Bases and Alkalis
What is a base?
A substance that neutralises an acid to form a salt and water.
What colour does red litmus paper turn in an alkaline solution?
Blue.
What are the two products when an acid reacts with a base?
A salt and water.
Which gas is produced when an ammonium salt reacts with an alkali?
Ammonia, NH₃.
Name one common alkali used in school chemistry laboratories.
Sodium hydroxide solution.
What is the difference between a base and an alkali?
An alkali is a base that dissolves in water.
What colour is methyl orange in an alkaline solution?
Yellow.
Complete the word equation: hydrochloric acid + sodium hydroxide → ?
Sodium chloride + water.
What is the name of the salt formed when ammonium chloride reacts with sodium hydroxide?
Sodium chloride.
A student adds an acid to an alkali. The final solution has pH 4. Has complete neutralisation occurred? Explain.
No. The final solution is acidic, so acid is still in excess.
Which ions are responsible for the alkaline properties of an aqueous solution?
Hydroxide ions, OH⁻.
What colour is thymolphthalein in an alkaline solution?
Blue.
Write the balanced symbol equation for hydrochloric acid reacting with sodium hydroxide.
HCl+NaOH → NaCl+H2O
Complete the word equation: ammonium chloride + sodium hydroxide → ?
Sodium chloride + water + ammonia.
An unknown solution turns red litmus blue, methyl orange yellow, and thymolphthalein blue. What do all three results tell you?
The solution is alkaline. All three indicator results support that conclusion.
Is copper(II) oxide a base, an alkali, or both?
A base, but not an alkali, because it is insoluble in water.
Solution X turns methyl orange yellow, while solution Y turns it red. Compare their acidity.
X is alkaline or has a pH above methyl orange's transition range; Y is acidic.
What is Neutralisation?
an acid reacts with a base to form salt and water
Why must damp litmus paper be used to test for ammonia gas?
Ammonia must dissolve in water to produce hydroxide ions, which cause the alkaline colour change.
A student adds an acid to an alkali until the final solution has pH 4. Has the solution been completely neutralised? Explain.
No. The solution is acidic, indicating that acid is in excess.
A substance neutralises hydrochloric acid but does not dissolve in water. Can it be classified as a base? Explain why it is not an alkali.
Yes. It neutralises acid, so it is a base. It is not an alkali because it is insoluble in water.
A student tests an unknown solution. Methyl orange turns yellow, but thymolphthalein remains colourless. Explain why these results do not necessarily contradict each other.
The indicators change colour over different pH ranges. The solution could be weakly alkaline, with a pH high enough for methyl orange to be yellow but below the range where thymolphthalein turns blue.
Complete the ionic equation: H⁺ + OH⁻ → ?
H++OH- → H2O
Complete the net ionic equation: NH₄⁺ + OH⁻ → ?
NH₄⁺ + OH⁻ → NH₃ + H₂O
An alkali has a pH of 12. Dilute it with water. What happens to its pH, and why?
Its pH decreases towards 7 because dilution reduces the concentration of hydroxide ions.