Chapter: 5
NCERT TEXT BOOK EXERCISES
Q. 1. State differences between acids and bases.
Ans:
| Acids | Bases |
| 1. Acids have sour taste. | 1. Bases have bitter taste. |
| 2. Acids are not soapy to touch. | 2. Bases feel soapy to touch. |
| 3. Acids turn blue litmus to red. | 3. Bases turn red litmus to blue. |
Q. 2. Ammonia is found in many household products, such as window cleaners. It turns red litmus blue, what is its nature?
Ans: Basic.
Q. 3. Name the source from which litmus solution is obtained. What is the use of this solution?
Ans: Litmus solution is extracted from lichens. It is used to determine the given solution is acidic or basic.
Q. 4. Is the distilled water acidic/basic/ neutral? How would you verify it?
Ans: The distilled water is neutral. Put a drop of the distilled water on a strip of the red litmus paper with the help of a dropper. We observe that colour of the litmus paper is not change.
The solutions which do not change the color of either red or blue litmus are known as neutral solutions. These substances are neither acidic nor basic.
Q. 5. Describe the process of neutralization with the help of an example.
Ans: The process due to which an acid completely reacts with a base to form salt and water as the only products is called neutralization.
Acid + Base → Salt + Water
Example: Hydrochloric acid (HCI) + Sodium hydroxide (NaOH)
Sodium chloride (NaCl)+ Water (H₂O)
Q. 6. Make T if the statement is true and ‘F” if it is false.
(i) Nitric acid turns red litmus blue.
Ans: F.
(ii) Sodium hydroxide turns blue litmus red.
Ans: F.
(iii) Sodium hydroxide and hydrochloric acid neutralize each other and form salt and water.
Ans: T.
(iv) Indicator is a substance which shows different colors in acidic and basic solutions.
Ans: T.
Q.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?
Ans: Dorji can decide with the help of litmus paper:
(i) The drink which would turn a red litmus blue would be basic.
(ii) If the drink turns a blue litmus to red would be acidic.
(iii) The drink which would not affect both red and blue litmus would be neutral.
Q. 8. Example, why?
(a) An antacid tablet is taken when you suffer from acidity.
Ans: (a) Our stomach contains hydrochloric acid. It helps us to digest food. But too much of acid in the stomach causes indigestion.
Sometimes indigestion is painful. To relieve indigestion, we take an antacid such as milk of magnesia, which contains magnesium hydroxide. It neutralizes the effect of excessive acid.
(b) Calamine solution is applied on the skin when an ant bites.
Ans: The sting of an ant contains formic acid. When an ant bites, it injects the acidic liquid into the skin. The effect of the sting can be neutralized by rubbing moist baking soda (sodium hydrogen carbonate) or calamine solution, which contains zinc carbonate.
(c) Factory waste is neutralized before disposing it into the water bodies.
Ans: The wastes of many factories contain acids. If they are allowed to flow into the water bodies, the acids will kill fish and other organisms. The factory wastes are, therefore, neutralized by adding basic substances.
Q. 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.
Ans:
| Name of the liquid | Effect on turmeric paper |
| (i) Hydrochloric acid | Yellow to blue |
| (ii) Sodium hydroxide | Yellow to red |
| (iii) Sugar solution | No change |
Q. 10. Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.
Ans: The nature of the solution is neutral, because the solutions which do not change the color of either red or blue litmus are known as neutral solution. These substances are neither acidic nor basic.
Q. 11. Consider the following statements:
(a) Acids and bases both change colour of all indicators.
(b) If an indicator gives a color 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 b.
(iii) b and c.
(iv) only d.
Ans: (ii) a and b.
ADDITIONAL IMPORTANT QUESTIONS
Very Short Type Questions Answer
Q. 1. Name two natural indicators.
Ans: Litmus and turmeric.
Q. 2. Name two synthetic indicators which are used to test for acids and bases.
Ans: Methyl orange and phenolphthalein.
Q. 3. What is the colour of litmus solution when it is neither acidic nor basic?
Ans: Purple.
Q. 4. Name the acid present in bee-sting that causes pain.
Ans: Methanoic acid.
Q. 5. Name the acid present in sour milk or curd.
Ans: Lactic acid.
Q. 6. Name the acid present in oranges or lemon.
Ans: Citric acid.
Q. 7. What is meant by a neutralization reaction?
Ans: The reaction between an acid and a base to form salt and water is called neutralization.
Q. 8. What color do the following indicators turn when added to an acid?
(a) Litmus solution.
Ans: Red.
(b) China Rose indicator.
Ans: Magenta (Deep pink).
Q. 9. What color do the following indicators turn when added to a base?
(a) Litmus solution.
Ans: Blue.
(b) China Rose indicator.
Ans: Green.
