Exploring Substances: Acidic, Basic, and Neutral – Long Answer Type Questions

Q1. Define acids and bases. Describe their general properties and give any three examples of each from daily life.

Ans. Substances which usually taste sour and turn blue litmus paper red are called acids. Substances which taste bitter, feel soapy and turn red litmus paper blue are called bases.

Properties of acids:

• They generally have a sour taste (for example, lemon juice, curd).
• They turn blue litmus paper red.
• They may react with some metals to produce hydrogen gas.

Properties of bases:

• They usually have a bitter taste and feel soapy to touch.
• They turn red litmus paper blue.
• They react with acids in neutralisation reactions to form salt and water.

Examples of acids: lemon juice (citric acid), vinegar (acetic acid), curd (lactic acid).
Examples of bases: baking soda, soap solution, lime water. These characteristics help us to distinguish acids and bases in everyday life.

Q2. What are neutral substances? How are they different from acidic and basic substances? Give four examples.

Ans. Substances which are neither acidic nor basic in nature are called neutral substances. They do not change the colour of either red or blue litmus paper.

Neutral substances differ from acids and bases in the following ways:

• Acids are sour and change blue litmus to red, while bases are bitter and change red litmus to blue. Neutral substances do not show these changes.
• Acids and bases can react with each other in neutralisation reactions. Neutral substances generally do not take part in such reactions in a special way.
• Neutral substances are usually safe to handle in comparison to strong acids and bases, which may be corrosive.

Examples of neutral substances: pure water, common salt solution, sugar solution, cooking oil. These substances show that not everything around us is acidic or basic; some are neutral and help maintain balance.

Q3. Distinguish between strong and weak acids with suitable examples. Why is it important for students to know this difference?

Ans. Acids are divided into strong and weak based on how completely they show their acidic behaviour (in higher classes this is explained by ionisation).

Strong acids: They show very strong acidic properties. They can be highly corrosive and dangerous if mishandled. Examples are hydrochloric acid (HCl) and sulphuric acid (H₂SO₄) used in laboratories and industries.

Weak acids: They show mild acidic behaviour and are usually safer when used in small amounts in food. Examples are citric acid in lemon, acetic acid in vinegar and lactic acid in curd.

It is important for students to know the difference because strong acids must be handled carefully using safety rules, while weak organic acids are often part of food and daily life. This awareness prevents accidents in the laboratory and helps in making healthy choices in daily life.

Q4. Explain with examples how acids, bases and neutral substances are connected to our day-to-day life.

Ans. The concept of acids, bases and neutral substances is closely linked to many daily activities. In the kitchen, lemon juice, tamarind and orange contain acids which give them a sour taste. Curd and buttermilk also contain natural acids. On the other hand, baking soda and baking powder used in cakes are basic.

In cleaning, most soaps and detergents are basic in nature. They help remove dirt and oil from clothes and utensils. Toothpaste is also slightly basic to neutralise acids formed by bacteria in our mouth. Neutral substances like water and common salt solution help maintain balance in our body and are widely used.

Even in our stomach, hydrochloric acid helps in digestion, while antacids (basic substances) are taken to neutralise excess acid. Thus, understanding these three categories helps us link classroom learning to real-life situations.

Q5. “Not all sour substances can be tasted safely.” Justify this statement and explain how we should test substances instead.

Ans. It is true that many naturally sour substances like lemon and orange are safe to eat. However, some acids, especially laboratory acids such as hydrochloric acid and sulphuric acid, are very strong and corrosive. Even a small drop can burn the tongue or damage internal organs if swallowed.

Because we cannot tell by just looking whether a sour substance is safe or not, it is never safe to taste unknown substances. Instead of tasting, we use indicators such as litmus, turmeric or china rose extract to test whether a substance is acidic, basic or neutral. These indicators change colour in a specific way and allow us to identify the substance safely without tasting it.

Therefore, in science, safety comes first, and the use of indicators replaces the risky method of tasting unknown substances.

