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Question

A. The elements Li, Na, and K, each having one valence electron, are in period 2, 3 and 4 respectively of modern periodic table.

(i) To which group of the modern periodic table these elements belong? [1 mark]
(ii)Which one of them is least reactive? [1 mark]
(iii) Which one of them has the largest atomic radius? [1 mark]

B. What is a chemical symbol of an element? What information does it convey? [3 marks]

C. (i) During photosynthesis, plants use light energy to prepare food. How will you classify the photosynthesis process based on the release/absorption of energy? [1 mark]

(ii) Write the chemical reaction where the following changes are observed. [3 marks]

(a) Gas is evolved

(b) Colour change is noticed

(c) Precipitate is formed

(d) Physical state is changed



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Solution

A.(i)The given elements belong to Group 1 of the modern periodic table as all of them have one valence electron. [1 mark]
(ii) Li is least reactive because its outermost orbit is very near to the nucleus. So the removal of electron is difficult. [1 mark]
(iii) K has the largest atomic radius because it has more number of shells as compared to Na and Li. [1 mark]

B.
(i) A chemical symbol is the short form which stands for an atom of a specific element or the abbreviations used for the names of elements. [1 mark]
(ii) It represents a specific element. [0.5 mark]
(iii) It represents one atom of an element. [0.5 mark]
(iv) A symbol represents how many atoms are present in one gram (gm) atom. [0.5 mark]
(v) The chemical symbol of an element represents the mass of an atom of that element. [0.5 mark]

C. (i) During photosynthesis, the energy in the form of sunlight (a form of energy) is absorbed by the green plants in order to prepare their food.
Such a type of reaction in which energy is absorbed is called an endothermic reaction. [1 mark]

(ii) (a) Reaction between baking soda and vinegar. Baking soda on reaction with vinegar produces carbon dioxide.

The chemical equation for the overall reaction is:

NaHCO3(s) + CH3COOH(l) CO2(g) + H2O(l) + Na+(aq) + CH3COO(aq)

with s = solid, l = liquid, g = gas, aq = aqueous or in water solution

Another common way to write this reaction is:

NaHCO3 + HC2H3O2 NaC2H3O2 + H2O + CO2 [1 mark]

(b) Change in colour: Reaction between lead nitrate solution and potassium iodide solution.

Pb(NO3)2(aq) + 2Kl Pbl2(s) + 2KNO3(aq)

In this reaction, colour changes from colourless to yellow.
[0.5 mark]
(c) Formation of precipitate: Action of barium chloride on sodium sulphate.

BaCl2(aq) + Na2SO4(aq) BaSO4(s) + 2NaCl(aq)

BaSO4(s)- ppt
[0.5 mark]
(d) Physical state

Change in temperature: Action of dilute sulphuric acid on zinc.

Zn(s) + H2SO4(aq) ZnSO4(aq) + H2

In this reaction, heat is evolved. [1 mark]


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