NCERT Class 9 Science Chapter 9 Activities — Atomic Foundations of Matter
NCERT Class 9 Science — Atomic Foundations of Matter, Activities 9.1 to 9.4 on pages 163 to 177.
Also here. Solutions to the end-of-chapter questions for this chapter and the in-text questions are on their own pages.
Only four activities, and three of them are doing the same job: establishing the law of conservation of mass by weighing things before and after a change. The fourth, much later in the chapter, is about how ionic compounds behave.
The sequence is unusually careful, and worth preserving:
- Activity 9.1 weighs a physical change — salt dissolving — and finds the mass unchanged.
- Activity 9.2 weighs a chemical change twice, once in an open vessel and once closed, and gets two different answers.
- Activity 9.3 repeats the chemical test with a reaction that produces no gas at all.
- Activity 9.4 tests the solubility and electrical conductivity of a set of compounds.
Key insight. Activity 9.2 is designed to fail first. The open flask loses mass, which looks like a refutation of conservation of mass — and the closed one, done immediately afterwards, shows that the missing mass was carbon dioxide escaping into the room. A class that only ever sees the working version learns a law; a class that sees both learns why anyone doubted it.
Activity 9.1
Put a clean dry 100 mL beaker on a digital balance and press tare to zero it. Pour in about 50 mL of water and add a spatula full of common salt. Record the reading. Swirl until the salt dissolves, and record it again.
What you should observe. The number does not change when the salt disappears.
The mass of the solution equals the sum of the masses of the water and the salt. There is practically no change in mass during dissolving — and the chapter adds that this holds for all physical changes.
Why “the salt has gone” is the wrong reading. The salt has dissolved, not vanished. The balance is the evidence: if the salt had ceased to exist the reading would have dropped, and it does not. That the mass is unchanged is what tells you dissolving is a physical change.
Why tare first. Zeroing with the beaker in place means the reading is the contents alone, so the arithmetic is a straight addition with no beaker mass to subtract.
Other versions worth doing. The chapter suggests weighing a piece of paper before and after tearing it up, which makes the same point with nothing dissolved and no possibility of anything escaping.
What goes wrong. Spilling a few grains while adding the salt, or splashing while swirling. On a digital balance reading to 0.01 g, a grain matters. Add the salt carefully over the beaker and swirl gently.
Activity 9.2
Set-up 1: put a conical flask of vinegar and a balloon containing baking soda on the balance and record the total. Tip the baking soda into the vinegar, put flask and balloon back on the balance, and record again. Set-up 2: the same, but with the balloon fitted over the mouth of the flask so nothing can escape.
What you should observe. Two different answers from the same reaction.
Set-up 1 (open): brisk effervescence, and the final reading is lower than the initial one. Set-up 2 (sealed): the balloon inflates, and the final reading matches the initial one.
The difference is the carbon dioxide escaping in the first case. Mass is conserved — it just left the balance.
Why the failing version comes first. It is the honest order. Anyone weighing this reaction in an open flask will find the mass drops, and a student who is only ever shown the sealed version has no answer when that happens. Running the open one first turns the discrepancy into the question the second set-up answers.
Why the balloon stays on the balance in set-up 1. The chapter’s own note: traces of baking soda cling to the inside of the balloon, and if the balloon is taken off, that small amount leaves with it and confuses the result. Keep everything on the pan.
What goes wrong. A balloon that is not sealed tightly over the neck in set-up 2. Gas leaks out slowly and the reading drifts down — reproducing set-up 1’s result while appearing to be set-up 2, which is the worst possible outcome. Check the seal before tipping, and use a balloon that grips the neck firmly.
Activity 9.3
Place two conical flasks on a balance, one holding sodium sulfate solution and the other barium chloride solution, and record the total. Mix the two solutions, put the flasks back, and record again.
What you should observe. A white solid appears, and the balance does not move.
A white precipitate of barium sulfate forms, with sodium chloride left in solution — and the total mass is unchanged.
$$\text{Sodium sulfate} + \text{Barium chloride} \rightarrow \text{Barium sulfate} + \text{Sodium chloride}$$
This establishes that mass is conserved during a chemical change.
