Question

State the four observation which help us to know whether a chemical reaction has taken place. Give examples for any two of 'em.​

Answer 1

Answer :

The four observations which help us to determine whether a chemical reaction has taken place are:-

✒ Change in colour.

✒ Change in temperature.

✒ Evolution of a gas.

✒ Change in state.

Explanation (any two) :

1. Change in colour - Reaction between lead nitrate solution and potassium iodide solution.

• $\bold{Pb(NO_3)_{2(aq)}+2KI_{(aq)}\longrightarrow{}2KNO_{3(aq)}+PbI_{2(s)}}$

– In this reaction, colour changes from colourless to yellow.

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2. Change in temperature - The exothermic reaction between quicklime and water to form slaked lime.

• $\bold{CaO_{(s)}+H_2O_{(l)}\longrightarrow{}Ca(OH)_{2(aq)}+Heat}$

– In this reaction, heat is released along with the formation of slaked lime. This results in an increase in the temperature of the solution.

Answer 2

$\huge\sf\red{Answer}$

❍ The four observations which help us to determine whether a chemical reaction has taken place are:-

➝ Change in state.

➝ Change in colour.

➝ Evolution of a gas.

➝ Change in temperature.

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$\large\sf\bullet\green{\: Examples}$

$\large\circ\sf\pink{\:Change \ in \ State}$

$\sf\dashrightarrow H_2 (gas)+ O_2(gas) \longrightarrow H_2O(liquid)$

☞ Here 2 gases combine together and form a liquid.

$\large\circ\sf\pink{\:Change \ in \ Colour}$

$\sf\dashrightarrow Fe + CuSO_4 \longrightarrow FeSO_4 + Cu$

☞ Here the colour changes from Blue to Green.

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