The pH scale and salts (chemical changes)

Question 1

What occurs when substances dissolve in water?

Answer 1

• When substances dissolve in water, they dissociate into their individual ions: hydroxide ions make solutions alkaline and hydrogen ions make solutions acidic.

Question 2

What is the pH scale?

Answer 2

• The pH scale is a measure of the acidity or alkalinity of an aqueous solution. A solution with a pH of 7 is neutral. Aqueous solutions with a pH of less than 7 are acidic. The closer to the pH of zero, the stronger the acid. Aqueous solutions with a pH of more than 7 are alkaline. The closer to a pH of 14, the stronger the alkali.

Question 3

What are the two ways the pH of a solution be measured?

Answer 3

• The pH of a solution can be measured by a pH probe or universal indicator. Indicators are dyes that change colour depending on whether they are in acidic or alkaline solutions:
  1. Litmus changes colours from red to blue or vice versa.
  2. Universal indicator is a mixture of dyes that shows a range of colours to indicate how acidic or alkaline a solution is.

Question 4

What are soluble bases?

Answer 4

• Soluble bases are called alkalis. Acids can be neutralised by bases.

Question 5

What is produced from a reaction between an acid and metal hydroxide?

Answer 5

• Salt and water

Question 6

What is produced when an acid reacts with an alkali and why?

Answer 6

• Acids contain hydrogen ions and alkalis contain hydroxide ions.
• When an acid reacts with an alkali, the hydrogen and hydroxide ions react together to produce water, which has a pH of 7.

Question 7

What is a neutralisation reaction?

Answer 7

• The reaction between hydroxide and hydrogen ions is called neutralisation because: acid is neutralised by an alkali and the solution that remains has a pH of 7, showing it is neutral.

Question 8

What is produced when an acid reacts with metal oxides or metal carbonates in a neutralisation reaction?

Answer 8

• Acid reacts with metal oxide to produce salt and water.
• Acid reacts with metal carbonate to produce a salt, water, and carbon dioxide.

Question 9

When is a salt produced?

Answer 9

• A salt is produced when the hydrogen in the acid is replaced by a metal ion. The name of the salt produced depends on the acid used.
• Hydrochloric acid produces chloride salts.
• Nitric acid produces nitrate salts.
• Sulfuric acid produces sulfate salts.

Question 10

How can soluble salts be produced from insoluble bases?

Answer 10

• Soluble salts can be made by reacting acids with insoluble bases, such as metal oxides, metal hydroxides and metal carbonates.

Question 11

How do you carry out the preparation of a pure, dry sample of a soluble salt from an insoluble oxide or carbonate?

Answer 11

Sample method:
1. Add the metal oxide or carbonate to a warm solution of acid until not more will react.
2. Filter the excess metal oxide or carbonate to leave a solution of the salt.
3. Gently warm the salt solution so that the water evaporates and crystals of salt are formed.

Hazards and risks:
* Corrosive acid can cause damage to the eye, so eye protection must be used.
* Hot equipment can cause burns, so care must be taken when the salt solution is warmed.

Question 12

What happens to strong and weak. acids in water?

Answer 12

• Strong acids such as hydrochloric acid, nitric acid and sulfuric acid are completely ionised (split up into ions) in water. Weak acids such as ethanoic acid, citric acid and carbonic acid are only partially ionised in water.

Question 13

What is the pH of a solution a measure of?

Answer 13

• The pH of a solution is a measure of the concentration of hydrogen ions. A pH decrease in one unit indicates that the concentration of hydrogen ions has increased by a factor of 10. For a given concentration of acid, a strong acid will have a higher concentration of hydrogen ions and, therefore, a lower pH.