Science — CBSE Class 10 board question

Chemical Reaction: A chemical reaction is a process in which one or more substances (reactants) are transformed into new substances (products) with different properties. It involves breaking and making of bonds between atoms.

(i) Change of Colour:
Take lead nitrate solution in a test tube and add potassium iodide solution to it. A bright yellow precipitate of lead iodide is formed immediately. The change in colour (from colourless to yellow) indicates that a chemical reaction has taken place.

$$\text{Pb(NO}_3)_2 + 2\text{KI} \rightarrow \text{PbI}_2 \downarrow + 2\text{KNO}_3$$

(ii) Change in Temperature:
Take zinc granules in a conical flask and add dilute sulphuric acid. Bubbles of hydrogen gas are produced and the flask becomes warm on touching. The rise in temperature shows that a chemical (exothermic) reaction has taken place.

$$\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2\uparrow$$

Source: Chapter 1, Chemical Reactions and Equations, Activities 1.2 and 1.3

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• Define chemical reaction in 1–2 lines first (1 mark).
• Each activity is worth ~2 marks: name substances used, state the observation clearly, and write the equation.
• Examiners look for: correct activity description, the specific observation (colour change / temperature change), and the chemical equation.
• Use Activity 1.2 (lead nitrate + potassium iodide → yellow precipitate) for colour change and Activity 1.3 (Zn + H₂SO₄, flask becomes warm) for temperature change — these are the textbook examples.