NCERT Based Activity: Nature of Matter: Elements, Compounds, and Mixtures

Activity 8.1: Let us experiment

• Take a glass tumbler and fill it half with water.
• Add a small amount of calcium oxide (quick lime) slowly to it.
• What do you observe?
• Stir continuously to make a solution of calcium hydroxide. This solution is called lime water.
• Filter it and observe its colour.
• Leave this colourless solution in a petri dish for a few hours (Fig. 8.5a).
  
• Keep stirring the solution at regular intervals.
• What do you observe (Fig. 8.5b)?
• Does it turn milky?
• Can you explain why the solution has turned milky?

Answer: 

• Observation: When calcium oxide is added to water, it reacts vigorously and the mixture becomes warm. The filtered solution is initially colourless. After a few hours the solution turns milky.
• Explanation and chemical equations: Calcium oxide reacts with water to form calcium hydroxide (lime water):
  CaO + H₂O → Ca(OH)₂. Carbon dioxide from the air dissolves in lime water and reacts with calcium hydroxide to form insoluble calcium carbonate, which makes the solution milky:
  Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. The formation of the white precipitate CaCO₃ causes the milky appearance and demonstrates the presence of CO₂ in air.

Activity 8.2: Let us explore

• Take a black sheet of paper. Ensure that it is free from any visible dust particles.
• Place the black sheet of paper undisturbed near an open window (Fig. 8.6a), or in the garden, for a few hours.
  
• What do you observe?

Answer: 

• Observation: Tiny particles settle on the black sheet and become visible. Under a magnifying glass, these particles can be seen more clearly.
• Explanation: The particles are dust suspended in air. They settle out when the air is still. These dust particles are not a part of the air itself but are solid impurities or pollutants suspended in it. The experiment shows that air contains suspended particles which can be collected on surfaces.

Activity 8.3: Let us experiment (Demonstration activity)

• Collect two small test tubes, a beaker or a glass tumbler, and a 9 V battery.
• Fill 2/3rd of the beaker with water and add a few drops of dilute sulfuric acid to it.
• Fill both the small test tubes completely with water taken from the beaker (Fig. 8.8a).
  
• Place a 9 V battery inside the beaker (Fig. 8.8b).
  
• Without spilling the water, carefully place the water-filled test tubes on each of the terminals of the battery (Fig. 8.8c).
  
• Wait for a few minutes.
• Do you observe the formation of any gas bubbles at both the terminals inside the test tubes?
• Let it continue for 10-15 minutes.
• Observe the volume of gas collected in each test tube (Fig. 8.8d).
• Is the volume of the gas collected the same in both the test tubes?
• Remove these test tubes one-by-one carefully.
• Test these gases one-by-one by bringing a burning candle close to the mouth of the test tubes.
• What happens in each case?
• Which gas is present in each test tube?
• Can these collected gases be water vapour?

Answer: 

• Observation: Bubbles appear at both terminals; the volumes of gas collected in the two test tubes are not the same. On testing, one tube gives a characteristic 'pop' sound with a burning splint (hydrogen), while the other supports combustion or rekindles a glowing splint (oxygen).
• Explanation and chemistry: Electric current splits water into two gases: hydrogen collects at the negative terminal (cathode) and oxygen at the positive terminal (anode). The overall reaction for the electrolysis of water is:
  2 H₂O (l) → 2 H₂ (g) + O₂ (g). This gives a volume ratio of hydrogen to oxygen of 2:1, so the test tube at the negative terminal usually contains about twice the volume of gas as the positive terminal. The dilute H₂SO₄ acts only as an electrolyte to increase conductivity; it is not electrolysed in significant amounts in this demonstration. The gases are not water vapour because they do not condense back into water on cooling and they show the characteristic tests: hydrogen burns with a pop and oxygen supports glowing splints.

Activity 8.4: Let us experiment

• Put a teaspoon of sugar in a boiling tube.
• Heat it gently as shown in Fig. 8.12a.
  
