Download App

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.
✓

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.
$\mathrm{Zn}+\mathrm{H}_{2} \mathrm{SO}_{4} \longrightarrow \mathrm{ZnSO}_{4}+\mathrm{H}_{2}$

Need a full question paper?

Generate a complete, print-ready paper with questions like this in minutes — across 16+ boards, with answer keys.

Start Generating Free

Explore more

More "Answer the questions.[Che-3M]" from MODEL PAPER - 3 [2026]→MODEL PAPER - 3 [2026] — all question types→Science STD 10 question papers→CBSE Board STD 10 question papers→CBSE Board question paper generator→

Similar questions

i. Why are the chips packets puffed when we buy them from market?
ii. Paint is applied on articles made up of iron, why?
→
What happens when CO2 gas is passed through lime water?
→
Name the compound formed when ethanol is heated in excess of conc. sulphuric acid at 443 K. Also write the chemical equation of the reaction stating the role of conc. sulphuric acid in it. What would happen if hydrogen is added to the product of this reaction in the presence of catalysts such as palladium or nickel?
→
In terms of electronic configurations, explain the variation in the size of the atoms of the elements belonging to the same period and same group.
→
What are strong and weak acids? In the following list of acids, separate strong acids from weak acids. Hydrochloric acid, citric acid, acetic acid, nitric acid, formic acid, sulphuric acid.
→
What makes the candle flame yellow and luminous?
→
Explain the formation of a chlorine molecule on the basis of electronic theory of valency.
→
Complete the missing components/variables given as reactant and product in the following reactions:

  1. $\text{BaCl}_2+\text{X}\rightarrow+\text{BaSO}_4\downarrow$

  2. $\text{Cu}+\text{AgNO}_3\rightarrow\text{Cu}(\text{NO}_3)_2+\text{Y}$

  3. $\text{CaCO}_3\xrightarrow{\text{Z}}\text{CaO}+\text{CO}_2$

→
Name two oxidising agents that are used for the conversion of alcohols to acids. Distinguish between ethanol and ethanoic acid on the basis of,
  1. Litmus test.
  2. Reaction with sodium hydrogen carbonate.
→
Name two metals which react violently with cold water. Write any three observations you would make when such a metal is dropped into water. How would you identify the gas evolved, if any, during the reaction?
→