Q. 10. (a) Name one indicator which turns red on adding an acid.
Ans: Litmus.
(b) Name one indicator which turns red on adding a base.
Ans: Turmeric paper.
Q. 11. What can you say about the nature of a solution which turns blue litmus paper red?
Ans: Acidic.
Q. 12. Why does a turmeric stain on white shirt is turned to red when it is washed with soap?
Ans: It is because the soap solution is basic.
Q. 13. Name a substance which is used to neutralize the effect of excessive acid in stomach.
Ans: Milk of magnesia which contains magnesium hydroxide (base).
Q. 14. Name the salt formed when hydrochloric acid neutralizes sodium hydroxide solution.
Ans: Sodium chloride.
Q. 15. What is the color of litmus in a solution of ammonium hydroxide?
Ans: Red litmus turns black in solution of ammonium hydroxide.
Short Type Questions Answer
Q. 1. How can you test the presence of an acid in any substance?
Ans: Acid can be tested in any substance by the following methods:
(i) Take a drop of the dilute solution of the substance on the tips of your tongue. If tastes sour, it is acidic.
(ii) Dip a blue litmus paper in the solution. If the colour of the litmus paper turns red, the solution is acidic.
Q. 2. Why should-curd and sour substances not be kept in brass and copper vessels?
Ans: Both curd and sour substances contain some acids in them. They react with copper or brass vessels to form certain salts which are of poisonous nature. Therefore, it is not advisable to keep them in these containers.
Q. 3. Consider the following salts:
Sodium sulfate, Sodium carbonate, Ammonium sulfate.
The aqueous s solution of which of these salts will:
(a) turn blue litmus to red.
Ans: Ammonium sulfate solution because it is acidic in nature.
(b) turn phenolphthalein to pink.
Ans: Sodium carbonate solution because it is basic in nature.
(c) turn china rose indicator to green
Ans: Sodium carbonate solution because it is basic in nature.
(d) have no effect on any indicator.
Ans: Sodium sulfate solution because it is neutral in nature.
Q. 4. Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.
Ans: The solution is either basic or neutral. This is because basic and neutral solutions do not change the colour of blue litmus paper (only acids turn blue litmus paper to red).
Q.5. You have been provided with three test tubes. One of these test-tubes contains distilled water and the other two contain an acidic and a basic solution respectively. If you are given only blue litmus paper, how will you identify the contents of each test- tube?
Ans: Acidic solution will turn blue litmus red. Thus red litmus will turn blue in basic solution. Distilled water will have no effect in any type of litmus paper.
Q. 6. What is an ant bite? What is its effect?
Ans: The sting of an ant contains formic acid. When an ant bites, it infects the acidic liquid into the skin. It causes irritation and burning effect on skin. To relieve sting should be rubbed by moist baking soda or calamine which are basic in nature.
Q. 7. Explain why farmers add slaked lime to acidic soils.
Ans: The bases such as slaked lime neutralize the excess acid present in the soil and reduce its acidic nature.
Q. 8. A person is suffering from indigestion due to the intake of hot spicy food. What remedy will you prescribe to the patient? Give the name of a chemical that can give relief to him.
Ans: The patient should take antacids. Antacid reacts with excess acid in the stomach and neutralize it. This gives relief to the person concerned. The two common antacids are: Magnesium hydroxide (Milk of magnesia) and sodium hydrogen carbonate ((Baking soda).
Q. 9. A milkman adds a very small amount of baking soda to fresh milk. Why does this milk take a longer time to set as curd?
Ans: Fresh milk is made slightly alkaline by adding a little of baking soda. The slightly alkaline milk makes a longer time to set into curd because the lactic acid being formed during curdling has to first neutralize the alkali in it. present.
Q.10. The soil in a field is highly acidic. Name two materials which can be added to this soil to reduce its acidity. Give reason for your choice.
Ans: The two materials which can be added to a highly soil to reduce its acidity lime (calcium oxide) and slaked lime. Quick lime and slaked lime are bases which will neutralise are the excess acid present in the soil and reduce its acidity.
Short Type Questions Answer
Q. 1. A white shirt has a yellow stain of curry. When soap is rubbed on this shirt during washing, the yellow stain turns red. On rinsing the shirt with plenty of water, the red stain turns yellow again.
(a) Name the natural indicator present in curry stain.
Ans: Turmeric. Soap solution.
(b) What is the nature of soap (acidic or basic) as shown by the indicator present in curry stain?
Ans: Basic.
(c) Name a synthetic indicator which will give pink color with.
Ans: Phenolphthalein.
Q. 2. What happens during a honey-bee sting? What is its remedy?
Ans: When a honey-bee stings a person, it injects an acidic liquid into the skin which causes immense pain and irritation. The remedy is to rub a mild base like baking soda on the stung area. Being a base, baking soda neutralizes the acidic liquid injected by the bee-sting and gives relief.
