Dorji has a few bottles of soft drink in his restaurant. But, unfortunately, these are not labelled. He has to serve the drinks on the demand of customers. One customer wants acidic drink, another wants basic and third one wants neutral drink. How will Dorji decide which drink is to be served to whom?

Arun, a seventh grade student, was taking a bath one hot summer day when he slipped due to the soap and unintentionally tasted some soap. He remembered that he had experienced a similar taste after consuming baking soda.

a) What kind of taste did he experience?

b) Which substance in the soap is responsible for this kind of taste?

c) What kind of colour change is seen when drops of soap solution is poured on a litmus paper?

The Thomas Jefferson Memorial in Washington, D.C. is one of the most recognisable buildings in the United States, and Rohan went there with his parents while on vacation. Rohan liked the beauty of the building. According to his father, the structure purportedly suffered acid rain damage, losing part of its former splendour before being restored more recently.

a) Why rain becomes more acidic in some places?

b) Why are limestone or marble buildings easily damaged by acid rain?

Vanika learned in her previous class that the substances those are sour in taste are called acids. She has been provided with two chemical compounds represented as $\mathrm{HX}$ and $\mathrm{YOH}$. She could not taste them because she does not know whether they are edible or not. It was then explained to her that the chemical compounds that have replaceable hydrogen are called acids. According to Vanika both the compounds given to her are acids as they both have hydrogen atoms.

Explain what is meant by a replaceable hydrogen atom in a compound. On the basis of this, help Vanika to identify which compound will actually act as an acid.

Twin brothers Raju and Ramu are in the seventh grade. One day, their mother made a solution of baking soda for a cake.

Raju and Ramu decided to examine the nature of the solution after transferring a small amount of the baking soda solution to another cup. They were advised by their teacher the day before that we may use China rose as an indicator to determine the nature of the solution. In their garden, they noticed China roses, and they collected them for an experiment. Answer the questions below:

a) How is China rose used as an indicator?

b) What colour change will be visible in the baking soda solution if the China rose indicator is added?

Jaspreet was given with few solutions and he noted their colour change on blue and red litmus paper.

Which of the solution will react with solution D to give a neutral substance? Explain why solution C did not show any colour change when it is placed on either of the litmus papers.

A group of students is given two beakers labelled A and B by the teacher. One beaker was filled with dilute nitric acid, and the other with dilute sodium hydroxide solution. The teacher then instructed students to use the phenolphthalein indicator to determine which beaker contained nitric acid and which beaker contained sodium hydroxide solution. Students used phenolphthalein to test both beakers and obtained the results. Beaker A contains sodium hydroxide solution, while Beaker B contains the nitric acid solution.

Based on the above understanding, how does the phenolphthalein indicator work? Is phenolphthalein a natural or human-made indicator? Explain your answer.

Rajiv was eating "curd curry with rice" popularly known as Kadhi chawal. Mistakenly the curry was spilt on the white shirt of Rajiv, and it got a yellow stain of curry. The next day, when Rajiv's mother was doing laundry, he observed that as soon as her mother rubbed the curry stain with the soap cake, the yellow stain turned reddish-brown.

On rinsing the shirt with plenty of water, the reddish-brown stain turned yellow again. Name the natural indicator which is possibly present in curry stain on Rajiv's shirt. Also help Rajiv to understand the reason behind these colour changes.