Class 7, Science, Chapter:- 5 Acids, Bases and Salts Notes, NCERT Solutions & Frequently Asked Questions

Science

Class:-7, Chapter:- 5

Acids, Bases and Salts 

Notes, NCERT Solutions & Frequently Asked Questions

-- Notes --

⭐ Acids:- An acid is a chemical substance that has a sour taste.

• Many food items such as lemons, curd, vinegar and orange taste sour because of the presence of acid in them.

• Acidic Substances are the substances that contain acid in them.

• Natural Acids are the acids that occur in nature, for example, acids found in fruits are natural acids.

⭐ Bases:- A base is a chemical substance that has a bitter taste and a soapy texture.

• Bases are found in different substances such as bleach, ammonia, washing powder and soap.

• Bases are also called Alkaline.

• Basic Substances are the substances that contain a base in them.

Figure 1.- Acids and Bases found in Nature

⭐Neutral Substance:- These are any substance which is neither acidic nor basic in nature.

⭐ Indicators:- We cannot taste every object and find its nature. Therefore, we use indicators.

• An indicator is a substance that can determine if another substance is acidic or basic in nature.

• The indicators indicate the presence of an acid or base in a substance by changing their colour. 

• For Example Turmeric, China rose petals and Litmus are some natural indicators. Natural indicators the indicators that occur in nature.

⭐ Litmus:- Litmus is a natural indicator which is obtained from Lichens.

• Litmus is available in a solution form and paper strips (red litmus and blue litmus paper).

Figure 2.- Litmus test

Figure 3.- Testing Solutions with Litmus Paper

⭐ Turmeric as an indicator:- Turmeric is generally mixed with water to form a paste which is then put on blotting paper and dried to form thin strips of turmeric paper.

• The turmeric paper is then put into the solutions in order to determine their acidity or alkaline nature.

• Sometimes turmeric solution is also used as an indicator.

Figure 4.- Testing Substances with Turmeric Solution

⭐ China Rose:- China Rose petals are kept in warm water and a coloured solution is obtained from that. 

• This coloured solution is used as an indicator to test other substances.

Figure 5.- Using China rose as an Indicator

Figure 6.- Testing solutions with China rose 

⭐ Acid Rain:- When the rainwater has increased amounts of acids in it, it is called Acid Rain.

• The acid rain is formed because of the presence of air pollutants such as Nitrogen dioxide, Carbon dioxide and Sulphur dioxide in the air.

• These pollutants mix with the rainwater and form acids such as Nitric acid, Sulphuric acid and Carbonic acid respectively.

• The acid rain in severely affect the vegetation, animal life and even buildings of the region where it falls.

Figure 7.- Acid Rain formation

⭐ pH scale:- The measure of acidity or basic nature of a substance can be determined by its pH value.

• The pH value range from 1 to 14 with 1 being the most acidic substance and 14 being the most basic substance while 7 is a neutral substance.

• The pH value is generally determined by using pH strips or solutions

Figure 8.- pH scale showing different colors

Note:- 

Acids are never stored in metal containers. They are rather stored in glass containers. This is so because acids are generally reactive in nature. If we keep them in metal containers they may react with the metal and erode them. Glass, on the other hand, does not react with acids at all.

⭐ Neutralization:- Neutralization is a process or a chemical reaction in which an acidic and basic substance is mixed with each other in order to neutralize their acidic and alkaline nature.

• The product that is formed after the neutralization process is called a Salt.

• The salt can have basic, acidic or neutral nature.

• The neutralization process results in the generation of heat which raises the temperature of the reacting mixture.

• A synthetic indicator often used for testing neutralization reactions is Phenolphthalein solution. It is pink in color.

• When an acid is added to Phenolphthalein solution, the solution turns colorless, indicating the presence of an acid.

• When a base is added to Phenolphthalein solution, the solution retains its pink color, indicating the presence of a base. 

Figure 9.- Neutralization using Phenolphthalein

Figure 10.- Formation of Salt

⭐ Neutralization in Everyday Life:- 

1.- Indigestion:- Our stomach produces hydrochloric acid which helps in the digestion of food.

• But sometimes the stomach releases too much of acid which leads to indigestion or sometimes hyperacidity.

• Hence, we need to neutralize this acid by taking substances that are basic in nature commonly known as antacids.

• For Example, milk of magnesia is a basic substance that can neutralize the acid of the stomach. 

Figure 11.- Indigestion caused in stomach

2.- Ant Bite:- The irritation of the skin due to ant bite is caused because of the presence of formic acid that the ant injects into the skin while biting.

• Hence we use a basic substance to neutralize the effect.

• For Example, baking soda or hydrogen carbonate, calamine solution or zinc carbonate are generally used to treat ant bites

3.- Soil Treatment:- Plants need a soil which is neutral in nature but using chemical fertilizers on soil can turn it into acidic.

• To treat acidic soil we use quicklime (calcium oxide) or slaked lime (calcium hydroxide).

• Basic soil can be treated by adding organic substances to it as they release acids while decomposing into the soil. 

Figure 12.- Soil treatment

4.- Factory Wastes:- The factory waste is acidic in nature and cannot be directly dumped anywhere. Hence bases are added to it before it falls off into a river or stream so that the aquatic life does not get affected.

-- NCERT Solutions -- 

Question 1.- State differences between acids and bases.

Solution:-

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

Solution:- The answer is it is basic in nature.

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

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?

Solution:- Distilled water is neutral in nature, and this can be tested by using red and blue litmus paper. In either of the cases, colour remains unchanged.

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

Solution:- Neutralisation is a reaction between an acid and a base. Here, both acids and bases get neutralised. For example, when sodium hydroxide (NaOH) is added to hydrochloric acid (HCl), sodium chloride (NaCl) and water (H₂O) are obtained.

NaOH + HCl → NaCl + H₂O + Heat

Question 6.- Mark ‘T’ if the statement is true and ‘F’ if it is false.

i) Nitric acid turns red litmus blue.

ii) Sodium hydroxide turns blue litmus red.

iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water.

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

v) Tooth decay is caused by the presence of a base.

Solution:-

i) False

ii) False

iii) True

iv) True

v) False

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

Solution:- Dorji can taste a few drops out of soft drinks bottles; the acidic solution is sour in taste, the 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 the soft drinks. He should use blue litmus paper to test the acidic solution. Dorji has to put a drop of solution on blue litmus. If it turns red, then the solution will be acidic in nature.

Similarly, he can use red litmus paper to test the basic solution. He has to put a drop of solution on red litmus. If it turns blue, then the solution will be basic in nature.

Question 8.- Explain why

a) An antacid tablet is taken when you suffer from acidity.

Solution:- The antacid tablet contains base-like milk of magnesia, which neutralises the acid produced in the stomach. Hence, it is used while suffering from acidity.

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

Solution:- When ant bites, it injects formic acid inside the skin. Calamine consists of Zinc carbonate which is basic in nature. Hence, calamine neutralises the effect of formic acid to bring relief for the affected person.

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

Solution:- Factory wastes are acidic in nature which may cause harm to aquatic life. Hence, they are neutralised by using a base before disposing it into the water bodies.

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

Solution:- 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 neutralised 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.

Solution:- The above solution may be neutral or basic in nature as both will not change the colour of the blue litmus paper.

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

Solution:- iv) Only d is correct

- By Durgesh Pandey

(Eklavya Coaching Institute)

📞 8376976688, 9310533915