Q. 3. What is a neutralization reaction? Illustrate with two examples.
Ans: The reaction between an acid and a base to form salt and water is called a neutralization reaction. That is:
Acid + Base ― Salt + Water:
The examples of neutralization reactions are as follows:
(i) Sodium hydroxide reacts with sulphuric acid to form sodium sulfate and water.
(ii) Magnesium hydroxide reacts with hydrochloric acid to form magnesium chloride and water.
Q. 4. Write any three properties of an acid.
Ans: (i) Acids have a sour taste.
(ii) Acids turn blue litmus red.
(iii) Acids react with bases (or alkalis) to form salt and water.
Q. 5. Write any three properties of a base.
Ans: (i) Bases have bitter taste.
(ii) Bases feel soapy to touch.
(iii) Bases turn red litmus to blue.
Long Type Questions Answer
Q. 1. Write five uses of sulphuric acid.
Ans: (i) It is used as a dehydrating agent.
(ii) It is used in the manufacture of fertilizers such as ammonium sulfate and superfate.
(iii) It is used in the manufacture of celluloid, artificial silk, paints, plastics, drugs, detergents, etc.
(iv) It is used in petroleum refining.
(v) It is used in textile, paper and leather industries.
Q. 2. How will you treat the following:
(i) high acidity in the stomach.
Ans: We generally take antacid tablet or gel. The antacid tablets or gels contain aluminum hydroxide or magnesium hydroxide. Aluminum hydroxide and magnesium hydroxide are weekly basic in nature. Therefore antacids neutralize the excess acid present in the stomach and provide some relief.
(ii) pain caused by a bee sting.
Ans: Pain caused by a bee sting: When an ant or bee stings, formic acid gets injected into the skin. This acid causes irritation and swelling.
To get relief from the irritation and swelling, baking soda paste is rubbed at the sting site.
Baking soda is basic and therefore neutralizes the formic acid left behind after the sting.
(iii) too much acidic soil?
Ans: If the soil is too acidic, it can be corrected by adding quicklime (calcium oxide) or slaked lime (calcium hydroxide). Both quick time and slaked lime are basic and thus neutralize the excess acid of the soil.
Q. 3. Mention two uses each of the following:
(i) Ammonium hydroxide.
Ans: Uses of ammonium hydroxide:
(a) To remove grease from window panes.
(b) To remove ink spots from clothes.
(ii) Hydrochloric acid.
Ans: Uses of hydrochloric acid:
(a) It is used for the purification of common salt.
(b) It is used for cleaning of metals, before soldering, tinning and galvanisation.
(iii) Magnesium hydroxide.
Ans: Uses of magnesium hydroxide:
(a) It is used in making antacid formula- tions.
(b) It is used as a reagent in the chemistry laboratory.
HOTS QUESTIONS
Q.1. If someone is suffering from the problem of acidity after overeating, which of the following would you suggest as remedy?
Lemon juice, Vinegar. Baking soda solution: Give reason for your choice.
Ans: Baking soda solution. Being basic, it neutralizes excess acid in the stomach.
Q. 2. While diluting an aeld, why is it recommended that the acid should be added to water and not water to the acid?
Ans: Dilution of concentrated acid with water is a highly exothermic process. The concentrated acid must always be added slowly to water with constant stirring. If water is added to a concentrated acid, then the heat generated is so larger that it many cause the mixture to splash out and cause burns.
Q.3. Tooth enamel is one of the hardest substances in our body. How does it undergo damage due to eating chocolates and sweets? What should we do to prevent it?
Ans: Bacteria present in the mouth produce acids by degradation of sugar (from chocolates and sweets) and food particles remaining in the mouth after eating. Due to this, the enamel gets corroded.
Prevention of tooth decay:
(i) Cleaning mouth after eating food.
(ii) Using tooth particles which are generally basic as they can neutralize the excess acid and prevent tooth decay.
SKILL BASED QUESTIONS
Q.1. (a) Draw a diagram to show the preparation of greeting card.
Ans:
(b) What is the effect of soap solution on turmeric paste?
Ans: The soap solution turns turmeric paste into red color.
Q. 2. Draw a diagram to show the china rose and indicators formed from it.
Ans:
FORMATIVE ASSESSMENT
1. Fill in the following blanks with suitable words:
(i) Excessive use of chemical fertilizers make the soil _______.
Ans: Acidic.
(ii) When an acid is mixed with a base, a ________ reaction takes place.
Ans: Neutralization.
(iii) The products formed in a neutralization reaction of an acid and a base are ______ and __________.
Ans: Salt, water.
(iv) The acidic or basic nature of a substance is tested by using an _______.
