CBSE Class 10 Science Practical Skills – pH of Samples

CBSE Class 10 Science Practical Skills – pH of Samples

BASIC BUILDING CONCEPTS
The acidic, basic or neutral character of any solution can be expressed by expressing its H⁺ ion concentration. A Danish biochemist Peter Sorensen suggested a method of expressing the concentration of H⁺ ions in terms of pH. In Danish language, pH stands for protenz de hydrogen which means power of H⁺ ions. pH of a solution is given by
pH = -log [H⁺] = log \(\frac { 1 }{ { [H }^{ + }] }\)
where [H⁺] = concentration of hydrogen ions in solution.
To report the pH, a scale from 0 to 14 was devised.

You can also download Class 10 Science NCERT Solutions to help you to revise complete syllabus and score more marks in your examinations.

• For an acidic solution, pH is less than seven.
• For a basic/alkaline solution, pH is greater than seven.
• For a neutral solution, pH is equal to seven.

The pH of a solution is measured accurately using an apparatus known as pH meter. An approximate value of pH can be obtained by using universal indicator/pH paper.
Indicators: are the substances which change the colour at a certain pH. Some common examples of indicators are litmus, phenolphthalein and methyl orange. Out of these, litmus is a natural indicator while phenolphthalein and methyl orange are synthetic indicators. The colour of these indicators in different medium is given in the following table:Universal indicator is a homogeneous mixture which is prepared by mixing a number of common indicators together. It can undergo series of colour change over a pH value ranging from 1-14 to indicate acidity or alkalinity of solutions.
Colour changes observed using universal indicator are given in the following table:
TABLE: Colour of Universal Indicators at Different pH Values
pH paper is a strip of paper which is coated with universal indicator. It is used to measure the acidity or alkalinity of aqueous solutions. To know the pH value of a solution, a drop of solution is placed on the pH strip. The colour produced tells the pH value of the test solution.

NATURE OF CHEMICALS USED IN THIS EXPERIMENT

1. Hydrochloric acid: The chemical formula of hydrochloric acid is HCl. This is a strong acid which is formed by dissolving hydrogen chloride gas in water. In aqueous solution, it gets ionized completely and forms hydrogen ions and chloride ions. The pH of hydrochloric acid is less than 7.
   
2. Sodium hydroxide: The chemical formula of sodium hydroxide is NaOH. In laboratory, it is found in the form of pellets or flakes. Sodium hydroxide is a strong base. On dissolving in water, sodium hydroxide gets ionized completely into sodium ion and hydroxide ion. The pH of sodium hydroxide solution is more than 7.
   
3. Ethanoic acid: Ethanoic acid is also known as acetic acid. Its chemical formula is CH₃ It is a weak acid. In aqueous solution, it gets ionized partially to give hydrogen ion and acetate ion. The pH of ethanoic acid solution is less than 7.
   
4. Lemon juice: Lemon juice is a mixture of acids, carbohydrates and minerals. Due to presence of acids, it is acidic in nature and has pH less than 7.
5. Water: The chemical formula of water is H₂ It is neutral in nature and thus, its pH value is 7.
6. Sodium bicarbonate: It is also known as baking soda or sodium hydrogen carbonate. Its- chemical formula is NaHCO₃. It is a weak base. On dissolving in water, it gets ionized partially to give sodium ion and bicarbonate ion. The pH of solution of sodium bicarbonate is greater than 7.
   

AIM
To find the pH of the following samples by using pH paper/universal indicator:

1. Dilute hydrochloric acid (HCl)
2. Dilute sodium hydroxide  (NaOH) solution
3. Dilute ethanoic acid (CH₃COOH) solution
4. Lemon juice
5. Water
6. Dilute sodium bicarbonate (NaHCO₃) solution.

MATERIALS REQUIRED
Test tubes, test tube stand, droppers or glass rod, pH paper/universal indicator, standard colour chart, glazed white tile and samples of dil. HCl, dil. NaOH, dil. ethanoic acid (acetic acid/vinegar), lemon juice, distilled water and dil. sodium bicarbonate solution.

