Exploring Substances: Acidic, Basic, And Neutral

Hello my dear students! Welcome to today's science lesson. I am so happy to see you all here, ready to learn something new and exciting. Today, we are going to explore a chapter that is absolutely wonderful — Chapter 2: Exploring Substances: Acidic, Basic, and Neutral. Are you ready for this adventure? Let's begin!

Imagine you are walking into a science fair. That's exactly what happened to two siblings, Ashwin and Keerthi, on the 28th of February, which was National Science Day at their school. At the entry gate, they were given a white sheet of paper. How strange! Why would anyone give them just a plain white paper? They walked a few steps ahead and saw a volunteer spraying some liquid on these sheets of paper. And guess what happened next? The words 'Welcome to the Wonderful World of Science' appeared on the papers the moment the liquid was sprayed! Wasn't that amazing? The children were so excited and curious to know how this magic happened. Well, my dear students, by the end of this chapter, you will not only understand how this happens but also be able to do it yourself! Isn't that wonderful? So let's join Ashwin and Keerthi on this learning adventure together.

Now, let's start with the very first concept. Our nature is like a big science laboratory. There are so many substances around us — some are sour, some are bitter, some feel slippery, and some taste sweet. But how do we know whether a substance is acidic, basic, or neutral? We need some special substances that can tell us about the nature of other substances. These special substances are called indicators. Let's learn about one very common indicator called litmus.

For this, we are going to do a very interesting activity. Collect samples of various substances from your home — lemon juice, soap solution, amla juice, tamarind water, vinegar, baking soda solution, lime water, tap water, washing powder solution, sugar solution, and salt solution. Now, take a strip of blue litmus paper and cut it into small pieces. Spread these pieces on a clean and dry white tile. Using a dropper, put one drop of each of the samples on these litmus paper pieces, one by one. Do you observe any change in the colour of the blue litmus pieces? Record your observations in a table. Now, repeat the same activity with pieces of red litmus paper and record your observations.

Now, let's analyze what we observed. Some substances turned the blue litmus paper to red. These substances are acidic in nature. Some other substances turned the red litmus paper to blue. These substances are basic in nature. And some substances did not change the colour of either litmus paper. These are neutral substances.

So, let me tell you what litmus actually is. Litmus is a natural substance obtained from lichens. Lichens are formed by the association of two living organisms — a fungus and an alga. They grow on rocks and trees in regions that have abundant rainfall and clean air. You might find lichens on trees in your neighbourhood too! Litmus is available both as a solution and in the form of paper strips, known as litmus paper. The litmus paper is available in two colours — blue and red. Substances that turn blue litmus paper to red are acidic in nature, while those that turn red litmus paper to blue are basic in nature. Since litmus shows different colours in acidic and basic solutions, it is called an acid-base indicator. Some other substances, both natural and synthetic, can also be used as indicators. Synthetic indicators are made in laboratories, and you will learn more about them in higher grades.

Now, let's classify the substances we tested. Group A includes substances that turn blue litmus paper to red — like lemon juice, amla juice, tamarind water, and vinegar. These are acidic substances. Group B includes substances that turn red litmus paper to blue — like soap solution, baking soda solution, lime water, and washing powder solution. These are basic substances. Group C includes substances that do not affect either litmus paper — like tap water, sugar solution, and salt solution. These are neutral substances.

Now, let me ask you something. Are all the substances in Group A edible? Have you ever tasted these edible substances? Can you recall their taste? You will find that all these substances taste sour. For example, lemon juice is sour, amla is sour, tamarind is sour, and vinegar is sour. So, we can say that substances that taste sour tend to contain acids and are acidic in nature. But remember, my dear students, do not taste anything until asked to do so. Do not taste any unknown substance. This is very important for your safety.

Now, let's look at some common edible substances and the acids present in them. Lemon contains citric acid. Curd contains lactic acid. Tamarind contains tartaric acid. Vinegar contains acetic acid. Can you find out and write the names of the most common acids present in the following substances — Lemon, Curd, Tamarind, and Vinegar? Do try this at home!

Now, let's take one of the substances from Group B — baking soda solution. Rub the baking soda solution between your fingers. What do you observe? It feels soapy or slippery, isn't it? Basic substances are generally slippery to touch. Also, bases generally taste bitter. But remember, everything that tastes bitter may not contain a base. For example, bitter gourd, which we call karela, has a bitter taste but is not basic in nature.

