Exploring Substances Acidic Basic and Neutral Class 7 Notes Science Chapter 2

Class 7 Science Chapter 2 Notes Exploring Substances Acidic Basic and Neutral

In a Nutshell

→ Substances around us may be classified as acidic, basic, and neutral in nature.

→ Extracts of lichen, red rose, red hibiscus, purple cabbage, turmeric, etc., 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.

On 28 February, the school hosted a science fair to celebrate National Science Day. At the entry gate, siblings Ashwin and Keerthi were greeted with a white sheet of paper. They were curious to know why a white sheet of paper was given to them!

A few steps ahead, there was a volunteer spraying a liquid on these sheets of paper. The siblings also got their white sheets sprayed. To their surprise, the words ‘Welcome to the Wonderful World of Science’ appeared on the papers the moment the liquid was sprayed (Fig. 2.1). They were excited and eager to know how this happened and the reason behind it.

Fig. Scene of a science fair
Their curiosity was partly satisfied at the ‘Colourful World of Substances’ stall. They saw many activities showing colour changes on mixing different substances. They decided to explore these changes further. Let us join them on this learning adventure.

Nature – Our Science Laboratory

Litmus as an indicator
Collect samples of lemon juice, soap solution, amla juice, tamarind water, vinegar, baking soda solution, lime water, tap water, washing powder solution, sugar solution, and salt solution.

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, one-by-one, on these litmus paper pieces, as shown in Figure (a).

Fig. (a): Colour change in blue litmus paper

Fig. (b): Colour change in red litmus paper

How to prepare lime water?
Do not confuse lime water with the word lime, which is a fruit similar to lemon.
Lime water (solution of calcium hydroxide in water) can be easily prepared by mixing lime (chuna, i.e. calcium oxide) in water and leaving it undisturbed for some time, say an hour. Filter the liquid into another container and use it as lime water.

• Group A with samples that turn the blue litmus paper to red.
• Group B with samples that turn the red litmus paper to blue.
• Group C with samples that do not aff ect either of the two litmus papers

Let us find out!
Litmus is a natural substance obtained from lichens. It 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, as shown in Figure.

Fig. Blue and red litmus paper strips

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 diff erent 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.

Holistic Lens
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. Do you find lichens on trees in your neighbourhood?

Now, let us classify the substances sorted in Group A, Group B, and Group C.

• The substances in Group A, such as lemon juice, amla juice, tamarind water, and vinegar turned the blue litmus paper to red, implying that these substances are acidic in nature.
• The substances in Group B, such as soap solution, baking soda solution, lime water, and washing powder solution turned the red litmus paper to blue. Hence, these substances are basic in nature.
• The substances in Group C, such as tap water, sugar solution, and salt solution, did not change the colour of either litmus paper. Can you predict their nature?
• These substances are said to be neutral because they are neither acidic nor basic

Let us relate and explore

Are all the substances in Group A of Table 2.2 edible? Have you ever tasted these edible substances? Can you recall their taste? You will find that all these substances taste sour. Thus, we can say that substances that taste sour tend to contain acids and are acidic in nature.

Caution  — Do not taste anything until asked to do so. Do not taste any unknown substance.
Some common edible substances and the names of the most common acids present in them are given in Figure.

Fig. The most common acids present in some edible substances

Now, let us take one of the substances — baking soda solution — from Group B. Rub the baking soda solution between your fingers. What do you observe?
It feels soapy or slippery. Basic substances are generally slippery to touch.

Also, bases generally taste bitter, but everything that tastes bitter may not contain a base. For example, bitter gourd (karela) possesses a bitter taste but is not basic in nature.

Red rose as an indicator
You might have observed many coloured flowers in your surroundings. Try making your indicators using these flowers.

Let us prepare

Fig. Red roses

Fig. Red rose petals immersed in hot water

• Collect some fallen petals of red roses available in your surroundings (Figure). It is advised not to pluck flowers. You may pick petals or flowers fallen on the ground.
• Take a fistful of the collected petals of red roses 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.

