Today, we are going to discuss an exam-based question (similar to those in GCE O-Level Cambridge Chemistry Exam) on Acids.
Question:
Hydrogen fluoride has melting point of -83.6 degree Celsius and a boiling point of 19.5 degree Celsius. It is highly corrosive and can cause blindness by rapid destruction of the cornea.
(a) With reasons, determine if hydrogen fluoride is covalent or ionic.
(b) Hydrogen fluoride dissolves in water to form a colourless aqueous solution.
(i) Write the formula of the ions formed in the aqueous state.
(ii) Write the dissociation equation, with state symbols for aqueous hydrogen fluoride.
(iii) Predict whether aqueous hydrogen fluoride is an acid or alkali. Give a reason for your prediction.
(iv) Describe what is observed when chlorine gas is bubbled into aqueous hydrogen fluoride. Explain your observation.
Now, go and work on the questions first before you scroll down to look at my suggested solutions.
Suggested Solution:
(a) It is a covalent compound since it consists of only hydrogen and fluorine which are both non-metals.
(b)(i) H+, OH– and F–.
(b)(i) HF(aq) -> H+(aq) + F–(aq)
(b)(iii) It is an acid since it dissociates in water to produce hydrogen ions.
(b)(iv) No visible change since chlorine is less reactive than fluorine. Chlorine is unable to displace fluorine from its aqueous hydrogen fluoride solution. [we are considering Halogen Displacement Reactions of Periodic Table & Group Trends in this part of the question]
I am sure you have learned something useful today.
Feel free to share it with your friends who need help in Basic Chemistry.
Cheers! 
Note:
Do note that for Advanced Chemistry (i.e. IB program and GCE A-Level Program), HF(aq) is in fact considered to be a weak acid due to the strength of H-F covalent bond. You can find out more about it at either:
1) wikipedia
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