THEORY

1. pH is the measure of hydrogen ion concentration of a solution.
2. The hydrogen ion concentration (H⁺) for an acidic solution is always greater than 10^-7 mol L^-1 and its pH is, therefore, always less than 7.
3. The hydrogen ion concentration of a basic solution is always less than 10^-7 mol L^-1 and, therefore, its pH is always greater than 7.
4. The hydrogen ion concentration of a neutral solution is 10^-7 mol L^-1 and, therefore, its pH is 7.
5. The pH of a solution can be measured by using a pH paper, universal indicator or pH meter.

PROCEDURE

1. Take the given solutions in separate test tubes marked as A, B, C, D, E and F and keep them in the test tube stand.
   
2. Take six strips of pH paper and place them on a glazed white tile.
3. Using a dropper or a glass rod, place a drop of the test solution on the pH paper.
4. Note the colour developed on the pH paper and compare it with the colour chart of the pH paper.
5. Record the pH value corresponding to the colour.
6. Similarly, using a fresh dropper each time, perform the experiment with the remaining test samples using a fresh strip of pH paper.
7. Record your observations as indicated in the table below.
8. For using universal indicator, add a few drops of universal indicator to each of the test tubes with the test solutions.
9. Note the colour of each solution and compare it with colour on the indicator bottle.
10. Record your observations.

OBSERVATION TABLE

RESULT

1. The pH of dilute solutions of hydrochloric acid, ethanoic acid and lemon juice is less than 7 and therefore, they are acidic in nature.
2. The pH of dilute solutions of sodium hydroxide and sodium bicarbonate is more than 7 and therefore, these solutions are basic in nature.
3. The pH of water is 7 and therefore, it is neutral in nature.

PRECAUTIONS

1. Mark the test tubes carefully.
2. Use distilled water for preparing solutions.
3. Use only standard colour charts supplied with the pH paper for assessing the pH value.
4. Do not touch the pH paper with unclean and wet hands.
5. Keep the pH paper away from chemical fumes. .
6. Do not touch or taste the solutions.
7. Clean the glass rod/dropper properly before reusing.
8. Do not waste pH paper.

INTERACTIVE SESSION

Question 1:
What is the aim of your experiment?
Answer:
The aim of my experiment is to determine the pH of some sample solutions.
Question 2:
What do you understand by pH?
Answer:
pH is the measure of acidity or alkalinity of a sample.
Question 3:
Define pH.
Answer:
pH is defined as the negative logarithm of the hydrogen ion concentration in moles per litre.
Question 4:
What is the pH of an acidic solution?
Answer:
It is less than 7.
Question 5:
What is the pH of a solution with [H⁺] = 10^-9? Is the solution acidic or basic?
Answer:
Since pH = 9, therefore, the solution is basic in nature.
Question 6:
What happens to the pH of pure water if a small amount of acid is added to it?
Answer:
The pH becomes less than seven as the [H⁺] concentration increases.
Question 7:
For a neutral solution, what is the concentration of [H⁺] and [OH^–] in solution?
Answer:
Since for a neutral solution [H⁺] = [OH^–], therefore, their concentrations are 10 ^-7 mol L^-1.
Question 8:
What is an acid-base indicator?
Answer:
Indicator is a substance which tells us whether a substance is acidic or basic by change in colour.
Question 9:
Name any two indicators.
Answer:
Litmus and methyl orange

Question 10:
What is the pH of a soap solution?
Answer:
It lies within the basic region, i.e., > 7.

Question 11:
What is the pH of rainwater and why?
Answer:
pH of rainwater < 7 due to dissolution of acidic gases like NO₂ and SO₂.

Question 12:
What are bases?
Answer:
Substances furnishing OH” ions in solution are called bases.

Question 13:
Which out of NH₄OH and NaOH will have a higher pH and why?
Answer:
NaOH will have a higher pH value as it is a strong base, whereas NH₄OH (ammonium hydroxide) is a weak base.