Now, let's learn about another interesting indicator — the red rose! You might have observed many coloured flowers in your surroundings. Let's try making our own indicator using red roses.

Collect some fallen petals of red roses. Remember, it is advised not to pluck flowers. You may pick petals or flowers fallen on the ground. Take a fistful of the collected petals and wash them with water. Crush the petals using a mortar and pestle. Place them in a glass tumbler. Pour some hot water into the glass tumbler to ensure that the crushed flower petals are completely immersed. Be careful while doing this — perform this step under the supervision of an adult. Cover the glass tumbler with a lid. Wait for 5 to 10 minutes till the water becomes coloured, and filter it. The filtrate, which is the liquid after filtration, is the required flower extract to be used as an acid-base indicator.

Now, let's test various substances using this red rose extract. Place 10 to 20 drops of the prepared red rose extract in each of two small transparent bottles or test tubes. Mark them A and B. Add 20 to 30 drops of lemon juice in test tube A and 20 to 30 drops of soap solution in test tube B. Observe and record any colour changes. Repeat the same with the other samples we used earlier.

What do we observe? The red rose extract gives a red colour in acidic solutions and a green colour in basic solutions. Neutral substances do not change the colour of the extract. So, the samples that change the colour of the flower extract to a shade of red are the same as those that changed the colour of blue litmus paper to red, which means they are acidic. The samples that change the colour of the flower extract to a shade of green are the same as those that changed the colour of red litmus paper to blue, which means they are basic. And the samples that do not change the colour of the flower extract are the same as those that did not change the colour of red and blue litmus papers, which means they are neutral.

So, we can conclude that the red rose extract can also be used to test the nature of substances. It is another example of an acid-base indicator. You can also try using other vegetables, fruits, or flowers such as beetroot, purple cabbage, turmeric, Indian blackberry which we call jamun, and red hibiscus flower. They can also act as acid-base indicators.

Here's a fascinating fact for you! Hydrangea is a plant that grows in cooler climates in the Himalayan region and the North-eastern states. It gives flowers of different colours depending on the nature of the soil. Acidic soil produces blue-coloured flowers, whereas in basic soil, the flowers are pink or red. So, gardeners can alter the colour of hydrangea flowers by adjusting the acidic or basic nature of the soil. Isn't that interesting?

Now, let's learn about another very common indicator — turmeric, which we call haldi. We have used blue and red litmus paper strips. Can we also make paper strips with some other natural indicators? Let's find out!

Take a spoonful of turmeric in a petri dish or container and add a little water to make a paste. You may also grind a piece of fresh turmeric. Carefully dip a piece of filter paper in the turmeric paste until it gets yellow colour. Take it out and allow it to dry. Cut this yellow paper into thin strips, which are used as turmeric paper. Again, perform this step under the supervision of an adult.

Now, using a dropper, put a drop of each of the samples we used before, one by one, on separate pieces of turmeric paper. Record your observations.

What did you observe? Let's see! The yellow colour of turmeric turns red in basic solutions but remains unchanged in acidic and neutral solutions. So, turmeric paper can be used to test basic substances. However, it cannot differentiate between acidic and neutral substances. That's an important point to remember!

Now, here's something interesting. Have you ever got a curry stain on your white shirt and noticed that on applying soap, it changed its colour? Now you know the reason! The curry stain had turmeric in it, and soap, being basic, turned the yellow colour to red.

Turmeric is known as a 'Golden' spice. It is a member of the ginger family, grown in India and other countries. It is a common spice in daily households and is being researched for benefits beyond the taste and colour it provides to the food! In the Ayurvedic system of medicine, turmeric is considered to have numerous health benefits and is commonly used in several traditional home remedies.

Now, let me tell you about another interesting type of indicator. Are there any substances whose odours change on adding acidic or basic substances? Yes, there are! These are called olfactory indicators. Let us explore this!

Take some finely chopped onions in a container, along with some strips of clean cotton cloth or filter paper. Tightly close the container and leave it overnight. Take two of the cotton cloth or filter paper strips from the container and check their odour. Keep them on a clean surface and put a few drops of tamarind water on one strip and a few drops of baking soda solution on the other. Allow the drops to spread on the strips. Check the odour again. Do you notice any change in the odour of the onion strips before and after putting tamarind water and baking soda solution on them? Note your observations. Similarly, test the change in the odour with other acidic and basic substances and record your observations. This is a great way to identify acids and bases if you cannot see the colour changes!

