3 marksShort AnswerSection A

2 mL of sodium hydroxide solution is added to a few pieces of granulated zinc metal taken in a test tube. When the contents are warmed, a gas evolves which is bubbled through a soap solution before testing. Write the equation of the chemical reaction involved and the test to detect the gas. Name the gas which will be evolved when the same metal reacts with dilute solution of a strong acid.

Acids, Bases and Salts

Reaction of Metals with Bases and Acids

Official Answer

When granulated zinc reacts with a warm sodium hydroxide solution, sodium zincate and hydrogen gas are formed.

Chemical Equation:

$2NaOH(aq) + Zn(s) \rightarrow Na_2ZnO_2(s) + H_2(g)$

Test for Hydrogen Gas ( $H_2$ ):

When a burning candle is brought near the soap bubbles filled with the evolved gas, the gas burns with a characteristic 'pop' sound, which confirms the presence of hydrogen gas.

Gas evolved with a strong acid:

When the same metal (zinc) reacts with a dilute solution of a strong acid (such as dilute hydrochloric acid, $HCl$ , or dilute sulphuric acid, $H_2SO_4$ ), hydrogen gas ( $H_2$ ) is evolved.

sodium zincateNa2ZnO2hydrogen gasH2pop soundburning candle

Marking Scheme

11 mark
21 mark
31 mark

Hint

Recall Activity 2.3 and 2.4. The gas that burns with a pop sound is hydrogen. The salt formed with $NaOH$ is sodium zincate.

Quick Oral Answer

When zinc reacts with sodium hydroxide, sodium zincate and hydrogen gas are produced. We test the hydrogen gas by bringing a burning splinter near it, which produces a pop sound.

Analysis & Explanation

This question tests the understanding of how metals react with both bases and acids.

Reaction with Base: Zinc is an amphoteric metal that reacts with strong bases like sodium hydroxide ( $NaOH$ ) to liberate hydrogen gas and form a complex salt, sodium zincate ( $Na_2ZnO_2$ ). This reaction requires warming to proceed at a reasonable rate.
Testing Gas: Hydrogen gas is insoluble in water, so bubbling it through a soap solution traps the gas in bubbles. Since hydrogen is highly flammable, it burns rapidly with a characteristic 'pop' sound when exposed to a flame.
Reaction with Acid: Active metals like zinc displace hydrogen from dilute strong acids to form a metal salt and hydrogen gas:

$Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)$

In both cases (reaction with a strong base and reaction with a strong acid), the gas evolved is hydrogen.

Common Mistakes

1Writing an incorrect chemical formula for sodium zincate (e.g., NaZnO2 instead of $Na_2ZnO_2$ ).
2Forgetting to mention that the candle must be burning/lit for the pop-sound test.
3Confusing the gas evolved with carbon dioxide.

Interesting Facts

Zinc is one of the few transition metals that can react with both acids and strong bases because of its amphoteric nature.

Spotted a mistake or something unclear?

Tell us — we fix reported answers fast.

Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2018?

This question carries 3 marks in the CBSE Class 10 Science 2018 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Acids, Bases and Salts" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Reaction of Metals with Bases and Acids" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2018 paper?

This is a Short Answer question from Section A in the CBSE Class 10 Science 2018 paper.