Question 14:
Why is NH₄OH a weak base?
Answer:
It is a weak base because it ionizes partially in solution.
NH₄OH(aq) \(\)\leftrightharpoons /latex] NH₄⁺(ag) + OH^– (aq).

Question 15:
What is the pH of fluids in our stomach?
Answer:
2

Question 16:
Why?
Answer:
Due to secretion of HCl in gastric juice.

Question 17:
How do we cure hyperacidity?
Answer:
By using an antacid.

Question 18:
What is an antacid?
Answer:
Which increases the pH of stomach fluids as it is basic in nature.

Question 19:
Name any two antacids.
Answer:
Milk of magnesia, Sodium bicarbonate.

NCERT LAB MANUAL QUESTIONS

Question 1:
What is the pH of pure water at 25 C (298K)?
Answer:
The pH of pure water at 298K is 7.

Question 2:
What according to you should be the pH of dil. HCl and dil. NaOH solutions? Observe and explain your findings.
Answer:
Dil. HCl is an acid as it furnishes hydrogen ion in solution and, therefore, its pH will be less than 7.
HCl(aq) ——> H⁺(aq) + Cl^–(aq)
Dil. NaOH on the other hand, is a base as it furnishes hydroxyl ions in solution and therefore, has pH greater than 7.
NaOH(aq) —–> Na⁺(ag) + OH^–(aq)

Question 3:
On opening the soda water bottle the dissolved CO₂ comes out. Would the pH of the solution increase or decrease as the gas comes out? Explain your answer either way.
Answer:
CO₂ in soda water bottle exists as carbonic acid (H₂CO₃). On opening the bottle, CO₂ escapes out. Therefore, the concentration of carbonic acid decreases and pH increases.

PRACTICAL BASED QUESTIONS

Multiple Choice Questions/VSA

Question 1:
On putting a few drops of an unknown liquid on the pH strip as shown here, the colour of pH strip changed to violet. The liquid taken is likely to be  [CBSE 2012, AI 2010]

(a) dilute hydrochloric acid
(b) dilute sodium hydroxide
(c) water
(d) dilute acetic acid.

Question 2:
Bottle ‘A’ contains oxalic acid and bottle ‘B’ contains sodium carbonate solution. When pH paper is dipped in each of the solutions, the colour seen in ‘A’ and ‘B’ respectively be  [CBSE Sample Paper 2009]
(a) orange, blue
(b) blue, orange
(c) green, blue
(d) orange, green

Question 3:
In an experiment to test the pH of a given sample using pH paper, four students recorded the following observations: [CBSE 2012]

Which one of the above observations is incorrect?
(a) I (b) II (c) III (d) IV

Question 4:
To test the presence of a base with a strip of blue litmus you would
(a) moisten the strip with water and dip in the given solution
(b) dip the strip directly into the given solution
(c) dip the strip firstly into an acidic solution and then use it to test the sample
(d) dip the strip into an alkaline solution and then use it to test the sample.

Question 5:
Which of the following solutions with same concentrations will have lowest pH?
(a) CHgCOOH
(b) H₂C0₃
(c) HCl
(d) HCOOH (formic acid)

Question 6:
The pH of NaOH solution is 10.6. On addition of water to this solution its pH
(a) remains same       (b) increases
(c) decreases              (d) becomes 7

Question 7:
Which one is correct for a universal indicator?
(a) It is a solution of phenolphthalein and methyl orange
(b) It is a solution of aq. HCl and aq. NaOH
(c) It is a solution of methyl orange in water
(d) It is a mixture of many indicators

Question 8:
The correct method of finding the pH of a solution is to [CBSE 2012, Delhi 2007]
(а)  heat the solution in a test tube and expose the pH paper to the vapours formed
(b)  pour few drops of the solution from the test tube on the pH paper
(c) drop the pH paper in the solution
(d) put a drop of the solution on the pH paper using a dropper

Question 9:
Two solutions A and B were found to have pH values of 6 and 8 respectively. The inference which can be drawn is [Delhi 2007C]
(а) the strength of solution B is higher than that of A
(b) A is an acid while B is a base
(c) both are acidic solutions
(d) both are basic solutions

Question 10:
Four students (A), (B), (C) and (D) separately measured the pH values of each one of the given samples of distilled water, acetic acid, dilute hydrochloric acid and a solution of sodium hydroxide using pH papers. [CBSE 2011]

Which one of the following represents correct pH value?
(a) A (b) B (c) C id) D

Question 11:
Solid sodium bicarbonate was placed on a strip of pH paper. The colour of the strip [Delhi 2008]
(a) turned blue
(b) did not change
(c) turned green and suddenly yellow
(d) turned light pink.

