Chemical Reactions Signs and Observations - UNSOLVED PRACTICE SET

Q7. List any four observable changes that indicate a chemical reaction has taken place. Give one example for each.

Q8. When copper powder is heated in a china dish, it turns black. What chemical change has occurred? Write the name of the black substance formed.

Q9. Your mother adds baking soda to sour buttermilk while making dhokla. She notices bubbles forming and the batter rising. What type of change is this? Name the gas responsible for the bubbles and explain why the batter rises.

Q10. Distinguish between a physical change and a chemical change on the basis of: (i) reversibility, and (ii) formation of new substances.

Q11. When dilute sulphuric acid is added to granulated zinc taken in a test tube, a gas is evolved. What are the two observations you would make to confirm that a chemical reaction has occurred?

Q12. A student mixes two colourless solutions and observes that a yellow precipitate forms immediately. List three other signs he should look for to confirm that a chemical reaction has taken place.

Q13. Describe at least five different signs or observations that help us identify whether a chemical reaction has occurred. For each sign, give a specific example from everyday life or the school laboratory. Explain why each observation indicates a chemical change rather than a physical change.

Q14. A student performs the following experiments in the laboratory and records her observations:

Experiment 1: Burning of magnesium ribbon — dazzling white flame, white ash formed

Experiment 2: Adding vinegar to baking soda — brisk effervescence, hissing sound

Experiment 3: Heating blue copper sulphate crystals — colour changes to white

Experiment 4: Dissolving sugar in water — clear solution formed, no temperature change

(a) Identify which experiments show chemical reactions and which show physical changes.

(b) For each chemical reaction, identify the sign(s) that indicate a chemical change.

(c) Why is Experiment 4 considered a physical change and not a chemical reaction?

(d) What would happen if the student added water to the white substance obtained in Experiment 3?

Q15. "Chemical reactions are happening all around us, often unnoticed, but they shape our daily lives in profound ways." Discuss this statement by giving three examples from Indian daily life: (i) cooking food, (ii) digestion in our body, and (iii) the burning of incense sticks (agarbatti). For each example, describe the observable signs of the chemical reaction and explain why it is a chemical change.

Q16. In a school science exhibition, a group of students sets up a demonstration showing various chemical reactions and their observable changes:

Table

Reaction Initial State Final Observation

A Zinc granules + dilute HCl Bubbles, test tube feels warm

B Iron nail + copper sulphate solution Nail turns brown, blue solution fades

C Barium chloride + sodium sulphate White precipitate forms

D Ice cubes in a glass Water droplets on the outside

E Burning of camphor Flame, no residue left

F Mixing sand and salt Heterogeneous mixture

(a) Identify which of these are chemical reactions and which are physical changes.

(b) For each chemical reaction, name the specific sign(s) observed.

(c) Classify Reaction A as exothermic or endothermic. Give a reason.

(d) Why is Reaction E considered a chemical reaction even though no residue is left?

(e) If Reaction B is left for 24 hours, calculate the approximate mass of copper deposited if 5.6 g of iron completely reacts. (Atomic masses: Fe = 56, Cu = 63.5)

Q17. A chemistry teacher asks students to design an experiment to distinguish between a physical change and a chemical change using common household materials.

(a) Suggest an experiment using lemon juice and baking soda. What signs would indicate a chemical reaction?

(b) Suggest an experiment using salt and water. Why is this a physical change?

(c) If a student uses 10 g of baking soda (NaHCO₃) and excess lemon juice (containing citric acid), calculate the maximum mass of CO₂ gas that can be produced. (Molar mass of NaHCO₃ = 84 g/mol, CO₂ = 44 g/mol)

(d) Why is it important for students to be able to distinguish between physical and chemical changes in real life? Give two practical examples.

Q18. In a village in rural India, a farmer notices that his iron tools left out in the monsoon rain develop a reddish-brown coating, while his aluminium utensils do not.

(a) What is the reddish-brown coating on the iron tools? Is it a result of a physical or chemical change?

(b) List three observable signs that indicate this is a chemical reaction.

(c) If the farmer's iron plough weighs 50 kg and develops a rust coating that is 2% of its mass, calculate the mass of rust formed.

(d) Why do aluminium utensils not develop a similar coating? Explain the chemical reason.

(e) Suggest two practical measures the farmer can take to prevent his iron tools from rusting.