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Home > Chemistry > Acids > Acids and alkalis

Chemistry

Acids and alkalis

What happens when an acid and an alkali are diluted (General/Credit)

(General)

Diluting an acid by adding more water increases the pH of the solution towards 7 making it less acidic.

Diluting an alkali by adding more water decreases the pH of the solution towards 7 making it less alkaline.

Example question

Now try a typical exam question. Look at the answer after you have tried the question.

Question

The chart below shows the pH of some common substances.

pHs of some substances on the pH scale: lemon juice - pH between 2 & 3, vinegar pH between 3 & 4, water pH 7, salt solution pH 7, milk of magnesia pH 9, ammonia solution pH between 11 & 12

Identify the two substances which will show a decrease in pH when they are diluted with water.

Hint: Remember -Acids have pH values below 7. Alkalis have pH values above 7.

Answer

ammonia solution and milk of magnesia

The question clearly asks for two correct answers.

Both milk of magnesia and ammonia solution have pH values above 7 so they are examples of alkaline solutions. Diluting an alkali results in a decrease in pH towards 7.

(Credit)

Diluting an acidic solutiondecreases the concentration of H+(aq) ions. This causes the pH to increase towards 7.

Diluting an alkaline solutiondecreases the concentration of OH-(aq) ions.This causes the pH to decrease towards 7.

Example question

Now try a typical exam question. Look at the answer after you have tried the question.

Question

Acid solutions contain H+(aq) ions, so does pure water.

  1. Explain why pure water is not an acid.
  2. When water is added to an acid solution the pH rises. Explain why this happens.
Answer
  1. Pure water is not an acid as it contains equal numbers of H+(aq) and OH-(aq) ions which balances out the effect of each other.
  2. When an acid is diluted by adding water to it the concentration of H+(aq) ions decreases. If the concentration of H+(aq) ions decreases the pH will increase.

    pH is a measure of H+(aq) ion concentration.

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