New
New
Year 10
Edexcel
Foundation

pH scale and indicators

I can explain the pH scale and appropriately use universal indicator, phenolphthalein, methyl orange, and litmus and pH meters to identify different solutions.

New
New
Year 10
Edexcel
Foundation

pH scale and indicators

I can explain the pH scale and appropriately use universal indicator, phenolphthalein, methyl orange, and litmus and pH meters to identify different solutions.

Lesson details

Key learning points

  1. The pH scale ranges from 0 to 14, indicating how acidic or basic a substance is.
  2. Universal indicator provides a full range of colours corresponding to different pH levels, and this can be a limitation.
  3. Phenolphthalein changes from colourless to pink in alkaline conditions.
  4. Methyl orange shifts from red in acidic conditions to yellow in basic conditions.
  5. Litmus paper turns red in acidic conditions and blue in basic conditions, the solution will be purple when neutral.

Common misconception

All indicators provide precise pH values and give the same information as pH meters.

Indicators do not provide exact pH values; they only indicate whether a solution is acidic, neutral, or alkaline by changing colour. A pH meter, on the other hand, measures the exact pH value of a solution and is more accurate.

Keywords

  • PH scale - a scale of acidity or alkalinity, often shown to be between values of 0 and 14

  • Indicator - a solution or paper which will change a different colour depending on pH of a solution

  • PH meter - an electric device which measures pH and provides a value after being calibrated with reference samples

  • Acidity - the concentration of hydrogen ions (H⁺) present in a solution; acids have a pH value less than 7

  • Alkalinity - the concentration of hydroxide ions (OH⁻) present in a solution; alkalis have a pH value greater than 7

Encourage students to understand that while indicators are useful for quick assessments, pH meters provide more precise measurements, especially when an exact pH value is needed.
Teacher tip

Equipment

spotting tiles, UI solution, phenolphthalein solution, methyl orange solution (fresh), litmus paper (red and blue), pH meter (optional)

Content guidance

  • Risk assessment required - chemicals
  • Risk assessment required - equipment

Supervision

Adult supervision required

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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6 Questions

Q1.
What is the pH range of acids?
Correct answer: 0 to 7
0 to 14
7 to 14
4 to 10
Q2.
Which of the following is common laboratory equipment used to measure pH?
burette
volumetric pipette
Correct answer: pH meter
beaker
Q3.
What colour does litmus paper turn in an acidic solution?
blue
green
Correct answer: red
yellow
purple
Q4.
What does a pH value of 7 indicate?
acidic solution
basic solution
Correct answer: neutral solution
strong acid
Q5.
Which ion's concentration is high in acidic solutions?
OH⁻ (hydroxide)
Na⁺ (sodium)
Correct answer: H⁺ (hydrogen)
Cl⁻ (chloride)
Q6.
What should you always wear to protect your eyes in a laboratory?
gloves
lab coat
mask
Correct answer: safety goggles

6 Questions

Q1.
How does the pH scale indicate whether a substance is acidic or basic?
By showing the colour change in universal indicator.
Correct answer: By the number ranging from 0 to 14.
By measuring the temperature of the solution.
By indicating the volume of the solution.
Q2.
What colour does methyl orange turn in acidic conditions?
Correct answer: red
yellow
green
blue
Q3.
What is the pH of a neutral solution?
Correct Answer: 7
Q4.
Which indicator changes from colourless to pink in alkaline conditions?
litmus paper
methyl orange
Correct answer: phenolphthalein
bromothymol blue
Q5.
How does litmus paper indicate a neutral solution?
turns blue
turns red
Correct answer: remains unchanged
turns green
Q6.
What is a limitation of using universal indicator?
It only shows one colour for all pH levels.
Correct answer: It provides a full range of colours but might not pinpoint the exact pH.
It cannot be used with strong acids or bases.
It changes colour too slowly.