NCERT Class 9 Science Exploration Chapter 09 Atomic Foundations of Matter PDF Download

Chapter 9: Atomic Foundations of Matter

In Chapter 8, Journey Inside the Atom, you explored the structure of the atom. You also studied subatomic particles, viz., electrons, protons and neutrons, in terms of their discoveries, properties and locations in the atom. You also learnt that atoms with an octet of electrons in their valence shell are stable. Atoms can lose, gain or share electrons to achieve an octet of valence electrons.

You have also learnt that many properties of the elements are not retained when they form a compound. For example, hydrogen and oxygen are gases, whereas the substance formed when they combine is water, which is a liquid at ordinary temperatures.

Interestingly, water does not have the same properties as hydrogen or oxygen. Hydrogen gas is combustible and oxygen supports combustion, whereas water neither burns nor helps in burning; rather, it extinguishes fire. However, it was found that the mass of the water formed equals to the sum of the masses of the hydrogen and oxygen that combined to form it. Let us explore whether the mass remains unchanged during physical and chemical changes.

Think It Over

• Water can be obtained from various sources. Are all these samples of water chemically identical?
• Oxygen is sometimes represented as O and sometimes as \( \ce{O2} \). What is the difference between these symbols?
• Why does dissolved salt in water conduct electricity, but sugar does not?

Activity 9.1: Let us investigate a physical change

1. Place a clean and dry 100 mL beaker on a digital weighing balance.
2. Set the balance reading to zero by pressing the tare or reset button.
3. Pour about 50 mL of water into the beaker.
4. Add a spatula full of common salt to the water contained in the beaker.
5. Record the reading on the weighing balance (Fig. 9.1a).
6. Swirl until the added salt dissolves and record your observations (Fig. 9.1b).

[Figure 9.1: (a) Weight of water and undissolved salt; (b) Weight of solution of salt and water, See in your textbook]

What do you observe? You may notice that the mass of the solution is equal to the sum of the masses of water and salt taken. This shows that there is practically no change in the mass during the formation of a solution, which is a physical change. This is true for all physical changes. You can repeat the above activity by weighing a piece of paper before and after tearing it into pieces, and observe whether its mass changes or not.

Now, let us find out whether this is true for chemical changes as well.

Teacher's Note

When you do Activity 9.1, focus on reading the balance before and after dissolving the salt. The key observation is that mass stays the same even though you cannot see the salt anymore - it is dissolved, not gone. This is why physical changes do not create or destroy matter, they just change its appearance.

Activity 9.2: Let us investigate a chemical change

You have learnt about various chemical changes. Do you remember what happened when baking soda was added to vinegar? A gas, carbon dioxide, was formed during this chemical change and the reaction is represented as -

Vinegar + Baking soda (Sodium hydrogencarbonate) \( \rightarrow \) Carbon dioxide + Other substances

Let us explore whether the mass remains the same before and after the change.

Experimental set-up 1

1. Place a clean, dry 100 mL conical flask and a medium-sized balloon on a weighing balance.
2. Set the balance reading to zero by pressing the tare or reset button.
3. Pour about 20 mL of vinegar or lemon juice into the conical flask.
4. Take about 2 g of baking soda (sodium hydrogencarbonate) and put it into the balloon.
5. Keep the balloon filled with baking soda on the weighing balance next to the conical flask. Record the initial reading (Fig. 9.2a).
6. Carefully transfer the baking soda (sodium hydrogencarbonate) from the balloon into the conical flask containing vinegar (Fig. 9.2b).
7. Place the conical flask and balloon back on the weighing balance, and record the final reading (Fig. 9.2c).
8. What do you observe?
9. Are the initial and the final readings same?

[Figure 9.2: (a) Weight of vinegar and baking soda; (b) Pouring baking soda into the conical flask containing vinegar; (c) Weight of the final reaction mixture, See in your textbook]

A brisk effervescence is observed. The final reading does not match the initial reading. What can be the reason for this?

Repeat the above experiment in a slightly modified way as explained below.

Note

Keep the conical flask and the balloon on the digital weighing balance. This prevents errors caused by small traces of baking soda that may remain stuck to the balloon.

Experimental set-up 2

1. Place a clean, dry 100 mL conical flask and a medium-sized balloon on a weighing balance.
2. Set the balance reading to zero by pressing the tare or reset button.
3. Pour about 20 mL of vinegar or lemon juice into the conical flask.
4. Place about 2 g of baking soda (sodium hydrogencarbonate) in the balloon.
5. Fix the balloon to the mouth of the conical flask using a thread, without allowing the baking soda to mix with the vinegar.
6. Weigh the conical flask containing vinegar and the balloon containing baking soda, and record the reading (Fig. 9.3a).
7. Lift the other end of the balloon upwards, allowing the baking soda (sodium hydrogencarbonate) to fall into the vinegar (Fig. 9.3b).
8. What do you observe?

[Figure 9.3: (a) Weight of vinegar and baking soda; (b) Pouring baking soda into the conical flask containing vinegar, See in your textbook]

Note

Usually in any measurement, there is uncertainty of \( \pm 1 \) in the last digit. Therefore, the variation in readings of digital weighing balance is within experimental error and the weight can be taken as constant.

Teacher's Note

In Experimental set-up 1, the mass decreases because carbon dioxide gas escapes into the air - it is not trapped. In Experimental set-up 2, when the balloon is tied at the mouth, the carbon dioxide stays inside the balloon, so the total mass remains constant. This shows that mass is conserved in chemical reactions, but you must keep all substances together to measure it correctly.

Key Points

• In physical changes like dissolving salt in water, the total mass remains the same because no new substances are created - the salt just becomes invisible as it mixes with water.
• In chemical changes, if all products are kept together (such as using a tied balloon), the total mass before and after the reaction is constant, showing that matter is neither created nor destroyed.
• If a gas escapes during a chemical reaction and is not collected, the measured mass of the remaining mixture decreases, but the escaped gas still has mass.

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