Now, before we move ahead, let me tell you about a great scientist. Acharya Prafulla Chandra Ray is known as the 'Father of Modern Indian Chemistry'. He earned a doctorate in chemistry from the United Kingdom and returned to India. He contributed towards advancing scientific research in India. He also established India's first pharmaceutical company in 1901. He was a person rooted in Indian culture and knowledge traditions. Through his writings on the history of chemistry in India, he highlighted the achievements and expertise of ancient Indian scientists to the modern world. Ray, a social reformer, also advocated the use of the mother tongue as a medium of instruction in educational institutions. We should all be proud of such great Indian scientists!

Now, let's learn about what happens when acidic substances mix with basic substances. This is a very important concept, so pay attention!

Let's perform an experiment. Take one drop of lemon juice in a test tube and add around twenty drops of water to it. Observe the colour. Add a drop of blue litmus solution to it. Do you observe any colour change? The colour turns red because lemon juice is acidic. Now, slowly add drops of lime water to this test tube with the help of a dropper and swirl it well. What do you observe? Is there any change in the colour of the solution? A stage comes when the colour of the solution changes from red to blue. Again, add one drop of lemon juice to the above solution. Can you predict why there is a change in colour?

Initially, when a drop of blue litmus solution is added to the lemon juice solution, the colour of the solution turns red. When lime water is added to this test tube, the colour of the solution eventually changes from red to blue. This shows that the solution in the test tube is no longer acidic. Lime water has neutralised the effect of the acid.

When the solution of an acid is mixed with the solution of a base in sufficient quantity, we find that the resulting solution is neither acidic nor basic. Such reactions are called neutralisation reactions. In a neutralisation reaction, salt and water are formed with the evolution, that is, release, of heat. We can write this as: Acid + Base → Salt + Water + Heat.

There are many examples in everyday life where you can observe the use of neutralisation processes. Let us find out!

Let's look at Situation 1. Keerthi was observing a butterfly in the garden with her hand resting on a tree trunk. Suddenly, a red ant bit her, leaving her skin red with stinging pain. Her brother helped her by applying moist baking soda to the affected area, which relieved the pain. What do you think might be the reason for this? When an ant bites, it injects an acidic liquid called formic acid into the skin. The effect of the acid can be neutralised by rubbing moist baking soda, which is a base. So, the next time someone gets an ant bite, you know what to do!

Now, Situation 2. On the Farmer's Portal, a farmer stated that his plants were not growing well. After a detailed discussion, it was found that the excessive use of chemical fertilisers made the soil acidic. What remedy might be provided to him? When the soil is too acidic, the plants do not grow well. It can be treated with lime, which is a base. If the soil is basic, organic matter like manure and composted leaves are added to it. Organic matter releases acids that neutralise the basic nature of the soil. Sometimes, the soil may be neutral, but the plants growing in it may still show poor health, which can be due to the deficiency of nutrients in the soil.

Situation 3. Ashwin's friend Gurbir stays near an industrial area. He shared that the fish population in his neighbourhood lake was declining day by day! What do you think might be the causes for this? It may be due to factory waste being released into the lake. If the factory waste is acidic in nature, what could be done to save the fish in the lake? The factory waste can be neutralised by adding basic substances before releasing it into the lake. This is how neutralisation is used to protect our environment!

Now, can you explain why the words 'Welcome to the Wonderful World of Science' appeared on Ashwin and Keerthi's paper sheets when the liquid was sprayed on them? Do you think that one possibility could be using a turmeric solution for the spraying liquid and a soap solution for writing on the paper? Yes! Because when you write on turmeric paper with a basic substance like soap solution, the yellow colour turns to red, making the writing visible!

Let me summarize what we have learned so far:

Substances around us may be classified as acidic, basic, and neutral in nature. Extracts of lichen, red rose, red hibiscus, purple cabbage, turmeric, and so on can be used to indicate the nature of substances. Substances that show different colours in acidic and basic solutions are called acid-base indicators. Acids turn the colour of blue litmus to red. Bases turn the colour of red litmus to blue. Extract of red rose gives red colour in acidic solutions and green colour in basic solutions. The yellow colour of turmeric turns red in basic solutions but remains unchanged in acidic and neutral solutions. An acid and a base neutralise each other, forming salt and water, along with the evolution of heat. Many day-to-day problems like a red ant bite, acidic or basic nature of soil, and industrial waste can be attempted to be managed by the process of neutralisation.

