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:

$N a H C O_{3 (s)}$ + $C H_{3} C O O H_{(l)}$ $\to$ $C O_{2 (g)}$ + $H_{2} O_{(l)}$ + $N a_{(a q)}^{+}$ + $C H_{3} C O O^{-} (a q)$

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

Another common way to write this reaction is:

$N a H C O_{3}$ + $H C_{2} H_{3} O_{2}$ $\to$ $N a C_{2} H_{3} O_{2}$ + $H_{2} O$ + $C O_{2}$ [1 mark]

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

$P b (N O_{3})_{2 (a q)}$ + $2 K l$ $\to$ $P b l_{2 (s)}$ + $2 K N O_{3 (a q)}$

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

$B a C l_{2 (a q)}$ + $N a_{2} S O_{4 (a q)}$ $\to$ $B a S O_{4 (s)}$ + $2 N a C l_{(a q)}$

$B a S O_{4 (s)}$ - ppt
[0.5 mark]
(d) Physical state

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

$Z n_{(s)}$ + $H_{2} S O_{4 (a q)}$ $\to$ $Z n S O_{4 (a q)}$ + $H_{2}$

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

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