Q6. What are indicators? Describe different types of natural indicators used in Class 7 with their colour changes in acids and bases.

Ans. Indicators are substances which show different colours in acidic and basic solutions. They help us identify the nature of a solution without tasting or touching it.

In Class 7, we mainly study the following natural indicators:

• Litmus: Obtained from lichens. Blue litmus turns red in acids, while red litmus turns blue in bases. In neutral solutions, there is no colour change.
• Turmeric: A common kitchen spice. It remains yellow in acids and neutral solutions, but turns reddish-brown when a base like soap solution or baking soda is added.
• China rose (hibiscus) extract: The coloured extract may appear pink. It usually turns bright red in acidic solutions and greenish in basic solutions.

These natural indicators are easy to prepare, safe to use and make experiments interesting. They show how plants can help us in scientific testing.

Q7. Explain how you would perform an activity using litmus paper to classify vinegar, soap solution and sugar solution as acidic, basic or neutral.

Ans. To classify vinegar, soap solution and sugar solution, we can perform the following activity:

Step 1: Take three clean test tubes and label them A, B and C. Put vinegar in A, soap solution in B and sugar solution in C.

Step 2: Dip a strip of blue litmus paper into each test tube and observe the colour change.

• If blue litmus turns red, the solution is acidic.
• If there is no change, it may be basic or neutral.

We observe that blue litmus turns red in vinegar (A), showing that vinegar is acidic. In soap solution (B) and sugar solution (C), blue litmus may not change.

Step 3: Now dip a strip of red litmus paper into B and C.

• Red litmus turns blue in a basic solution.
• Red litmus shows no change in a neutral solution.

Red litmus turns blue in soap solution (B), so soap solution is basic. Sugar solution (C) does not affect either blue or red litmus, so it is neutral. Thus, using litmus we classify vinegar as acidic, soap solution as basic and sugar solution as neutral.

Q8. “Turmeric and china rose indicators make learning about acids and bases easy and interesting.” Explain this statement with suitable examples.

Ans. Turmeric and china rose are natural indicators that can be prepared using common plant materials, so they make learning about acids and bases easy and fun.

Turmeric: Turmeric powder mixed with water forms a yellow paste. When we drop soap solution or baking soda on it, the spot turns reddish-brown, showing that these substances are basic. If we add lemon juice, there is almost no change, which shows that acids do not turn turmeric red. Students can try this activity at home and understand basic substances easily.

China rose (hibiscus): When petals of china rose are boiled in water, we get a coloured extract. This extract turns bright red in acidic solutions like lemon juice and greenish in basic solutions like soap water. Such clear and attractive colour changes make the concept of acids and bases very visual and memorable.

Because these indicators use familiar plants and show quick colour changes, students enjoy the experiments and develop more interest in science.

Q9. Why should we use more than one indicator sometimes while testing unknown solutions? Explain with a suitable example.

Ans. Different indicators may show different sensitivities and colour ranges for acids and bases. Sometimes a single indicator may not give clear information, so we use more than one indicator to confirm the result.

For example, if we test an unknown colourless solution with litmus paper and observe that blue litmus turns red, we know it is acidic. However, we do not know whether it is a strong acid or a weak acid. If we then use china rose indicator, we may see very intense red colour in a strong acid and a lighter shade in a weak acid, which gives us more information.

Also, some solutions themselves may have colour that makes it hard to observe changes with one indicator. Using a second indicator with a different colour change helps confirm the nature of the solution. Thus, using more than one indicator increases accuracy and confidence in our conclusion.

Q10. Describe an activity using turmeric to show that soaps and detergents are basic in nature.

Ans. We can perform a simple activity with turmeric to show that soaps and detergents are basic:

Step 1: Take a small piece of white cloth or paper and apply a thin layer of turmeric paste on it. Let it dry to form a yellow patch. This acts like turmeric indicator paper.

Step 2: Dip a cotton bud in a solution of soap or detergent and gently touch it on the turmeric patch. Observe the change in colour. The yellow patch turns reddish-brown, indicating the presence of a base.

