Question

A student added dilute HCL to a test tube containing zinc granules and made following observations:

I. The zinc surface became dull and black.

II. A gas evolved which burnt with a pop sound.

III. The solution remained colourless.

(a) Which is the correct observation?

(b) Justify your answer.

Answer 1

The correct observation is (2) as hydrogen is formed when an acid +metal combines and gives salt+H2

HCL+Zn--^ZnCl2+H2

Answer 2

The questions are answered below:

(a) The correct observation is - II. A gas evolved which burnt with a pop sound.

(b) Justification for the above statement - Hydrogen gas is evolved when zinc reacts with dilute HCl, which burns with a pop sound.

The reaction which will take place when the student adds dilute HCl to a test tube containing zinc granules is -

$Zn(s)+2HCl(aq) \rightarrow ZnCl_{2}(aq) +H_{2}(g)$

Let us discuss the observations that can be made after this reaction -

• Observation II (correct): As we can see, hydrogen gas is evolved along with the formation of zinc chloride. Hydrogen gas can burn with a pop sound, hence observation II is correct.

• Observation III (incorrect): Initially, the solution has HCl, which is colourless. But when the reaction proceeds, zinc chloride is formed, which is of white colour. Thus, the solution will not remain colourless but will change into white colour. Hence, observation III is not correct.

• Observation I (incorrect): Zinc is of grey to blue colour in appearance. After the reaction, zinc forms a compound, zinc chloride. Thus, observation I. is also not correct.