1. Our Food -
ü In our daily life, we consume a large number of substances.
ü Previously we learned the difference between edible and non edible substances.
ü The edible substances which we use on daily basis includes lemon, baking soda, tamarind, common salt, sugar, curd and vinegar. Some of these substances taste sour, some taste bitter, some taste sweet and some taste salty.
Substance | Taste (Sour/Bitter/Any other) |
Lemon Juice | Sour |
Orange Juice | Sour |
Vinegar | Sour |
Curd | Sour |
Tamarind(imli) | Sour |
Sugar | Sweet |
Common Salt | Salty |
Amla | Sour |
Baking Soda | Bitter |
Grapes | Sweet/Sour |
Table - Food and its taste
2. Acids and Bases-
ü The word acid has been derived from a Latin word ‘acidus’ which means ‘sour’. Thus, all sour substances essentially contain acids.
ü Substances like lemon juice, orange juice, unripe mango and curd taste sour. They taste sour because they contain substances called acids in them.
ü The chemical nature of such substances is acidic.
ü The acids in these substances are natural acids.
ü However, there are other substances like baking soda it does not taste sour.
ü It means that it has no acids in it. It is bitter in taste.
ü And if prepare a solution of baking soda in water and rub it between your fingers, it feels soapy.
ü Substance like these which are bitter in taste and feel soapy on touch are known as bases.
ü The chemical nature of such substances is said to be basic. All the acids mentioned in table occur in nature.
Name of Acid | Found in |
Acetic Acid | Vinegar |
Formic Acid | Ant’s Sting |
Citric Acid | Citrus fruits such as oranges, lemons, etc.. |
Lactic Acid | Curd |
Oxalic Acid | Spinach |
Ascorbic Acid(Vitamin C) | Amla, Citrus fruits |
Tartaric Acid | Tamarind, grapes, unripe mangoes, etc.. |
Table - Source of acids
Name of Base | Found in |
Calcium Hydroxide | Lime Water |
Ammonium Hydroxide | Window Cleaner |
Sodium Hydroxide/Potassium Hydroxide | Soap |
Magnesium Hydroxide | Milk of Magnesia |
Table - Source of base
3. Natural Indicators -
ü It is not safe to taste every substance to find out if it is acidic or basic.
ü There are some substances that have tendency to show different colors in acidic and basic mediums.
ü These substances are known as indicators.
ü The indicators change their colour when added to a solution containing an acidic or a basic substance.
ü Some naturally occurring indicators are litmus, turmeric, China rose petals and red cabbage juice. They are used to test whether a substance is acidic or basic in nature.
ü Litmus- A Natural Dye. A naturally occurring indicator, i.e. litmus is obtained from certain lichens (small plants) and used as a dilute solution. Litmus has mauve (purple) colour in water. In an acidic solution, it turns red. When it is added to a basic solution, it turns blue. Usually, it is available as a red and blue litmus paper.
ü Turmeric - It is another natural indicator - Turmeric is a bright yellow powder obtained from a plant. It is is called ‘Haldi’ in Hindi. Turmeric contains a yellow dye. Turmeric turns red in basic solution. It is used as indicator in the form of turmeric paper.
ü China Rose- It is a natural indicator. It is called ‘Gudhal’ in Hindi. It is a extracted from the red flowers of China rose plant with water.
4. Acid Rain-
ü The rain containing excess of acids called an acid rain.
ü The rain becomes acidic because carbon dioxide, sulphur dioxide and nitrogen dioxide dissolve in rain drops to form carbonic acid, sulphuric acid and nitric acid respectively.
ü It can cause damage to buildings, historical monuments, plants and animals.
ü This happens as follows -
n Acid rain makes the water of lakes, ponds and rivers too acidic due to which fish and other aquatic animals get killed.
n Acid rain eats up the leaves of the trees gradually. By losing leaves, the trees die.
n Acid rain also damages crop plants in the fields.
n Acid rain damages the metal structures like steel bridges, etc when it falls on them.
n Acid rain damages the surfaces of buildings and monuments made up of marble.