Ans: Indicator.
(v) The substance which show different colors in acidic, basic and neutral solutions are called ________.
Ans: Indicators.
(vi) Acetic acid is present in ________ whereas citrus fruits contain _________.
Ans: Vinegar, citric acid.
(vii) The bases turn _______ litmus_____.
Ans: Red, blue.
(viii) The acids turn ______ color of red cabbage juice to ________.
Ans: Purple, red.
(ix) The indicator that gives green color with a base is _________.
Ans: China rose.
(x) The reaction between an acid and a base to form a salt and water is called _________ reaction.
Ans: Neutralization.
2. State whether the following statements are true or false:
(i) Nitric acid turns red litmus blue.
Ans: False.
(ii) Sodium hydroxide turns blue litmus red.
Ans: False.
(iii ) Tooth decay is caused by the presence of a base.
Ans: False.
(iv) Change of color in acid or base depends on the type of indicator used.
Ans: True.
(v) Indicator is a substance which shows different colour in acidic and basic solutions.
Ans: True.
(vi) Acids turn blue litmus red.
Ans: True.
(vii) An acidic solution gives green color with china rose indicator.
Ans: False.
(viii) Battery acid is dilute sulphuric acid.
Ans: True.
(ix) Antacids are acidic substance.
Ans: False.
(x) A salt solution that turns red-cabbage juice to red is basic in nature.
Ans: False.
3. Match the following:
| Column – A | Column – B |
| 1. Baking soda | (a) Neutralizes acidic soil |
| 2. Calcium hydroxide | (b) Neutralizes acidity in the stomach |
| 3. Organic matter. | (c) Reliefs insect stings |
| 4. Antacid. | (d) Neutralizes basic soil |
Ans:
| Column – A | Column – B |
| 1. Baking soda | (c) Reliefs insect stings |
| 2. Calcium hydroxide | (a) Neutralizes acidic soil |
| 3. Organic matter. | (d) Neutralizes basic soil |
| 4. Antacid. | (b) Neutralizes acidity in the stomach |
4. Write one word for the following:
(i) Acids obtained from minerals.
Ans: Mineral acids.
(ii) The base which dissolve in water.
Ans: Alkalis.
(ii) A base used in white washing.
Ans: Slaked lime.
(iv) A substance that changes its color when added to an acid or a base.
Ans: Indicator.
(v) A natural indicator which is pink in neutral solution and changes to green in basic solution.
Ans: China rose.
(vi) A reaction between an acid and a base to form salt and water.
Ans: Neutralisation.
(vii) A class of substances which have a sour taste and turn blue litmus red.
Ans: Acid.
(viii) A class of substances which have a bitter taste and turn red litmus blue.
Ans: Base.
(ix) A new substance obtained by neutralization of an acid with a base.
Ans: Salt.
(x) Acids present in fruits and vegetables.
Ans: Organic acids.
5. Multiple Choice Questions:
(i) Acids:
(a) give substances a sour taste
(b) give substances a bitter taste
(c) give substances a sweet taste
(d) none of the above
Ans: (a) Give substances a sour taste.
(ii) Bases are bitter to taste and are also:
(a) rough to touch
(b) soapy to touch
(c) sour in taste
(d) used as indicators
Ans: (b) Soapy to touch.
(iii) An example of a natural indicator is:
(a) litmus
(b) methyl orange
(c) phenolphthalein
(d) All of the above
Ans: (a) Litmus.
(iv) Baking soda turns:
(a) turmeric to red
(b) purple-cabbage juice to green
(c) red litmus to blue
(d) all of the above
Ans: (d) All of the above.
(v) Substances that are produced when acids react with bases are:
(a) salts
(b) indicators
(c) litmus
(d) antacid
Ans: (a) Salts.
(vi) Which of the following is not an acid- base indicator?
(a) litmus
(b) methyl orange
(c) phenolphthalein
(d) sulphur
Ans: (d) Sulphur.
(vii) Which of the following is acidic in nature?
(a) lime juice
(b) human blood
(c) lime water
(d) antacid.
Ans: (a) Lime juice.
(viii) An aqueous solution turns red litmus solution blue. Excess addition of which of the following solution would reverse the change?
(a) Baking powder
(b) Lime
(c) Ammonium hydroxide solution
(d) Hydrochloric acid.
Ans: (d) Hydrochloric acid.
(ix) To protect tooth decay, we are advised to brush our teeth regularly. The nature of the tooth paste commonly used is:
(a) acidic
(b) neutral
(c) basic
(d) corrosive
Ans: (c) Basic.
(x) Which of the following is not a mineral acid?
(a) Hydrochloric acid
(b) Citric acid
(c) Sulphuric acid
(d) Nitric acid
Ans: (b) Citric acid.