Question 12:
A student was given four unknown colourless samples labelled A, B, C and D and asked to test their pH using pH paper. He observed that the colour of pH paper turned to light green, dark red, light orange and dark blue with samples A, B, C and D respectively. The correct sequence of increasing order of the pH value for samples is
(a) A  < B < C  < D
(b) A < D <  C < B
(c) C < B  < A  < D        
(d) B < C <  A  < D

Question 13:
A student has four samples A, B, C, D containing dil. HCl, aq. KCl, dil. NaOH and distilled water respectively. The solutions with equal pH are
(a) A and B
(b) B and C
(c) C and D
(d) B and D

Question 14:
What is the correct order for increasing values of pH?
(a) Water < fruit juice < soap solution
(b) Fruit juice < soap solution < water
(c) Fruit juice < water < soap solution
(d) Soap solution < water < fruit juice

Question 15:
On adding methyl orange to solution ‘A’, it imparts a red colour and on adding it to solution ‘B’ a yellow colour is obtained. Solutions ‘A’ and ‘B’ are respectively
(a) neutral, acidic
(b) acidic, basic
(c) basic, acidic
(d) neutral, basic

Short Answer Questions
1. A solution ‘X’ gives orange colour when a drop of  universal indicator is added to it. On the other hand, another solution ‘Y’ gives bluish-green colour when a drop of universal indicator is added to it.
What are the types of solution ‘X’ and ‘Y’ and what type of pH would they have?

2. Hydrochloric acid is a strong acid while acetic acid is a weak acid. Why?

3. How can the pH of a solution be determined?

4. What is an acid-base indicator? Give any two examples of synthetic indicators.

5. Five solutions P, Q, R, S and T when tested with universal indicator showed pH of 13, 8, 1, 7 and 5 respectively.
(a) Which solution is (i) strongly alkaline (ii) weakly acidic?
(b) Arrange the pH in the increasing order of hydrogen ion concentration.

 ANSWER KEY

Multiple Choice Questions /VSA
1. (b)
2. (a)
3. (a)
4. (c)
5. (c)
6. (c)
7. (d)
8. (d)
9. (b)
10. (d)
11. (b)
12. (d)
13. (d)
14. (c)
15. (b)

Short Answer Questions

1. pH is a number which indicates acidic or basic nature of a solution.
Solution X’ is acidic in nature, i.e. pH < 7.
Solution Y is basic in nature, i.e., pH > 7.

2. The strength of acids depends on the number of H⁺ ions produced in them. Acids which on dissolving in water give rise to more H⁺ ions are called strong acids and acids which give less H⁺ ions are called weak acids. In aqueous solution, hydrochloric acid ionises completely to give more H⁺ ions and therefore, hydrochloric acid is a strong acid. In aqueous solution, acetic acid ionises partially to give less H⁺ ions and therefore, it is a weak acid.

3. pH of a solution can be determined by two methods:

1. By using universal indicator: To measure the pH of a solution, a paper strip impregnated with universal indicator called pH paper can be used
2. By using pH meter: With the help of pH meter, pH of a solution can be measured accurately.

4. A substance that indicates the acidity or basicity of a solution through characteristic colour changes is called acid-base indicator. Methyl orange and phenolphthalein are the two examples of synthetic indicators.

5. (a) (i) Solution- P (ii) Solution- T
(b) The solution with highest pH (13) will have minimum hydrogen ion concentration whereas solution having the least pH (1) will have maximum hydrogen ion concentration. So, we can arrange the given solutions in increasing order of their hydrogen ion concentrations as follows:

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