1. Acids and bases F Acids and bases pH — measuring acidity
The pH of laboratory solutions
Acids
Hydroxides
More basic substances

2. Release H+ ions in water Have pHs below 7
ACIDS
Taste sour
Turn litmus red
Release H+ ions in water Have pHs below 7
Neutralise bases
React with carbonates or bicarbonates giving CO2 gas
Examples: hydrochloric acid, citric acid, vinegar.
BASES (alkalis)
Taste bitter
Turn litmus blue
Release OH– ions in water
Have pHs above 7
Neutralise acids
Examples: sodium hydroxide, ammonia, baking soda
3F1 Testing pH

3. Indicators
To test if a substance is acidic or basic we use indicators (often plant extracts) that change colour
as the pH changes.
Indicator
Colour in acid
Colour in base
Universal indicator
red, orange, yellow
blue, purple
Litmus
red
blue
Bromothymol blue
yellow
Phenolphthalein
colourless
pink

4. pH — measuring acidity
The pH scale measures acidity by measuring the concentration of hydrogen ions (H+) and hydroxide ions (OH–) ions in solution.
When the concentration of H+ is high, the concentration of OH– is low.
When the concentration of H+ is low, the concentration of OH– is high.

5. In acidic solutions there are more H+ ions than OH– ions.
In basic solutions there are more OH– ions than H+ ions.
In neutral solutions the numbers of H+ and OH– ions are equal.

6. The mathematical formula used to calculate pH from H+ ion concentration is:
pH = –log[H+]
where [H+] means hydrogen ion concentration.
All you need to know is that this formula generates numbers from 0 to 14, with low numbers for acids and high numbers for bases.
A change of 1 on the pH scale represents a 10-fold change in hydrogen ion concentration.
Acid
Neutral
Base 7

7. This soft drink has a pH of 3.19.
It is mildly acidic.

8. The scale goes from 0 to 14, with 7 being neutral.
pH: measuring acidity
The pH scale measures how acidic or basic a solution is from the concentration of hydrogen ions in solution.
The scale goes from 0 to 14, with 7 being neutral.
Solutions with more H+ ions than OH– are acidic and have pHs below 7.
Solutions with fewer H+ ions than OH– are basic and have pHs above 7.
If the concentrations of H+ and OH– are equal the solution is neutral and the pH = 7.
Acid
Neutral
Base 7
3F2 Acid, base or neutral F3 Experimental design: soil testing

9. This indicator is blue at high pH.
The pH of laboratory solutions
Universal indicator is a mixture of several different indicators. It is used to measure pH.
Full-range universal indicator goes purple at high pH (12–14). Many schools use a cheaper brand of universal indicator that does not change colour above a pH of 11.
This indicator is blue at high pH.
The colours of full-range universal indicator:
Acidic Neutral Basic

10. HCl(g) H+(aq) + Cl–(aq)
Acids
Substances are acidic when they react with water to release H+ ions.
When hydrogen chloride molecules react with water they form H+ and Cl– ions.
HCl(g) H+(aq) + Cl–(aq)
Because all the HCl molecules react, HCl releases lots of hydrogen ions in solution.
A dilute solution of HCl has a pH of 1.
Chemists say HCl is a strong acid because it releases all of its H+ ions in solution. Sulfuric acid and nitric acid are also strong acids.
HCl solution

11. Many acids, particularly those found in fruits, do not release all of their H+ ions in water. Chemist say they are weak acids.
Weak acids, such as ethanoic acid (in vinegar), citric acid (in lemons) and tartaric acid (in grapes), form solutions with higher pHs than strong acids.
hydrochloric acid pH =1
ethanoic acid pH = 3
These solutions have the same concentration but the weaker acid contains 100 times fewer H+ ions.

12. Hydroxides
Substances are basic, or alkaline, when they mix with water to release OH– ions.
Solid sodium hydroxide dissolves in water to form OH– and Na+. We say sodium hydroxide is a strong base because it dissolves easily in water so all the hydroxide ions are released.
NaOH(s)
water
pH = 12

13. Other metal hydroxides, such as magnesium hydroxide, are not very soluble in water. Because only a little solid dissolves, the solution contains relatively few hydroxide ions. We say that magnesium hydroxide is a weak base.

14. Solid magnesium hydroxide (left) and sodium hydroxide (right) are placed on spatulas over beakers of water containing universal indicator.

15. Drop the solids into the beakers.
water
magnesium hydroxide
sodium hydroxide
Drop the solids into the beakers.

16. water
magnesium hydroxide
sodium hydroxide
The magnesium hydroxide is slow to dissolve, but the sodium hydroxide starts dissolving immediately.

17. The NaOH has fully dissolved and formed a solution with pH = 13.
water
magnesium hydroxide
sodium hydroxide
The NaOH has fully dissolved and formed a solution with pH = 13.
The Mg(OH)2 has only partly dissolved (the mixture is cloudy). pH = 9.5.

18. Looking down into the magnesium hydroxide solution, you can see the undissolved powder at the bottom of the beaker.

19. The pH of laboratory solutions
Acids have low pHs because they release H+ ions in water. Laboratory acids (HCl, H2SO4, HNO3) release all their H+ ions in solution and are strong acids with low pHs.
Fruit acids only release a few H+ ions in solution. They are called weak acids and form solutions with higher pHs.
Hydroxides are basic because they dissolve to release OH– ions. Sodium hydroxide is a strong base because it dissolves easily, releasing all its OH– ions.
Magnesium hydroxide is a weak base because it does not dissolve well in water, so releases few OH– ions in solution.

20. More basic substances Oxide ions react with water to produce OH–:
O2– + H2O → 2OH–
A few metal oxides dissolve fully in water. They form strongly basic solutions. Some others, such as magnesium oxide, are slightly soluble and form weakly basic solutions.
Most metal oxides are insoluble in water.

21. Pea-sized samples of magnesium oxide and calcium oxide are put on spatulas.

22. Drop the solids into the beakers.
water
magnesium oxide
calcium oxide
Drop the solids into the beakers.

23. The magnesium oxide and calcium oxide start to dissolve.
water
magnesium oxide
calcium oxide
The magnesium oxide and calcium oxide start to dissolve.

24. water magnesium oxide slightly soluble pH = 8 calcium oxide soluble

25. Carbonate ions also react with water to produce OH–, but only a fraction of the carbonate ions react, which means only a small amount of OH– is released even when the carbonate dissolves easily in water. That is why soluble carbonates (eg sodium carbonate) are weak bases.
CO32– + H2O → HCO3– + OH–

26. A small amount of washing soda (sodium carbonate) dissolves in water to form a solution of pH = 9.
However, most carbonates — such as calcium carbonate — are insoluble. They do not change the pH of water.

27. Bicarbonate (hydrogen carbonate) ions also form weakly basic solutions:
HCO3– + H2O → H2CO3 + OH–
A small amount of baking soda, sodium hydrogen carbonate, forms a solution with a pH of about 7.5.

28. Other basic substances
If metal oxides are soluble, the oxide ion reacts with water to form OH–:
O2– + H2O → 2OH–
Calcium oxide is soluble in water and forms a strongly basic solution. Magnesium oxide is slightly soluble in water and forms a slightly basic solution.
The carbonate and hydrogen carbonate ions also react with water to release a few hydroxide ions.
Sodium carbonate (washing soda) dissolves in water to form a solution with a pH of about 10.
Sodium hydrogen carbonate (baking soda) forms a solution with a pH of about 8.

29. Proceed to next chapter
End of chapter
Proceed to next chapter