Step 3: To compare, put a drop of lemon juice on another turmeric patch. The colour remains yellow, showing that lemon juice, an acid, does not turn turmeric red.

From this activity we conclude that soaps and detergents are basic in nature because they change turmeric from yellow to reddish-brown. This simple test also shows how natural indicators like turmeric can be used to study the properties of substances at home or school.

Q11. Define neutralisation. Explain the process with a labelled word equation and a simple activity.

Ans. Neutralisation is the chemical reaction in which an acid reacts with a base to form salt and water. During this process, the effects of both acid and base are reduced or cancelled.

Word equation: Acid + Base → Salt + Water (+ heat)

Activity: Take some dilute hydrochloric acid (HCl) in a test tube and add a few drops of phenolphthalein or another suitable indicator. Now slowly add dilute sodium hydroxide (NaOH) solution from a dropper while stirring. Initially, the solution is acidic. As NaOH is added, the colour changes, showing that the acid is being neutralised. When enough base has been added, the solution becomes neutral.

The reaction taking place is:
Hydrochloric acid + Sodium hydroxide → Sodium chloride (salt) + Water.
This activity shows how an acid and a base combine to form neutral products (salt and water).

Q12. Describe in detail how neutralisation is used in treating indigestion. Mention cause, effect and remedy.

Ans. Our stomach normally produces hydrochloric acid to help in digestion of food. When we eat too much spicy food, irregular meals or due to tension, the stomach may produce excess acid.

Cause: Excess secretion of hydrochloric acid in the stomach.
Effect: The stomach becomes highly acidic, causing pain, burning sensation and discomfort. This condition is called indigestion or acidity.

Remedy using neutralisation: To reduce the effect of extra acid, doctors recommend substances called antacids. Antacids are mild bases, such as milk of magnesia (magnesium hydroxide). When swallowed, the antacid reacts with the excess hydrochloric acid in a neutralisation reaction to form salt and water, reducing acidity.

As the acid is neutralised, the pH in the stomach moves closer to normal, and the burning sensation decreases. Thus neutralisation inside our body helps to treat indigestion and protect the delicate inner wall of the stomach.

Q13. How is neutralisation used by farmers to improve soil quality? Explain the complete process with examples.

Ans. Soil is one of the most important resources for farming. For healthy growth of crops, soil should not be too acidic or too basic. Sometimes, due to excessive use of chemical fertilisers or acid rain, soil may become too acidic.

Plants do not grow well in highly acidic soil. To correct this, farmers use the concept of neutralisation. They add basic substances to the soil to reduce its acidity.

Process:

• Farmers spread quicklime (calcium oxide) or slaked lime (calcium hydroxide) over the acidic soil.
• These basic substances react with the acids present in the soil and neutralise them, forming neutral salts.
• As a result, the soil pH becomes more suitable for crop growth.

In some regions, if the soil is too basic, mild acidic materials (such as organic matter) may be added to neutralise the extra base. Thus, neutralisation helps farmers maintain soil health and ensures better crop yields.

Q14. Explain how the concept of neutralisation is used in case of bee sting and wasp sting. Why are different remedies used?

Ans. When a bee stings, it injects an acidic substance into the skin. This causes pain and irritation. According to the concept of neutralisation, the effect of an acid can be reduced by using a mild base. Therefore, a paste of baking soda (a basic substance) or calamine lotion is applied on the sting area. The base neutralises the acid and gives relief.

In contrast, the sting of a wasp is usually basic in nature. To neutralise a base, we require a mild acid. Therefore, substances like vinegar (acetic acid) or lemon juice (citric acid) are gently applied on a wasp sting. These weak acids react with the basic venom and reduce pain.

Thus, different remedies are used because the nature of the chemicals injected by bees and wasps is different. Neutralisation helps us choose the correct treatment: a base for an acidic sting and an acid for a basic sting.

Q15. “Neutralisation is an important chemical process in industries.” Explain this statement with at least three examples.

