Acids, Bases and Salts
Acids are substances that taste sour and can be harmful.
They turn blue litmus paper to red.
Examples include orange juice, vinegar, and hydrochloric acid.
They are chemically acidic.
Bases are substances that feel slippery and taste bitter when dissolved in water.
They turn red litmus paper to blue.
Examples include soap, ammonia solution, and calcium hydroxide.
They are chemically basic.
When an acid and a base react, they form salt, water, and heat.
This reaction is called neutralization.
It neutralizes the acidic and basic properties of the substances.
For example, when hydrochloric acid reacts with sodium hydroxide, it forms sodium chloride and water:
HCl + NaOH → NaCl + H2O
Salts are the products of neutralization reactions.
In the reaction between hydrochloric acid and sodium hydroxide, sodium chloride is formed:
HCl + NaOH → NaCl + H2O
Salts can be acidic, basic, or neutral.
Substances used to test if a solution is acidic or basic.
Acidic Solution does not change red litmus but turns blue litmus to red.
Basic Solution turns red litmus to blue and doesn’t change blue litmus.
Natural indicators come from plants, like China Rose and turmeric.
Litmus changes color: red for acid, blue for base.
Phenolphthalein is synthetic and turns pink in a base.
Olfactory indicators change smell, like onions or cloves.
Visual indicators change color, such as litmus and red cabbage.
Neutralization in Daily Life:
Milk of magnesia neutralizes stomach acid during indigestion.
Baking soda neutralizes ant stings caused by formic acid.
Soil is treated with acids or bases to help plants grow.
Factories neutralize acidic waste before disposal to protect the environment.
Safety Measures While Using Acids:
Pour acid into water when diluting, not the other way around.
Wear safety gloves when working with acids or bases.
Uses and Applications:
pH of Soil: Adjusting soil pH with acids or bases for better plant growth.
Food Preservation: Citric acid preserves food.
Aerated Drinks: Carbonic acid gives fizz to sodas.
Baking Powder: Tartaric acid helps cakes rise.
Cooking: Acetic acid is found in vinegar.
Manufacture of Soaps: Sodium hydroxide is used.
Manufacture of Bleaching Powder: Calcium hydroxide is used.
Fire Extinguishers: Aluminium hydroxide is used as a foaming agent.
Read More: Heat | Class 7 Notes | Science | Chapter 4
Differences between Acids and Bases:
- Taste sour.
- Turn blue litmus paper red.
- Do not change the color of red litmus.
- Do not change the color of turmeric.
- Taste bitter.
- Do not change the color of blue litmus.
- Turn red litmus paper blue.
- Turn turmeric red.
Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?
Ammonia is basic in nature because it turns red litmus paper blue.
Litmus solution is indeed extracted from lichens, and its primary use is as an indicator to determine whether a solution is acidic or basic.
Distilled water is considered neutral, and you can verify this by testing it with red and blue litmus paper. If the color remains unchanged in both cases, it indicates neutrality.
Neutralization is a chemical reaction between an acid and a base, resulting in the formation of a salt and water, along with the release of heat. Your example involving sodium hydroxide (NaOH) and hydrochloric acid (HCl) is a perfect illustration of this process.
NaOH + HCl → NaCl + H2O + Heat
Mark ‘T’ if the statement is true and ‘F’ if it is false. (i) Nitric acid turns red litmus blue. (T/F) (ii) Sodium hydroxide turns blue litmus red. (T/F) (iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. (T/F) (iv) Indicator is a substance which shows different colours in acidic and basic solutions. (T/F) (v) Tooth decay is caused by the presence of a base. (T/F)
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?
Dorji can indeed use taste and litmus paper to determine the nature of the soft drinks and serve them to the customers accordingly:
- For the customer who wants an acidic drink:
- Dorji can taste a few drops; if it's sour, it's likely acidic.
- He can also use blue litmus paper. If it turns red, the drink is acidic.
- For the customer who wants a basic drink:
- Dorji can taste a few drops; if it's bitter, it's likely basic.
- He can also use red litmus paper. If it turns blue, the drink is basic.
- For the customer who wants a neutral drink:
- Dorji can taste a few drops. If it has no strong taste (neither sour nor bitter), it's likely neutral.
This way, Dorji can accurately serve the drinks to his customers based on their preferences for acidity, basicity, or neutrality.
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 tablet for acidity:
- Antacid tablets contain bases like milk of magnesia.
- They neutralize the excess stomach acid, providing relief from acidity.
(b) Calamine solution for ant bites:
- Ants inject formic acid when they bite.
- Calamine solution contains Zinc carbonate, which is basic.
- It neutralizes the formic acid's effects, relieving discomfort.
(c) Neutralizing factory waste:
- Factory waste is often acidic, which can harm aquatic life.
- To protect the environment, these wastes are neutralized with a base before disposal into water bodies.
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.
- Initial Test:
- Place a drop of each liquid on the turmeric indicator.
- The one that turns the indicator red is sodium hydroxide, indicating its basic nature.
- Confirmation Tests:
- Take the remaining two liquids and add a drop of sodium hydroxide separately to each.
- Now, place a drop of each combination on the turmeric indicator.
- The mixture that changes the indicator to red contains a neutral solution of sugar.
- The mixture that doesn't change the indicator contains hydrochloric acid neutralized by sodium hydroxide.
Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.
When blue litmus paper remains blue after being dipped in a solution, it indicates that the solution is either neutral or basic in nature. Neither neutral nor basic solutions will cause a change in the color of blue litmus paper.
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