NCERT Based Activity: Nature of Matter: Elements, Compounds, and Mixtures | Science Curiosity Class 8 - New NCERT PDF Download

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: 

• Calcium oxide reacts vigorously with water, releasing heat and forming calcium hydroxide. 
• The filtered lime water is initially colourless. After a few hours, it turns milky (Fig. 8.5b). 
• This occurs because carbon dioxide in the air reacts with calcium hydroxide to form insoluble calcium carbonate and water, indicating the presence of carbon dioxide in the 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: 

• Tiny particles settle on the surface of the black sheet of paper. 
• Using a magnifying glass (Fig. 8.6b), these can be identified as dust particles suspended in the air, which are considered pollutants and not an integral part of the air.

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: 

• Gas bubbles form at both terminals inside the test tubes. 
• After 10–15 minutes, the volume of gas collected differs, with one test tube containing more gas than the other. 
• Testing with a burning candle produces a pop sound in one test tube (indicating hydrogen gas) and a brighter flame in the other (indicating oxygen gas). 
• The volumes are not the same, typically in a 2:1 ratio (hydrogen:oxygen). 
• These gases are not water vapour, as they do not condense back into water but are elements formed by the breakdown of water into hydrogen and oxygen.

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: 

• As sugar is heated, it turns brown (Fig. 8.12b), then chars and turns blackish (Fig. 8.12c). 
  
• Small droplets of water form inside the boiling tube near the open end, indicating that sugar decomposes into carbon (charcoal) and water (which contains hydrogen and oxygen). 
• This shows sugar is a compound, not an element.

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: 

• Sample A is a non-uniform mixture where iron and sulfur particles are visible separately. On heating, Sample A forms a black mass (Sample B), which is uniform in color and texture. 
• The magnet attracts iron filings in Sample A but not in Sample B. In the gas test, Sample A produces hydrogen gas (colorless, no smell, pops with a flame) due to iron reacting with hydrochloric acid, while sulfur remains unreacted. 
• Sample B produces hydrogen sulfide gas (colorless, rotten egg-like odor) when reacted with hydrochloric acid, indicating a compound (iron sulfide) with different properties from the original mixture.