Ans. Neutralisation reactions are widely used in many industries to control acidity or basicity.

Examples:

• Treatment of industrial waste: Many factories produce acidic waste liquids which, if released into rivers, can kill aquatic life. Before disposal, these wastes are treated with basic substances to neutralise the acids.
• Manufacture of fertilisers: In fertiliser industries, acids and bases are made to react in controlled neutralisation processes to produce various salts (fertilisers) needed for agriculture.
• Leather and paper industries: In leather tanning and paper making, materials often become strongly acidic or basic at different stages. Neutralisation is carried out to bring the pH to a suitable level so that the products do not get damaged and workers remain safe.

These examples show that neutralisation is not just a school experiment but a practical process used on a large scale to protect the environment and improve product quality.

Q16. Describe the role of acids in our daily life with examples from food, industry and our body.

Ans. Acids play many useful roles in our daily life. In food, natural acids give a pleasant sour taste. Lemon and orange contain citric acid, tamarind contains tartaric acid, and curd contains lactic acid. These acids also help in digestion and act as preservatives.

In our body, the stomach produces hydrochloric acid which helps in breaking down food and killing harmful bacteria. Without this acid, digestion would be incomplete and infections might increase.

In industry, strong acids like sulphuric acid and nitric acid are used in making fertilisers, paints, dyes and car batteries. Acetic acid in vinegar is used in pickles and as a food preservative.

Thus, acids are not always dangerous; when used in the correct amount and concentration, they are very useful in food, industry and our body.

Q17. How are bases useful to us? Explain any five uses of bases in homes and industries.

Ans. Bases are also very useful in our daily life and industries.

Uses of bases:

• Sodium hydroxide is used in making soaps and detergents which help us clean clothes and utensils.
• Calcium hydroxide (slaked lime) is used in whitewashing walls. It reacts with carbon dioxide to form calcium carbonate, giving a smooth white finish.
• Magnesium hydroxide is used in antacid medicines like milk of magnesia to neutralise excess acid in the stomach.
• Ammonium hydroxide is used in cleaning solutions for removing grease and in making fertilisers.
• Some basic substances are used to treat acidic soil in agriculture so that crops can grow better.

These examples show that bases, though sometimes dangerous, are very important when used correctly in various fields.

Q18. Explain why toothpaste is basic in nature. What will happen if a strongly acidic substance is used regularly to clean teeth?

Ans. Toothpaste is made slightly basic because acids in the mouth can damage our teeth. When we eat, food particles remain in the mouth. Bacteria act on these particles and produce acids. These acids attack the tooth enamel and cause tooth decay.

A basic toothpaste neutralises these acids in a neutralisation reaction. This protects the enamel from dissolving and prevents cavities. That is why most toothpastes have mild basic substances, such as calcium carbonate or other basic compounds.

If a strongly acidic substance is used regularly to clean teeth, it will further lower the pH in the mouth. The acid will slowly dissolve the tooth enamel, making teeth weak, sensitive and prone to decay. Therefore, basic toothpaste is preferred for healthy teeth.

Q19. How do acids and bases help in the whitewashing of walls? Describe the process and the changes taking place.

Ans. Whitewashing of walls uses the basic substance quicklime (calcium oxide). Quicklime is first mixed with water to form slaked lime (calcium hydroxide). This reaction is exothermic and releases heat. The slaked lime is a thin basic paste which is applied to walls as whitewash.

After application, slaked lime on the walls slowly reacts with carbon dioxide present in air. This reaction forms calcium carbonate, which is a hard, shiny, white material.

The overall change can be summarised as:
Calcium oxide + Water → Calcium hydroxide (whitewash)
Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Calcium carbonate sticks well to walls and gives a bright white appearance. Thus a basic substance and a reaction involving carbon dioxide result in a durable white layer on the walls.

Q20. Explain with examples how knowledge of acids and bases helps us in making healthy and safe choices in daily life.