Why this reaction and not the vinegar one. Nothing here is a gas. There is no possibility of anything escaping, so the result needs no sealed apparatus and no argument about leaks — the conclusion of Activity 9.2 stands on its own. Doing both is what makes the law convincing rather than an artefact of one set-up.
What goes wrong. Mixing so vigorously that a little slops out of the flask. The whole experiment is the balance reading, and a splash of a millilitre is enough to spoil it. Pour one into the other slowly, over the pan.
The extension the chapter sets. It asks you to design your own test of conservation using zinc and dilute hydrochloric acid, which produces hydrogen. That is the interesting case, because it forces the class to work out for themselves that a gas-producing reaction must be sealed — the lesson of Activity 9.2, applied rather than repeated.
Activity 9.4
Collect samples of camphor, sodium chloride, copper sulfate, sugar and naphthalene. Test each one’s solubility in water, kerosene and petrol. Then push two carbon or metal electrodes through a cardboard, wire them to a 9 V battery and a bulb, and test each sample for electrical conductivity — first as a solid, then dissolved in water. Record everything in Table 9.2.
What you should observe. The samples split into two groups on every test at once.
| Sodium chloride, copper sulfate | Camphor, naphthalene, sugar | |
|---|---|---|
| Soluble in water | yes | mostly no (sugar: yes) |
| Soluble in kerosene, petrol | no | yes |
| Solid conducts | no | no |
| Solution conducts | yes | no |
Why the solid does not conduct but the solution does. Sodium chloride and copper sulfate are ionic. The ions are locked in a rigid lattice in the solid and cannot move, so no current flows. Dissolve the solid and the ions are free to move towards the electrodes, and the bulb lights. Sugar dissolves too, but it dissolves as molecules, not ions — so its solution carries no charge and the bulb stays dark.
Why sugar is the sample that matters most. It is soluble in water like the ionic compounds, but non-conducting like the covalent ones. Without it, a student could reasonably conclude that anything which dissolves in water conducts. Sugar is the counter-example, and it is worth pointing at directly.
Why test kerosene and petrol as well. The pattern reverses: ionic compounds dissolve in water and not in these, while camphor and naphthalene do the opposite. “Like dissolves like” is the rule underneath, and the table shows it without stating it.
What goes wrong. Electrodes not rinsed between samples. A trace of salt solution carried into the sugar makes the bulb glow faintly and destroys exactly the comparison the activity exists to make. Rinse with distilled water and dry between tests — and keep the electrodes a constant distance apart, since moving them changes the brightness for reasons that have nothing to do with the sample.
Common mistakes
- Treating dissolved salt as “gone” in Activity 9.1. The balance says otherwise, and that is the entire point of weighing it.
- Concluding from Activity 9.2’s open flask that mass is not conserved. The gas escaped. The sealed set-up is the control that shows where it went.
- Removing the balloon from the balance in set-up 1. Traces of baking soda stuck inside it leave with it, and the chapter warns about this in its own note.
- A loose balloon in set-up 2. Gas leaks, the reading drifts down, and the result looks like set-up 1 while claiming to be set-up 2.
- Splashing in Activity 9.3. The reading is the whole experiment; a lost millilitre is a lost result.
- Concluding that anything soluble in water conducts. Sugar dissolves and does not conduct, because it dissolves as molecules rather than ions.
- Reusing electrodes without rinsing. Carry-over from an ionic sample makes the next solution appear to conduct.
Practise next
- The end-of-chapter questions, Revise, Reflect, Refine, where the conservation of mass from Activities 9.1 to 9.3 becomes numerical problems, and the ionic behaviour of Activity 9.4 becomes questions on formulae and bonding.
- The in-text Pause and Ponder questions of this chapter, twenty-four across ten boxes — the most of any Class 9 chapter — including the formula-writing that Activity 9.4’s compounds prepare for.
- The activities of Chapter 5, whose solutions, suspensions and colloids are the mixtures this chapter now explains in terms of particles.
Doubts are answered by Shiwam, usually within a day. Ask about this question specifically — a general question about the chapter is better asked in class.