Caution — Perform this step under the supervision of an adult.

• Cover the glass tumbler with a lid. Wait for 5-10 minutes till the water becomes coloured (Fig. 2.6), and filter it.
• The filtrate (liquid after filtration) is the required flower extract (Fig. 2.7) to be used as an acid-base indicator.

Fig. Test tube containing the red rose extract

Let us find out

• Place 10-20 drops of the prepared red rose extract in each of two small transparent bottles or test tubes. Mark them A and B.
• Add 20-30 drops of lemon juice in test tube A and 20-30 drops of soap solution in test tube B with the help of droppers.
• Observe and record any colour changes (Fig. 2.8) to the extract in Table 2.3.
• Repeat the same with the other samples used in Activity 2.1 and record your observations in Table 2.3.

Fig. The changes in colour of the red rose extract on adding lemon juice (A) and soap solution (B)

From the above activity, we can conclude that the red rose extract can also be used to test the nature of the substances; hence, it is another example of an acid-base indicator. We can conclude that the red rose extract seems to give red colour in an acidic solution and green colour in a basic solution.

We are sure that you will be excited about the above results. You may repeat the process of preparing the extract and testing substances with some vegetables, fruits, or flowers, such as beetroot, purple cabbage, turmeric, Indian blackberry (jamun), and red hibiscus (gudhal) flower. They can also act as acid-base indicators.

Fascinating Facts
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. Can gardeners alter the colour of hydrangea flowers by adjusting the acidic or basic nature of the soil?

Hydrangea flowers

Turmeric as an indicator
We have used blue and red litmus paper strips in Activity 2.1. Can you also make paper strips with some other natural indicators? Find out by performing the following activity.

Let us prepare

• Take a spoonful of turmeric (haldi) in a petri dish or container and add a little water to make a paste (Figure a). 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’ (Figure b).

Caution — Perform this step under the supervision of an adult.

Fig. Preparing turmeric paper

Based on the observations, we can conclude that turmeric paper can be used to test basic substances. However, it cannot differentiate between acidic and neutral substances.

Fascinating Facts

Why is turmeric known as a ‘Golden’ spice?
Turmeric is a member of the ginger family, which is grown in India and other countries.
A common spice in daily households, it 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 health1 benefits, and is commonly used in several traditional home remedies.

Ashwin created a greeting card to pay gratitude to his teacher on the day of Guru Purnima. He applied turmeric paste on white
paper and dried it. He wrote his wishes in the teacher’s mother tongue (Odia language) on the dried paper using one of the solutions tested in Table 2.4. Which solution can be used to write the message? His teacher applauded his creative use of this concept.

There are some substances whose odours change in an acidic or basic medium. These are called olfactory indicators.
Let us explore more!

Let us investigate

• 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.

What Happens When Acidic Substances Mix with Basic Substances?

Let us investigate by performing the following experiment.

Let us experiment

Figure (a): The colour of the solution on adding blue litmus solution

Figure (b): The colour of the solution on adding lime water

• 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 (Figure a)?
• 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 (Figure b).
• Again, add one drop of lemon juice to the above solution.

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 (i.e., release) of heat.
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!

Neutralisation in Daily Life
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 (Fig. 2.11). 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 (formic acid) into the skin. The effect of the acid can be neutralised by rubbing moist baking soda, which is a base.
What remedies do people use to treat ant bites in your region?

Situation 2: On the Farmer’s Portal (an online platform from the Department of Agriculture, Cooperation and Farmers Welfare), a query from a farmer states, “My plants are not growing well lately”. After a detailed discussion, it was found that the excessive use of chemical fertilisers (substances added to soil to help plants grow better) 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. (Figure).

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 with him 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 into the lake.
Let us wrap up!
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?

Class 7 Science Notes

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