Ans. Knowing about acids and bases helps us make better decisions regarding health, food and safety. For example, we understand that soft drinks and sour pickles contain acids. Consuming them in excess can harm teeth and stomach, so we should take them in limited amounts.

We also learn that strong cleaning agents like concentrated acids or bases used in bathrooms and kitchens must be handled carefully, using gloves and keeping them away from children. Understanding that toothpaste should be basic makes us prefer products that protect our teeth from acids.

Similarly, knowing about neutralisation helps us choose appropriate first-aid measures for insect stings or minor acid-base accidents. Overall, knowledge of acids and bases guides us to use household chemicals wisely and safely.

Q21. What is acid rain? Explain in detail how it is formed and mention its harmful effects on plants and animals.

Ans. When rainwater becomes more acidic than normal due to dissolved pollutants, it is called acid rain. Normally, rainwater is slightly acidic because carbon dioxide dissolves in it. However, when gases like sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) from factories and vehicles dissolve in rainwater, strong acids such as sulphuric and nitric acid are formed.

This makes the rainwater highly acidic. When this acid rain falls on the earth, it has several harmful effects:

• It damages leaves, stems and roots of plants, reducing their growth and crop yield.
• It increases the acidity of lakes and ponds, making them unsuitable for fish and other aquatic life, which may die.
• It can harm soil by destroying useful microorganisms and washing away important nutrients.

Thus, acid rain is a serious environmental problem which affects plants and animals and disturbs natural balance.

Q22. How does acid rain damage buildings and monuments? What can be done to reduce this damage?

Ans. Many buildings and monuments, such as the Taj Mahal, are made of marble or limestone. These materials contain calcium carbonate. When acid rain falls on these surfaces, the acids react with calcium carbonate and slowly dissolve it. This process is called corrosion.

Over time, the smooth and shiny surface of the monument becomes rough, dull and full of pits. The fine carvings are damaged and the beauty of the building is lost. Iron structures also rust faster in the presence of acid rain.

To reduce this damage, we must control air pollution by:

• Using cleaner fuels and maintaining vehicles properly to reduce emission of harmful gases.
• Installing pollution-control devices in factory chimneys.
• Planting more trees which absorb pollutants.

Regular cleaning and protective coatings on monuments can also slow down the effect of acid rain.

Q23. Suggest five ways by which we can help in reducing the problem of acid rain as responsible students and citizens.

Ans. As students and citizens, we can take the following steps to help reduce acid rain:

• Use public transport, bicycles or walk for short distances instead of using private vehicles for every small trip.
• Encourage our family to maintain vehicles properly so that they emit fewer harmful gases.
• Switch off lights, fans and other electrical appliances when not needed, to reduce the demand for electricity from power plants.
• Support tree plantation programmes because trees absorb carbon dioxide and other pollutants from the air.
• Spread awareness among friends and neighbours about the harmful effects of air pollution and acid rain.

By practising these small steps, we can help reduce the emission of gases that cause acid rain and protect our environment.

Q24. Relate the topic of acids and bases to environmental issues. How does understanding this chapter help us protect nature?

Ans. The topic of acids and bases is directly connected with several environmental issues. Acid rain, caused by acidic gases in the atmosphere, affects soil, water bodies, plants and buildings. Understanding how acids are formed and how they react helps us find ways to control and neutralise their harmful effects.

Knowledge of neutralisation teaches us how to treat acidic soil, industrial wastes and polluted water. It also helps in designing filters and scrubbers for factory chimneys. By knowing that some substances are corrosive, we can handle them carefully and prevent leaks and spills that might harm nature.

Thus, this chapter not only teaches basic chemistry but also makes us more responsible towards the environment by showing how chemical substances interact with nature.

Q25. What is pH in simple words? Explain the general range of pH values for acidic, basic and neutral solutions.

Ans. In simple words, pH is a number that tells us how acidic or basic a solution is. In higher classes, you will learn its exact definition, but at Class 7 level we use it as a scale of acidity and basicity.

