- Define and explain Acids in Everyday Life in your own words
- Use key terms such as acid accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
Lemons taste sour, soap feels slippery, and both can be explained by acids and alkalis. The pH scale measures just how acidic or alkaline something is, and indicators reveal it with colour. In this chapter you will test, measure and neutralise like a chemist.
This lesson focuses on Acids in Everyday Life: meet acids in citrus fruits, vinegar and your stomach.
Meet acids in citrus fruits, vinegar and your stomach.
Key ideas
The pH scale measures hydrogen ions
Acids release hydrogen ions (H⁺) in water, and the more hydrogen ions, the lower the pH and the stronger the acid. A solution of pH 3 is more acidic than one of pH 5 — each whole step on the scale is a tenfold change. Pure water sits at pH 7, exactly neutral.
Indicators change colour
An indicator is a dye that changes colour in acids and alkalis: blue litmus turns red in acid, red litmus turns blue in alkali. Universal indicator goes further, showing a rainbow of colours from red (strong acid) through green (neutral) to purple (strong alkali). This lets you estimate pH just by matching the colour.
Key term — acid: A substance with a pH below 7, such as lemon juice or vinegar; acids turn blue litmus red.
Four unlabelled solutions A, B, C and D give these results with universal indicator: A turns red, B turns green, C turns purple, D turns orange. Identify each as acidic, neutral or alkaline, and put the acids in order of strength.
Red means strongly acidic, so A is a strong acid. Orange means weakly acidic, so D is a weak acid. Green means neutral, so B is neutral. Purple means strongly alkaline, so C is a strong alkali. Lower pH means stronger acid, so A (red) is stronger than D (orange).
Answer: A: strong acid; D: weak acid; B: neutral; C: strong alkali. Acids in order of strength: A then D.
- Thinking all acids are dangerous Strength matters more than the word acid: citric acid in oranges and ethanoic acid in vinegar are weak acids we eat safely. It is concentrated strong acids, like concentrated sulfuric acid, that are hazardous.
- Saying pH 7 is an acid pH 7 is neutral — neither acidic nor alkaline. Acids are below 7 and alkalis are above 7; pure water is the classic example of pH 7.
Practice
(a) Red — vinegar is acidic. (b) It stays blue — soap solution is alkaline.
Lemon juice is more acidic — about ten thousand times more acidic (four pH steps of tenfold change each).
Hydrochloric acid + sodium hydroxide → sodium chloride + water.
Add an alkali such as lime, which neutralises the acid and raises the pH towards 7.
Quick check
Which of these best defines "acid"?
Hydrochloric acid and sodium hydroxide are mixed in exactly the right amounts. What is in the beaker at the end?
Explain why universal indicator is more useful than litmus for measuring pH.
- Acids in Everyday Life: meet acids in citrus fruits, vinegar and your stomach.
- The pH scale measures hydrogen ions: Acids release hydrogen ions (H⁺) in water, and the more hydrogen ions, the lower the pH and the stronger the acid.
- alkali: A soluble base with a pH above 7, such as soap solution; alkalis turn red litmus blue.
- Watch out for: thinking all acids are dangerous