Experiments Based on pH Change

Chemistry Experiment - Experiments Based on pH Change

A pH of the solution is the concentration of hydronium ions in a given solution. It is mathematically represented by the negative logarithm of the concentration of hydronium ions in a solution: pH= -log[H3O+]. A pH scale helps to understand whether a given solution is acidic or basic. An acidic solution contains more H3O+ ions as compared to OH- ions; a basic solution contains more OH- ions as compared to H3O+ ions; a neutral solution contains equal amounts of H3O+ and OH- ions. Knowing the pH of a solution is very important, as various vital processes of many organisms are dependent on the pH.

Table of Contents

• Aim

• Apparatus Required

• Theory

• Procedure

• Observations

• Result

• Precautions

• Lab Manual Questions

• Viva Questions

• Practical Based Questions

Aim 

To determine the pH of various solutions using pH paper. 

Apparatus Required

1. 0.1 M H2SO4

2. 0.1 M NaOH 

3. 0.1 M CH3COOH 

4. 0.1 M NH4OH

5. Soapy Water 

6. Orange Juice

7. Test Tubes 

8. Test Tube Stand

9. Dropper

10. pH Paper

11. pH Paper Colour Indicator.

Theory

1. pH is the amount of hydronium ions present in a given solution. 

2. The pH of the solution determines whether a solution is acidic or basic. 

3. The pH scale ranges between 0 and 14 from which 0-6 indicates that the solution is acidic, 8-14 indicates the solution is basic and 7 pH means the solution is neutral having an equal number of H3O+ and OH- ions. 

4. The pH paper or Universal indicator can be used to determine the pH of a solution, which indicates the acidity or basicity of a solution based on colour change. 

5. A pH indicator chart indicates colours and their corresponding pH.

Procedure

1. Take 6 test tubes and label them.

2. Add 1 ml of the given solution into each test tube.

3. Take pH paper and cut small strips of it.

4. Use forceps to catch the pH paper strips and dip them into test tubes containing solutions.

5. Observe the colour change of the pH paper strip.

6. Compare this colour change of the pH paper with the pH indicator chart.

7. Record your observations and note down the conclusion.

Observations and Result

This diagram shows the pH range and their corresponding colours

Precautions

1. Be careful with the chemicals used.

2. Wear a lab coat while working in the lab.

3. Do not waste pH paper strips or chemicals used.

Lab Manual Questions

1. Give examples of strong acids and strong bases.

Ans: Strong acids are H2SO4, HCl and HNO3. Strong bases are NaOH, KOH and Ca(OH)2.

2. What is the difference between strong acids and weak bases?

Ans: The difference between strong acids and weak bases are given below:

3. What is the pH of a solution?

Ans: pH of a solution is known as the amount of H+ ions present in a given solution. It can be determined by using pH paper, universal indicator etc. Mathematically, it can be determined by pH = -log[H3O+].

4. What will be the colour change when pH paper is dipped in HCl, KOH and H2O?

Ans: The Following colour change is seen when pH paper is dipped in the above solutions:

• HCl gives red colour since it is a strong acid.

• KOH gives a violet colour since it is a strong base.

• H2O gives green colour since it is neutral.

Viva Questions

1. The colour change is sharper at which pH?

Ans: The colour change is sharper when the pH is 10 as compared to other changes of colour in the pH observed through the pH paper.

2. Does the addition of water to a buffer or evaporation of water from a buffer solution change the pH of the buffer?

Ans: Upon the addition of water or elimination of water from the buffer solution, the pH of the buffer will not change.

3. In which case, pH will not change on dilution?

Ans: A buffer is an aqueous solution made between a weak acid and a weak base. The pH of such solutions does not change even if they are diluted. Hence, such solutions are used for preservation.

4. How is pH paper prepared?

Ans: pH paper is prepared by dipping a strip of paper into various basic and acidic indicators, followed by drying the paper.

5. What are other methods of determining the pH of a solution?

Ans: A universal indicator can be used for determining the pH of a solution, and it also shows the colour change based on the acidity or basicity of a solution.

6. What is a pH metre?

Ans: pH metre is an instrument which is used to measure the activity of hydrogen ions in a given solution. It is used to determine whether a given solution is acidic or basic.

7. Why does the NaCl solution show neutral pH?

Ans: Salts, which are formed from a combination of strong acid-HCl and strong base-NaOH, do not hydrolyse, but they form H2O molecules when dissolved in water. Therefore, their pH remains neutral.

8. What are pH indicators?

Ans: pH Indicators are chemical compounds used in minute quantities to determine the pH of a solution. These indicators, upon reaction with the solution, show colour change which helps in determining whether the solution is acidic or basic.

9. What are weak acids?

Ans: Weak acids are compounds which are weak electrolytes and partially dissociate in an aqueous solution, giving less amount of free Hydrogen ions. For example, H2S, CH3COOH, H2CO3 etc. 

10. What is a weak base?

Ans: Weak bases are compounds which are weak electrolytes and partially dissociate in an aqueous solution, giving less amount of free Hydroxide ions. For example, Carbonates, Bicarbonate, NH4OH, etc.

Practical Based Questions (MCQs)

1. pH + pOH=______

1. 15

2. 16

3. 14

4. 17

Ans: 14

2. Which of the following will show resistance to change in pH?

1. Hydrochloric acid

2. Sodium Hydroxide

3. Acetic acid

4. A mixture of Acetic acid and Sodium Acetate

Ans: Mixture of Acetic acid and Sodium Acetate

3. A buffer solution resists changes in pH____

1. In addition, only acid

2. In addition, acid or base

3. In addition, only weak base

4. In addition, only strong acids and water

Ans: Upon the addition of acid or base 

4. Experiments based on pH change do not involve which of the following:

1. Universal indicator

2. pH indicator chart

3. Burette

4. Dropper, test tube

Ans: Burette

5. What does ‘p’ in ‘pH’ indicate?

1. Price

2. Proportion

3. Power

4. Per

Ans: Power

6. Find the odd man out:

1. Sodium hydroxide

2. Sulphuric acid

3. Calcium hydroxide

4. Ammonium hydroxide

Ans: Sulphuric acid

7. Phenolphthalein indicator is _____in acidic solution and____ in basic solution?

1. Colourless, Pink

2. White, Yellow

3. Pink, Colourless

4. Yellow, White

Ans: Colourless, Pink

8. Which of the following are pH indicators?

1. Bromophenol blue

2. Methyl orange

3. Phenolphthalein

4. All of the above

Ans: All of the above

9. Universal indicator is_____

1. A mixture of acidic indicators

2. A mixture of basic indicators

3. A mixture of both acidic and basic indicators

4. A mixture of strong acids

Ans: Mixture of both acidic and basic indicators

10. Find the odd man out:

1. Hydrogen sulphide

2. Nitric acid

3. Acetic acid

4. Carbonic acid

Ans: Nitric acid

Conclusion

The pH of a given solution indicates the presence of the amount of hydronium ions present in that solution. The pH helps us to understand whether a given solution is acidic or basic. Acidic solutions show a pH below 7 and basic solutions show a pH above 7, pH 7 is neutral. Indicators such as phenolphthalein, methyl orange etc. are used to determine the pH of a solution.