NCERT Class 7 Science Curiosity Chapter 02 Exploring Substances Acidic Basic and Neutral PDF Download

Chapter 2: Exploring Substances: Acidic, Basic, and Neutral

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.

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.

[Figure 2.1: Scene of a science fair, See in your textbook]

2.1 Nature - Our Science Laboratory

2.1.1 Litmus as an indicator

Activity 2.1: Let us explore

• 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 Fig. 2.2a.
• Do you observe any change in the colour of the blue litmus pieces?
• Record your observations in Table 2.1.
• Repeat the same activity with pieces of red litmus paper as shown in Fig. 2.2b and record your observations in Table 2.1.

[Figure 2.2(a): Colour change in blue litmus paper, See in your textbook]

[Figure 2.2(b): Colour change in red litmus paper, See in your textbook]

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.

Teacher's Note

When you do this activity, be careful to use just one drop per sample and let the litmus paper dry completely before checking the colour. A common mistake is to use too much liquid, which can make the colour change unclear. If you are unsure whether the colour has changed, compare your piece with an unused litmus strip of the same colour.

Table 2.1: Testing the nature of samples with blue and red litmus papers

Now, let us analyse Table 2.1 and sort the samples into three groups as follows -

• 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 affect either of the two litmus papers.

Record the data in Table 2.2.

Table 2.2: Grouping of samples tested in Table 2.1

Teacher's Note

Look at your Table 2.1 results carefully. The three groups in Table 2.2 represent three different types of substances. Group A substances make blue litmus red - these are called acidic. Group B substances make red litmus blue - these are called basic. Group C substances that change neither colour are called neutral. You will learn the names of these three groups in the next section, but notice the pattern now from your own observations.

Key Points

• Litmus is a natural indicator that changes colour to show if a substance is acidic, basic, or neutral.
• Blue litmus paper turns red when placed on acidic substances, and red litmus paper turns blue when placed on basic substances.
• Substances that do not change the colour of either blue or red litmus paper are neutral.
• Common examples of acidic substances include lemon juice and vinegar, while soap solution and baking soda solution are basic.

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