The pH scale usually ranges from 0 to 14:

• Solutions with pH less than 7 are acidic. Lower pH means stronger acid.
• A solution with pH equal to 7 is neutral, like pure water.
• Solutions with pH greater than 7 are basic. Higher pH means stronger base.

Strong mineral acids have very low pH values, while strong bases have very high pH values. This simple idea of pH helps us compare solutions and understand which ones are more acidic or more basic.

Q26. Why is it important for our body fluids like blood and stomach juices to have proper pH values? Explain with examples.

Ans. Many chemical reactions in our body can work properly only when the pH of body fluids stays within a narrow range. If the pH becomes too low (too acidic) or too high (too basic), these reactions may slow down or stop, causing health problems.

For example, our blood is slightly basic and its pH is maintained around a fixed value. Large changes in blood pH can affect the working of the heart, lungs and brain. Our body has natural systems to keep this pH nearly constant.

In the stomach, hydrochloric acid helps in digestion. But excess acid lowers the pH too much and causes indigestion and pain. Antacids are given to bring the pH back towards normal by neutralising the extra acid. Thus, maintaining proper pH is essential for our health.

Q27. Explain how a simple understanding of pH helps us in choosing food and medicines wisely.

Ans. Knowing that highly acidic or highly basic substances can be harmful helps us choose food and medicines more carefully. For example, very sour and spicy food increases acid level in the stomach and may cause acidity. So, people with such problems should limit the intake of very sour food.

We also know that antacids are basic medicines used to neutralise extra acid in the stomach. They bring the pH closer to normal and provide relief. Similarly, toothpaste is chosen to be basic to neutralise the acids that damage teeth.

Understanding pH also explains why we should not drink very acidic or very basic cleaning liquids by mistake; they can damage internal organs. Thus, even simple knowledge of pH guides us to select safe foods and use medicines correctly.

Q28. List and explain at least six safety rules that must be followed while working with acids and bases in the school laboratory.

Ans. Working with acids and bases requires special care. Important safety rules are:

• Do not taste chemicals: Never taste or eat any substance in the lab, as acids and bases may be poisonous or corrosive.
• Wear safety goggles: Goggles protect eyes from accidental splashes of acids or bases.
• Handle bottles carefully: Always hold acid and base bottles with both hands and keep them closed when not in use.
• Use droppers or spatulas: Do not touch chemicals directly with hands; use proper instruments.
• Always add acid to water: While diluting an acid, add acid slowly to water and never water to acid, to avoid splashing.
• Wash spills immediately: If any acid or base spills on skin or table, wash with plenty of water and inform the teacher.

Following these rules prevents accidents and makes the laboratory a safe place for learning science.

Q29. Explain scientifically why we must always add acid to water and never water to concentrated acid while diluting it.

Ans. When we mix concentrated acid with water, the reaction releases a lot of heat. If we pour water into a small amount of concentrated acid, the upper layer of water immediately gets heated and may start boiling at once. This can cause the mixture to splash out of the container and may burn the skin or eyes.

On the other hand, if we take a large quantity of water in a beaker and slowly add acid to it while stirring, the heat produced is spread throughout the large amount of water. The temperature rises gradually and there is no sudden boiling or splashing.

Therefore, for safety reasons, the rule is: “Always add acid to water, never water to acid.” This rule is very important whenever acids are diluted in the laboratory or in industries.

Q30. How does the study of acids, bases and neutral substances help in developing a scientific attitude among students?

Ans. The study of acids, bases and neutral substances encourages students to think logically and rely on evidence. Instead of guessing by taste or appearance, students learn to use indicators and experiments to identify the nature of substances. This builds a habit of testing ideas scientifically.

Students also understand the importance of neutralisation in solving real-life problems such as indigestion, soil treatment and acid rain. They see how science provides practical solutions rather than superstition or guesswork.

Furthermore, following safety rules in the laboratory teaches discipline and responsibility. Overall, this chapter trains students to observe carefully, ask questions, collect evidence and draw conclusions, which are the main parts of a scientific attitude.