• What do you observe?

Answer: 

• Observation: On gentle heating, sugar first melts and turns brown, then chars and becomes black. Small droplets of water may form on the cooler parts of the tube near the open end.
• Explanation and chemical change: On heating, sucrose decomposes to form carbon (char) and water vapour. A simplified balanced decomposition is often written as:
  C₁₂H₂₂O₁₁ → 12 C + 11 H₂O. The brown and then black material is carbon (char), and the droplets indicate release and subsequent condensation of water formed during decomposition. This behaviour shows that sugar is a compound composed of carbon, hydrogen and oxygen; heating breaks the chemical bonds and produces new substances with different properties.

Activity 8.5: Let us experiment (Demonstration activity)

• Take 5.6 grams of iron filings (Fig. 8.13a) and 3.2 grams of sulfur powder (Fig. 8.13b) on a watch glass. Observe them carefully.
  
• Mix them thoroughly in a watch glass. Label this mixture as Sample A (Fig. 8.14).
  
• Observe it carefully.
• Is this a uniform or a non-uniform mixture? Can you still observe both iron and sulfur as separate substances?
• Take half of Sample A in a china dish and gently heat it (Fig. 8.15a) with continuous stirring until a black mass is formed.
  
• Let the content of the china dish cool.
• Place this black mass in a mortar and grind it with the help of a pestle.
• Now, put it on another watch glass and label it as Sample B (Fig. 8.15b).
• Now, you have two samples-Sample A and Sample B (Fig. 8.16a and 8.16b). Compare both the Samples A and B step by step and record your observations in Table 8.2.
  
• Step 1-Appearance: Compare the appearance of Sample A and Sample B like colour and texture.
• Step 2-Magnet test: Take a magnet and move it over the Samples A (Fig. 8.17a) and B (Fig. 8.17b), one by one.
  
• What do you observe?
• Step 3-Gas test: Take a small amount of Sample A in a test tube and add a few drops of dilute hydrochloric acid (Fig. 8.18a).
  
• What do you observe?
• Gently smell the evolved gas by wafting it towards your nose (Fig. 8.19).
  
• Test the evolved gas by bringing a burning splinter or a lighted candle near the mouth of the test tube (Fig. 8.20a).
  
• What do you observe?
• Repeat the above steps with Sample B as well (Fig. 8.18b and 8.20b).

Answer: 

• Observation for Sample A (mixture): The mixture of iron filings and sulfur is a heterogeneous (non-uniform) mixture. Iron particles and sulfur particles can be seen separately; the magnet attracts iron filings from the mixture. When dilute hydrochloric acid is added to the mixture, iron reacts with acid to produce hydrogen gas (which gives a pop with a burning splint) while the sulfur remains unreacted.
• When heated (formation of Sample B): Heating the mixture causes a chemical reaction between iron and sulfur to form a new substance, iron(II) sulfide (FeS), which appears as a black, uniform mass. Grinding it gives a fine, uniformly black powder. The magnet does not attract Sample B.
• Chemical equations and tests:
  Formation of iron sulfide on heating:
  Fe + S → FeS (one atom of Fe combines with one atom of S to form iron sulphide).
  Reaction of iron in the mixture with hydrochloric acid:
  Fe + 2 HCl → FeCl₂ + H₂↑ (hydrogen gas is produced and gives a pop on ignition).
  Reaction of iron sulphide with dilute hydrochloric acid:
  FeS + 2 HCl → FeCl₂ + H₂S↑ (hydrogen sulphide gas is produced; it is colourless but has a rotten-egg smell and can be detected by wafting).
  The change in properties-loss of magnetic attraction and production of H₂S on reaction with acid-confirms that Sample B is a compound with properties different from the original iron and sulfur.
• Conclusion: Sample A is a physical mixture of iron and sulfur (components retain their identities). Sample B is a chemical compound (iron sulphide) formed by a chemical change during heating; its properties are different from those of iron and sulfur.