Now, let's solve the exercises together. I'll explain each question step by step.

Question 1: A solution turns the red litmus paper to blue. Excess addition of which of the following solution would reverse the change? (i) Lime water (ii) Baking soda (iii) Vinegar (iv) Common salt solution

Let's think about this. The solution turns red litmus paper to blue, which means it is basic. To reverse this change, we need to add an acidic substance that will neutralise the basic nature. Lime water and baking soda are both basic, so adding more of them won't reverse the change. Common salt solution is neutral, so it also won't help. Vinegar is acidic, so adding excess vinegar will neutralise the basic solution and turn the litmus back to red. So the answer is (iii) Vinegar.

Question 2: You are provided with three unknown solutions labelled A, B, and C, but you do not know which of these are acidic, basic, or neutral. Upon adding a few drops of red litmus solution to solution A, it turns blue. When a few drops of turmeric solution are added to solution B, it turns red. Finally, after adding a few drops of red rose extract to solution C, it turns green.

Based on the observations, which of the following is the correct sequence for the nature of solutions A, B, and C? (i) Acidic, acidic, and acidic (ii) Neutral, basic, and basic (iii) Basic, basic, and acidic (iv) Basic, basic, and basic

Let's analyze each solution. Solution A turns red litmus to blue, so it is basic. Solution B turns turmeric solution to red, and turmeric turns red in basic solutions, so solution B is also basic. Solution C turns red rose extract to green, and red rose extract turns green in basic solutions, so solution C is also basic. Therefore, all three solutions are basic. The answer is (iv) Basic, basic, and basic.

Question 3: Observe and analyse the figures, in which red rose extract paper strips are used. Label the nature of solutions present in each of the containers.

This question refers to figures in your textbook. Based on what we have learned, if the red rose extract shows red colour, the solution is acidic. If it shows green colour, the solution is basic. If there is no change in colour, the solution is neutral. Please look at the figures in your textbook and label accordingly.

Question 4: A liquid sample from the laboratory was tested using various indicators:

Indicator: Red litmus - Change: No change Indicator: Blue litmus - Change: Turned red Indicator: Turmeric - Change: No change in colour

Based on the tests, identify the acidic or basic nature of the liquid and justify your answer.

Let's analyze this. Red litmus showed no change, which means the liquid is not basic. Blue litmus turned red, which means the liquid is acidic. Turmeric showed no change in colour, which is expected for acidic and neutral solutions, but since blue litmus confirmed it is acidic, we can say the liquid is acidic in nature.

Question 5: Manya is blindfolded. She is given two unknown solutions to test and determine whether they are acidic or basic. Which indicator should Manya use to test the solutions and why?

Since Manya is blindfolded, she cannot use visual indicators like litmus paper or red rose extract. She should use an olfactory indicator like onion strips, because she can detect the change in odour by smelling. This way, she can determine whether the solutions are acidic or basic without needing to see any colour change.

Question 6: Could you suggest various materials which can be used for writing the message on the white sheet of paper (given at the beginning of the chapter) and what could be in the spray bottle? Make a table of various possible combinations and the colour of the writing obtained.

This is a creative question. Let's think about it. The spray bottle contains the indicator, and the writing is done with a substance that will react with the indicator. Some possible combinations are: Writing with soap solution on turmeric paper, and spraying water — the writing will appear red. Writing with lemon juice on red rose extract paper, and spraying more red rose extract — the writing will appear darker red or may not be visible. Writing with baking soda solution on red rose extract paper, and spraying red rose extract — the writing will appear green. Writing with vinegar on blue litmus paper, and spraying water — the writing will appear red. There can be many such combinations. Try to create your own table with at least five combinations!

Question 7: Grape juice was mixed with red rose extract; the mixture got a tint of red colour. What will happen if baking soda is added to this mixture? Justify your answer.

Grape juice is likely acidic, which is why it gave a red colour with red rose extract. Baking soda is basic. When we add a base to an acidic solution, it undergoes neutralisation. So, the mixture will turn green because the base will neutralise the acid, and red rose extract gives green colour in basic solutions.

Question 8: Keerthi wrote a secret message to her grandmother on her birthday using orange juice. Can you assist her grandmother in revealing the message? Which indicator would you use to make it visible?

Orange juice is acidic. To reveal the message, we need an indicator that shows a colour change in acidic solutions. We can use blue litmus paper or red rose extract. If we sprinkle some blue litmus solution or red rose extract on the paper, the acidic orange juice will turn the indicator to red, making the secret message visible!