5. Neutralization -
ü Acids and bases are chemically opposite substances.
ü So, when an acid is mixed with a base, they neutralize (or cancel) the effect of each other.
ü When an acid solution and a base solution are mixed in suitable amounts, both the acidic nature of the acid and the basic nature of the base are destroyed.
ü The resulting solution is neither acidic nor basic. So, the reaction between an acid and base is known as neutralization.
ü In the process of neutralization, salt and water are produced with the evolution of heat.
ü Salt produced in the reaction may be acidic, basic or neutral in nature. The evolved heat raises the temperature of the reaction mixture.
Acid + Base → Salt + Water (Heat is evolved)
Hydrochloric acid (HCl) (Acid) + Sodium hydroxide (NaOH) (Base)
→ Sodium chloride (NaCl) (Salt) + Water (H2O)
ü If dilute sulphuric acid is added to lime water (which is a base), then neutralisation reaction takes place and the reaction mixture becomes hot.
ü We are going to use an indicator which you have not used so far. It is called phenolphthalein.
Note: Phenolphthalein is an indicator used in the neutralisation process. When the solution is basic, phenolphthalein gives a pink colour but if the solution is acidic, it remains colourless.
6. Neutralization In Day To Day Life -
ü The neutralization reactions involving acids and bases play a very important role in our everyday life.
ü The treatment of an ant’s sting, remedy for indigestion, soil treatment and the treatment of factory wastes, all involve neutralization reaction.
ü Indigestion -
n Our stomach produces hydrochloric acid.
n This hydrochloric acid helps in digesting our food.
n Sometimes, excess of hydrochloric acid is produced in the stomach which causes indigestion.
n Due to indigestion, sometimes a person feels pain in the stomach and irritation. T
n Relieve indigestion, we take an antacid such as milk of magnesia.
n Milk of magnesia contains a base called magnesium hydroxide.
n Magnesium hydroxide neutralizes the excess acid present in the stomach and cures indigestion.
n Another antacid is baking soda which contains a base sodium hydrogen carbonate.
ü Ant Bite -
n When an ant bites, it injects an acidic liquid into the skin of the person which causes burning pain.
n The sting of an ant contains an acid called formic acid. The effect of the acid can be neutralized by rubbing a mild base like baking soda solution (sodium hydrogen carbonate) or calamine solution. Calamine solution contains a base called zinc carbonate.
n Thus, being a base, baking soda solution or calamine solution neutralizes the acidic liquid injected by the ant and cancels its effect.
ü Soil Treatment-
n The soil may be acidic or basic naturally.
n The plants do not grow well, if the soil at a place is too acidic or too basic. Excessive use of chemical fertilizers makes the soil acidic.
n When the soil is too acidic, it is treated with bases like quicklime (calcium oxide) or slaked lime (calcium hydroxide).
n These bases neutralize the excess acid present in the soil and reduce its acidic nature.
n If the soil is basic, organic matter called manure or compost is added to it. The organic matter releases acids which neutralize the excess bases present in the soil and reduce its basic nature.
ü Factory Wastes-
n The waste substances discharged by many factories contain acids. If these factory wastes are allowed to flow into the water bodies (like rivers, ponds, lakes, etc), then the acid present in them will kill fish and other organisms which live in the water bodies.
n The factory wastes are therefore neutralized by adding basic substances before discharge.
NCERT PRACTICE QUESTIONS
1. State the differences between acids and bases.
Ans.-
Acids | Bases |
These are sour to taste. | These are bitter to taste. |
Acids turn blue litmus red. | Bases do not change the color of blue litmus. |
Acids do not change the color of red litmus. | Bases turn red litmus blue. |
With china rose indicator, these give dark pink color. | Bases give green color with china rose indicator. |
Acids do not change the color of turmeric indicator. | Bases turn the color of turmeric indicator to red. |
2. Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?
Ans.- Ammonia is basic in nature as it turns the color of red litmus paper to blue.
