Acid
An acid is a substance that gives hydrogen ions in water and shows acidic behaviour such as turning blue litmus red.
In one line: Acid: blue to red, pH goes down.
Practice This ConceptUnderstand it
In school chemistry, an acid is identified by its effect in water and on indicators. Acids usually have pH below 7, and they show characteristic changes with litmus and universal indicator. Many acids are found in daily life, such as lemon juice, curd, and vinegar, while mineral acids like hydrochloric acid are used in laboratories.
Example
Rule to remember
Acids release H+ ions in aqueous solution.
In common school tests, acids turn blue litmus red and usually have pH below 7.
See it worked out
Follow the example line by line, then use the same method on new questions.
Worked example
A student tests vinegar with blue litmus paper.
- 1The litmus turns red, so vinegar is identified as an acid.
- 2This matches the rule that acidic substances change blue litmus to red.
Method you can reuse
- 1First identify the sample or its test result.
- 2Then check whether the clue is litmus colour, pH value, or a daily-life example.
- 3Finally state that the substance is acidic because it gives hydrogen ions in water.
Visual tip
A simple test-tube sketch can show blue litmus paper at the mouth of a container and an arrow showing the colour change to red.
Check yourself
Answer the question in your head before opening the answer.
Try it yourself
Why does a blue litmus strip change when lemon juice is dropped on it?
Think first, then show answerHide answer
Common wrong answer
A common wrong answer is to call every sour liquid a strong acid.
Why it loses marks: Strength depends on ion release and pH, not only taste.
Don't mix this up
Exam tip
Acid diagram
Open full screen to read every label clearly.
Write it in the exam
Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.
How CBSE asks it
- 1
Enough for 1 mark
An acid is a substance that gives hydrogen ions in water and shows acidic behaviour such as turning blue litmus red.
- 2
Add for 2 marks
Example: Lemon juice is acidic. It turns blue litmus red and has a low pH because it contains acid particles in water.
- 3
Add for 3 marks
In school chemistry, an acid is identified by its effect in water and on indicators.
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