Question 9: How can natural indicators be prepared? Explain by giving an example.

Natural indicators can be prepared by extracting the coloured part of certain plants or flowers. For example, to prepare red rose extract, we collect red rose petals, crush them, soak them in hot water, and then filter the liquid. This filtered liquid can be used as an indicator. Similarly, turmeric paste can be used to make turmeric paper. Purple cabbage can be boiled in water to extract its juice, which can also be used as an indicator.

Question 10: Three liquids are given to you. One is vinegar, another is a baking soda solution, and the third is a sugar solution. Can you identify them only using turmeric paper? Explain.

Yes, we can identify them using turmeric paper. Vinegar is acidic. Turmeric does not change colour in acidic solutions, so the turmeric paper will remain yellow. Baking soda is basic. Turmeric turns red in basic solutions, so the turmeric paper will turn red. Sugar solution is neutral. Turmeric does not change colour in neutral solutions either, so the turmeric paper will remain yellow. But wait, both vinegar and sugar solution give the same result with turmeric! So, we can identify baking soda easily, but we cannot differentiate between vinegar and sugar solution using only turmeric paper. We would need another indicator like litmus paper to distinguish between acidic and neutral substances.

Question 11: The extract of red rose turns the liquid X to green. What will the nature of liquid X be? What will happen when excess of amla juice is added to liquid X?

Red rose extract turns green in basic solutions. So, liquid X is basic in nature. Amla juice is acidic. When excess amla juice is added to liquid X, the acid will neutralise the base. After neutralisation, if more acid is added, the solution will become acidic. So, initially the colour will change from green to some intermediate colour, and finally to red when the solution becomes acidic.

Question 12: Observe and analyse the information given in the following flowchart. Complete the missing information.

Imagine a garden with plants showing signs of poor health.

The soil can be acidic in nature. The soil can be basic in nature.

Which indicator can be used to test the nature of the soil? Litmus paper or any natural indicator like red rose extract or turmeric can be used.

The acidic soil can be treated with lime (calcium oxide or calcium hydroxide). The basic soil can be treated with organic matter like manure and composted leaves.

Now, let's look at the "Dive Deeper" question. Aman accidentally spilt vinegar on some pieces of an eggshell or marble and noticed bubbling. He then poured a soap solution on another piece of eggshell or marble, but no bubbles appeared. Why did bubbles occur with vinegar but not with soap solution?

Eggshell and marble contain calcium carbonate, which reacts with acids like vinegar (acetic acid) to produce carbon dioxide gas, which appears as bubbles. Soap solution is basic, but it does not react with calcium carbonate in the same way to produce gas. So, no bubbles appear with soap solution.

Now, let's look at the Exploratory Projects. You can create rangoli using acidic or basic substances and natural indicators. You can discuss in your class the acidic, basic, or neutral nature of water obtained from various sources. You may like to test the water samples available from sources such as rain, taps, rivers, and so on. You can also collect a soil sample of your area and find out whether it is acidic, basic, or neutral in nature.

Now, my dear students, we have covered the entire chapter. Let me give you a final summary of everything we learned today.

We started with the story of Ashwin and Keerthi at the science fair and learned about how indicators work. We learned that substances can be classified as acidic, basic, or neutral. We learned about litmus as an indicator — blue litmus turns red in acidic solutions, and red litmus turns blue in basic solutions. We learned that acidic substances taste sour, like lemon, amla, tamarind, and vinegar. We learned that basic substances feel slippery to touch and taste bitter, like soap, baking soda, and lime water. We learned about natural indicators like red rose extract, which gives red colour in acidic solutions and green colour in basic solutions. We learned about turmeric as an indicator, which turns red in basic solutions but remains yellow in acidic and neutral solutions. We learned about olfactory indicators like onion, which change their smell in acidic or basic solutions. We learned about the great Indian chemist Prafulla Chandra Ray. We learned about neutralisation reactions — when an acid reacts with a base, they form salt and water, and heat is released. We learned about many applications of neutralisation in daily life — treating ant bites, treating acidic soil, and treating factory waste. We also solved all the exercises from the chapter.

I hope you enjoyed this lesson as much as I enjoyed teaching you! Remember, science is all around us. Look for examples of acids, bases, and neutral substances in your daily life. Try making your own indicators at home. Keep exploring and keep learning!

Thank you so much for your attention. See you in the next lesson!