pH scale and indicators
Lesson details
Learning outcome
I can explain the pH scale and appropriately use universal indicator, phenolphthalein, methyl orange, and litmus and pH meters to identify different solutions.
Key learning points
- The pH scale ranges from 0 to 14, indicating how acidic or basic a substance is.
- Universal indicator provides a full range of colours corresponding to different pH levels, and this can be a limitation.
- Phenolphthalein changes from colourless to pink in alkaline conditions.
- Methyl orange shifts from red in acidic conditions to yellow in basic conditions.
- Litmus paper turns red in acidic conditions and blue in basic conditions, the solution will be purple when neutral.
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
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.
Teacher tip
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.
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
Lesson video
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Prior knowledge starter quiz
6 Questions
Q1.What is the pH range of acids?
Q2.Which of the following is common laboratory equipment used to measure pH?
Q3.What colour does litmus paper turn in an acidic solution?
Q4.What does a pH value of 7 indicate?
Q5.Which ion's concentration is high in acidic solutions?
Q6.What should you always wear to protect your eyes in a laboratory?
Assessment exit quiz
6 Questions
Q1.How does the pH scale indicate whether a substance is acidic or basic?
Q2.What colour does methyl orange turn in acidic conditions?
Q3.What is the pH of a neutral solution?
Q4.Which indicator changes from colourless to pink in alkaline conditions?
Q5.How does litmus paper indicate a neutral solution?
Q6.What is a limitation of using universal indicator?
To help you plan your 10 chemistry lesson on: pH scale and indicators, download all teaching resources for free and adapt to suit your pupils' needs...
To help you plan your 10 chemistry lesson on: pH scale and indicators, download all teaching resources for free and adapt to suit your pupils' needs.
The starter quiz will activate and check your pupils' prior knowledge, with versions available both with and without answers in PDF format.
We use learning cycles to break down learning into key concepts or ideas linked to the learning outcome. Each learning cycle features explanations with checks for understanding and practice tasks with feedback. All of this is found in our slide decks, ready for you to download and edit. The practice tasks are also available as printable worksheets and some lessons have additional materials with extra material you might need for teaching the lesson.
The assessment exit quiz will test your pupils' understanding of the key learning points.
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