Acids, Bases and Salts - Class 7 Science Chapter 5, NCERT Solutions and Notes


Class 7 Science Chapter 5 - Acids, Bases and Salts

NCERT Solutions - NCERT Answers - NCERT Notes
Important Terms (Definitions)  
Metals: Metals are generally lustrous solids, malleable, ductile and good conductors of heat and electricity.
Non-metals: Non-metals are generally non lustrous brittle and poor conductor of heat and electricity.
Acids: These are the compounds formed by the reaction of acidic oxide with water and have sour taste.
Mineral Acids: Acids which are capable of forming hydrogen ions when dissolved in water are mineral acids. Such types of acids are also called inorganic acidsmfor example, HCL H2SO4 HNO3 etc.
Strong Acids: These are the acids which are almost completely ionized in aqueous solution. Examples of strong acids - HCL, HNO3, H2SO4.
Weak Acids: Acids which are weakly ionized are called weak acids. For example CH3COOH, (COOH)2
Base: Substances whose molecules have hydroxide (OH-) ions which are set free in aqueous solution are termed as 'Base'. In short a 'Base' is a substance that contains hydroxyl group.
Dilute Acid: A dilute acid is one which contains only a small proportion of solute.
Concentrated Acid: A concentrated acid solution is one which contains high proportion of solute.
Salt: The product formed due to the reaction between an acid and a base.
Alkalies (Alkali): The Hydroxides of metals, which dissolve in water, are known as alkalies.
Indicator: The substances which give different colours with acid and base are called indicators. For example - Litmus, Methyl oranges, Phenolphthalein.
Corrosion: The process that takes place when metals and alloys that undergoes chemical reaction because of the presence of oxygen, water or acid in their immediate environment is known as corrosion. Corrosion is a slow process.
Neutralisation: It's a term for the chemical reaction that takes place between acid and base to form salt and water.
Strong Base: A base which is almost completely ionized in aqueous solution is called a strong base. For example: NaOH, KOH.
Weak Base: Those bases which are weakly or partially ionized are termed as weak bases. For example: Baking soda solution, NH4OH.
Water of Crystallisation: The crystals of some salts contain some water molecules associated with them. These water molecules are known as 'Water of Crystallisation'.
Slaked Lime: Slaked Lime is the other name of calcium hydroxide, Ca(OH)2. Calcium oxide is considered a strong base, because it is gets ionized almost completely.
Neutral Salt: The salt formed when a strong acid is neutralised by a strong base, is called Neutral Salt.
Deliquescent: Some salts have tendency to absorb water from atmosphere (air) and thereby change into a solution. Such substances are called deliquescent. image
Following are the answers of Class 7 Science NCERT Textbook Exercise Questions. For extra or additional important questions answers please refer to our CBSE Guide (CBSE Notes)  
Question 1: State the differences between acids and bases.
1. Acids are sour to taste.
2. Acids turn blue litmus red.
3. Acids contain hydrogen ion.
1. Bases are bitter to taste.
2. They turn red litmus blue.
3. They contain hydroxyl ion.

Question 2: Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is nature?
Answer: The nature of ammonia is basic.
Question 3: Name the source from which litmus solution is obtained. What is the use of this solution?
Answer: Litmus solution is obtained from lichens. Litmus solution is prepared by dissolving a natural dye which is extracted from lichens into distilled water.
It is used as an indicator to distinguish between acids and bases.
Question 4: Is the distilled water acidic / basic / neutral? How would you verify it?
Answer: Distilled water is neutral. This can be verified through litmus test as it will not change the colour of the litmus paper.
Question 5: Describe the process of neutralisation with the help of an example.
Answer: Neutralisation is the term used for the reaction between an acid and a base to form salt and water. It is an exothermic reaction where heat is evolved.
An ideal example of neutralisation is - taking antacids like Milk of Magnesia or Magnesium Hydroxide, Baking Soda (Sodium Bicarbonate) etc. which contains a base are used for reducing the acidity in stomach. It reduces acidity through neutralisation when excess acid present in the stomach gets neutralised.     
Question 6: Mark "T" if the statement is true and "F" if it's false.
(a) Nitric acid turns red litmus blue.
(b) Sodium hydroxide turns blue litmus red.
(c) Sodium hydroxide and Hydrochloric acid neutralise each other and form salt and water. 
(d) Indicator is a substance which shows different colours in acidic and basic solutions.
(e) Tooth decay is caused by the presence of a base.
Answer: (a)F. (b)F. (c)T. (d)T. (e)F.  
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?
Answer: Dorji can decide his drinks with the help of litmus paper:
a. the drink which will turn a red litmus paper blue would be basic.
b. the drink which will turn a blue litmus paper red would be basic and,  
c. the drink which remains unaffected by either red or blue litmus is neutral.
Question 8: Explain why -
(a) An Antacid tablet is taken when you suffer from acidity.
(b) Calamine solution is applied on the skin when an ant bites.
(c) Factory waste is neutralised before disposing it into the water bodies.
(a) Antacid tablets like Magnesium Hydroxide which contains a base are taken for reducing the acidity in stomach. It reduces acidity problems by neutralising the excess acid present in the stomach.
(b) When ant bites it injects an acidic liquid (formic acid) into the skin. This acid causes inflammation to the skin. The effect of the sting can be reduced or neutralised by applying calamine solution to that place. This happens because of the neutralisation reaction as calamine contains zinc carbonate which is a weak base and causes no harm to the skin.
(c) Factory wastes mostly contain acids. If these are allowed to go direct into the water bodies, the acids will kill the fish and other living things present in the water. Hence, factory waste is neutralised before disposing it into threw water bodies.        
Question 9: Three liquids are given to you. One is hydrochloric acid, another is sodium hydroxide and third is sugar solution. How will you identify them? You have only turmeric indicator.
Effect on turmeric indicator
1. Hydrochloric Acid
2. Sodium Hydroxide
3. Sugar solution
Yellow to blue
Yellow to red
No change

Question 10: Blue litmus paper is dipped in a solution. It remains blue. What is the n nature of the solution? Explain.
Answer: As the colour of the blue litmus paper remains unaffected hence, the solution must be basic.   
Question 11: Consider the following statements and tell which of these statements are correct -
(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 change colour with an acid.
(d) Change of colour in an acid and a base depends on the type of the indicator.
Answer: (a) and (d) are true.
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