3. Name the source from which litmus solution is obtained. What is the use of this solution?
Ans. - Litmus solution is obtained from lichens. A natural dye extracted from lichens is dissolved in distilled water to obtain litmus solution. It is used as an indicator to distinguish between acids and bases.
Solution | Color of litmus solution |
Acidic | Red |
Basic | Blue |
Neutral | No change |
4. Is the distilled water acidic/basic/neutral? How would you verify it?
Ans. - Distilled water is neutral in nature. The same can be verified by using red and blue litmus papers. Neither will show a colour change with distilled water. This proves that distilled water is neutral.
5. Describe the process of neutralization with the help of an example.
Ans. - The reaction between an acid and a base is known as neutralization reaction. In this reaction, both acid and base cancel each other’s effect. Neutralization reaction results in the formation of salt and water. During this reaction, energy in the form of heat is evolved.
Acid + Base → Salt + Water + Heat
For example, when sodium hydroxide (NaOH) is added to hydrochloric acid (HCl), sodium chloride (NaCl) and water (H2O) are obtained.
6. Mark ‘T’ if the statement is true and ‘F’ if it is false-
(i) Nitric acid turns red litmus blue. (False)
(ii) Sodium hydroxide turns blue litmus red. (False)
(iii) Sodium hydroxide and hydrochloric acid neutralize each other and form salt and water. (True)
(iv) Indicator is a substance which shows different colors in acidic and basic solutions. (True)
(v) Tooth decay is caused by the presence of a base. (False)
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. - Since the drinks are edible, Dorji can take the decision by tasting the drinks. Acidic drinks will be sour to taste whereas basic drinks will be bitter to taste and neutral drinks will have no taste. If Dorji has litmus indicator (solution or paper), then he can take its help. He should put one drop of each drink on blue litmus paper. If the color of the litmus paper changes to red, then it is an acidic drink. Out of the remaining drinks, some are basic and some are neutral. Again, he should put one drop of the remaining drinks on red litmus paper. If the color changes to blue, then it is basic and the others are neutral. In this way, he can serve all the three customers their respective drinks.
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 neutralized before disposing it into the water bodies.
Ans.-
(a) This is because during acidity, an excess of acids is produced in the stomach. An antacid contains base, such as milk of magnesia. These bases react with excess of acids and neutralize their effect, thus giving us relief.
(b) When an ant bites, it injects formic acid into the skin. Calamine solution contains zinc carbonate which is basic in nature. Therefore, it is applied on the skin to neutralize the effect of formic acid.
(c) Factory wastes contain acids. Therefore, these wastes, when thrown directly to water bodies, harm aquatic lives. Hence, these wastes are neutralized with basic chemicals before disposing to water bodies.
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.
Ans.-
We will put a drop each of hydrochloric acid, sodium hydroxide, and sugar solution on the turmeric indicator. The liquid which changes the color of turmeric indicator to red is basic in nature, that is, sodium hydroxide.
Now, we will put a drop of sodium hydroxide on a drop of each of the other two liquids separately. After that, we will put the drops of these mixtures on turmeric indicator. The drop which will change the color of the turmeric indicator to red contains sugar solution. This is because the mixture of basic and neutral solutions is basic in nature. On the other hand, the drop which will not change the color of turmeric indicator contains hydrochloric acid. This is because hydrochloric acid reacts with sodium hydroxide to form a neutral solution.
10. Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.
Ans. - We know that basic and neutral solutions do not change the color of blue litmus paper. Since blue litmus remains blue after dipping in the solution, the solution is either basic or neutral in nature. Put a drop of this solution on a red litmus paper. If it turns blue, then the above solution is basic in nature and if no color change occurs, then it is neutral.
11. Consider the following statements:
(a) Both acids and bases change color 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 color with an acid.
(d) Change of color 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 and c (iv) only d
Ans. - (iv) Only d is correct.
Junior Science Topic - Acid, Base And Salt. Chapter notes, video lectures, power point presentation and NCERT exercise solution. (Class 7, CBSE syllabus, chapter 05)
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