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MODEL PAPER - 3 [2026] — Science STD 10 — Question

CBSE BoardEnglish MediumSTD 10ScienceMODEL PAPER - 3 [2026]3 Marks
Question
Define a chemical reaction. State four observations which help us to determine that a chemical reaction has taken place. Write one example of each of the observations with a balanced chemical equation.
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Answer

Chemical reaction is the transformation of chemical substance into another chemical substance. Only a rearrangement of atoms takes place in a chemical reaction. Old bonds are broken and new bonds are formed. Some of the characteristics of chemical reactions are:
i. Change in colour: In some reactions, there is a change in colour after the reaction. For example, a chemical reaction between citric acid and purple-coloured potassium permanganate solution is characterised by a change in colour of potassium permanganate from purple to colourless.
$\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+2 \mathrm{KI} \longrightarrow 2 \mathrm{KNO}_{3}(\mathrm{aq})+\mathrm{PbI}_{2}(\mathrm{~s})$
$\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+2 \mathrm{KI}$ - Colourless
$\mathrm{PbI}_{2}(\mathrm{~s})$ - Yellow
ii. Formation of precipitate: The chemical reaction between sulphuric acid barium chloride solution. $\mathrm{BaCl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq})$
$\longrightarrow 2 \mathrm{HCl}(\mathrm{aq})+\mathrm{BaSO}_{4}(\mathrm{~s})$
iii. Change in temperature: Temperature change is the characteristic of many reactions. For example, the chemical reaction between quicklime and water to form slaked lime. In this reaction temperature of the reaction is increased.
$\mathrm{CaO}(\mathrm{s})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \longrightarrow \mathrm{Ca}(\mathrm{OH})_{2}(\mathrm{aq})+$ Heat
iv. Evolution of gas: Some reactions are characterised by evolution of gas as a result of chemical reaction. For example, the chemical reaction between sodium carbonate and hydrochloric acid is characterised by the evolution of carbon dioxide gas.
$\mathrm{Zn}+\mathrm{H}_{2} \mathrm{SO}_{4} \longrightarrow \mathrm{ZnSO}_{4}+\mathrm{H}_{2}$

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