Q22 of 216 Page 469

A student dips pH papers in solutions A and B and observes that the pH paper turns blue and orange respectively in them. What does he infer?

OR


A student took the following samples to find out their pH using pH paper: dilute hydrochloric acid, lemon juice, washing soda and soap solution. The teacher remarked that one of the samples taken was not proper. To which sample was the teacher referring to?

pH is a measure of acidity or basicity of an aqueous solution.


It is a mathematical notation that describes the power of an acid or a base. The concentration of the acid is represented as [H+] or [H3O+]


pH = – log [H+] (or)


pH = – log [H3O+]


The pH scale is the range of pH values from 0 to 14. The paper that indicates the complete range of pH value from 0 to 14 is called the universal indicator. pH paper is a strip of filter paper that is soaked in universal indicator and then dried.


When the pH value is between 0 and 7, it indicates the acidic range of the solution. When the value is between 7 and 14, it represents the basic or alkaline range of the solution. When pH = 7, it is called a neutral solution. The decrease of pH value indicates the increase of acidity and the increase of pH value shows the increase in alkalinity. Water is neutral and its pH is equal to 7.


Hence according to the question, he infers that A is sodium carbonate solution and B is acetic acid. The pH paper turns blue when dipped in solution A, it means A is a strong base having pH 9 and the pH paper turns orange when dipped in solution B, it means B is a strong acid having pH 4.


OR


pH is a measure of acidity or basicity of an aqueous solution.


pH paper is a strip of filter paper that is soaked in universal indicator and then dried.



From the samples what student has collected to test the pH value, washing soda is wrong selection.


Hence, the teacher was referring to washing soda because it is in powder form not in liquid form. To find pH using pH paper, the sample should be in liquid form.


Rest others - dilute hydrochloric acid, lemon juice, and soap solution are all in a liquid state and hence are good to be used for pH indication.


When the pH value is between 0 and 7, it indicates the acidic range of the solution. When the value is between 7 and 14, it represents the basic or alkaline range of the solution. When pH = 7, it is called a neutral solution. The decrease of pH value indicates the increase of acidity and the increase of pH value shows the increase in alkalinity.


More from this chapter

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20

i. Draw the pattern of magnetic field lines through and around a current-carrying solenoid. What does the magnetic field pattern inside the solenoid indicate?

ii. How can this principle be utilised to make an electromagnet?


iii. State two ways by which strength of this electromagnet can be increased.


OR


Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?

 21

i. (a) Water is an elixir of life, a very important natural resource. Your Science teacher wants you to prepare a plan for a formative assessment activity, “How to save water, the vital natural resource”. Write any two ways that you will suggest to bring awareness in your neighbourhood, on ‘how to save water.’

(b) Name and explain any one way by which the underground water table does not go down further.


ii. Explain two main advantages associated with water harvesting at the community level.

 23

When asked to set up an experiment to show that ‘light is necessary for photosynthesis’ , a student ran to the school garden and set up the experiment using a plant growing in the school garden. The experiment failed. His fellow student made the following suggestions to get success.

Student A: Safranin should be used instead of iodine.


Student B: The leaf should not be boiled in alcohol to remove chlorophyll before testing for starch.


Student C: Transparent paper strip should be used instead of black paper strip.


Student D: The plant should be de- starched before starting the experiment.


Which student made the right suggestion?

 24

What will happen if a student while studying the dependence of current on the potential difference keeps the circuit closed for a long time to measure the current and potential difference?