Why should a magnesium ribbon be cleaned before burning in air?

Balance the following chemical equations:
$(a)$ $HNO_{3} + Ca(OH)_{2} \longrightarrow Ca(NO_{3})_{2} + H_{2}O$
$(b)$ $NaOH + H_{2}SO_{4} \longrightarrow Na_{2}SO_{4} + H_{2}O$
$(c)$ $NaCl + AgNO_{3} \longrightarrow AgCl + NaNO_{3}$
$(d)$ $BaCl_{2} + H_{2}SO_{4} \longrightarrow BaSO_{4} + HCl$

What happens when dilute hydrochloric acid is added to iron filings? Tick the correct answer.

Translate the following statements into chemical equations and then balance them.
$(a)$ Hydrogen gas combines with nitrogen to form ammonia.
$(b)$ Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
$(c)$ Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
$(d)$ Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Why is the amount of gas collected in one of the test tubes in the image double the amount collected in the other? Name this gas.