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๐ฌ Think about it
Hydrogen is a fuel and oxygen helps things burn — yet together they make water, which puts out fire! When elements join chemically, the new substance can be totally different. That joined substance is a compound. Let's explore.
Why can't we separate hydrogen and oxygen in water by physical means?
- In water, hydrogen and oxygen particles are tightly attached.
- Physical methods cannot pull them apart.
- That is why water is a compound.
- Compounds form when elements join in fixed ratios.
Example:
In water, hydrogen and oxygen atoms are in a fixed 2:1 ratio.
How is a compound different from the elements that make it?
- A compound has properties different from its elements.
- Its elements cannot be separated by any physical method.
- The elements are combined chemically in a fixed ratio.
Elements (hydrogen, oxygen)
Compound (water)
Hydrogen is a fuel; oxygen supports burning.
Water puts out fire.
Exist as separate gases.
A liquid with new properties.
Can be split from water by electricity.
Cannot be split by physical methods.
Is common salt an element or a compound?
- Sodium is a soft metal; chlorine is a harmful gas.
- They combine to form sodium chloride (common salt).
- Sodium and chlorine are in a fixed 1:1 ratio.
- So common salt is a compound.
Example:
Dissolved common salt can be separated from water by evaporation, but not into sodium and chlorine.
๐ก A step further
- More than 45 different elements are used in a mobile phone.
- These include aluminium, copper, silicon, and cobalt.
- Lithium, gold, and silver are also used, among others.
- They go into its screen, battery, and other components.
In this Activity, we will heat sugar to show that it breaks down into simpler substances.
Safety first
This activity must be performed in the presence of a teacher.
Materials needed
A teaspoon of sugar, a boiling tube, a spirit lamp, a test tube holder, a watch glass.
Procedure
1. Put a teaspoon of sugar in a boiling tube.
2. Heat it gently.
3. Observe how the sugar changes colour.
4. Look for water droplets near the open end of the tube.
5. Observe what is left behind in the tube.
6. Scoop the black solid out onto a watch glass.
2. Heat it gently.
3. Observe how the sugar changes colour.
4. Look for water droplets near the open end of the tube.
5. Observe what is left behind in the tube.
6. Scoop the black solid out onto a watch glass.
Observation
The sugar turns brown, then black (charring). Small water droplets appear near the open end. Black charcoal (carbon) is left behind.
Explanation
Since we are heating, the water came from the dry sugar, not the air. Sugar decomposes on heating to give carbon and water. Water itself is made of hydrogen and oxygen. So sugar is a compound of carbon, hydrogen, and oxygen — not an element.
โ Summary
- Sugar heated
- Water + carbon form
- Sugar decomposes
- Sugar is a compound
In this Activity, we will compare a mixture of iron and sulfur with the compound formed when they are heated.
Safety first
Demonstrate under teacher supervision in a fume hood or well-ventilated area. Do not inhale the gases. Be careful with hydrochloric acid. Never smell anything directly — waft the fumes towards your nose.
Materials needed
5.6 g iron filings, 3.2 g sulfur powder, watch glasses, china dish, burner, tripod stand, mortar and pestle, magnet, test tubes, dilute hydrochloric acid.
Procedure
1. Take 5.6 g iron filings and 3.2 g sulfur powder on a watch glass; observe them.
2. Mix them thoroughly and label the mixture as Sample A.
3. Check if Sample A is uniform or non-uniform, and if iron and sulfur are still visible.
4. Take half of Sample A in a china dish and heat gently with continuous stirring until a black mass forms.
5. Let it cool, then grind it in a mortar with a pestle.
6. Put it on another watch glass and label it as Sample B.
7. Step 1 — Compare the appearance (colour and texture) of A and B.
8. Step 2 — Move a magnet over each sample and note what happens.
9. Step 3 — Add a few drops of dilute hydrochloric acid to each sample in a test tube.
10. Gently waft and smell the evolved gas; test it with a burning splinter.
11. Record all observations in Table 8.2.
2. Mix them thoroughly and label the mixture as Sample A.
3. Check if Sample A is uniform or non-uniform, and if iron and sulfur are still visible.
4. Take half of Sample A in a china dish and heat gently with continuous stirring until a black mass forms.
5. Let it cool, then grind it in a mortar with a pestle.
6. Put it on another watch glass and label it as Sample B.
7. Step 1 — Compare the appearance (colour and texture) of A and B.
8. Step 2 — Move a magnet over each sample and note what happens.
9. Step 3 — Add a few drops of dilute hydrochloric acid to each sample in a test tube.
10. Gently waft and smell the evolved gas; test it with a burning splinter.
11. Record all observations in Table 8.2.
Table 8.2 — Comparison of Samples A and B
| S.No. | Experiment | Observations | |
|---|---|---|---|
| Sample A | Sample B | ||
| 1. | Appearance (colour, texture) | Black and yellow particles seen separately | Black mass, same throughout |
| 2. | Magnet test | Iron attracted to magnet | Not attracted to magnet |
| 3. | Gas test (odour, burning) | Odourless gas, burns with a pop sound | Rotten-egg smell, no pop sound |
Observation
Sample A shows separate iron and sulfur; iron is pulled by the magnet; it gives an odourless gas that pops. Sample B is a uniform black mass; not magnetic; gives a rotten-egg-smelling gas.
Explanation
Sample A is a mixture — the elements keep their properties and can be separated by a magnet. Sample B is iron sulfide, a compound formed by heating; it has new properties, is not magnetic, and its elements cannot be separated. So heating turned a mixture into a compound.
โ Summary
- A is a mixture
- B is a compound
- Magnet separates A
- B has new properties
How do Sample A and Sample B react with dilute hydrochloric acid?
- Sample A: iron reacts to give iron chloride and hydrogen gas.
- The hydrogen gas is colourless, odourless, and burns with a pop.
- Sample B: iron sulfide reacts to give iron chloride and hydrogen sulfide.
- Hydrogen sulfide is colourless with a rotten-egg smell.
Acid Test: Mixture vs Compound
Sample A + dilute HCl
Iron chloride + hydrogen gas (pop sound).
↓
A is a mixture (iron stays iron)
↓
Sample B + dilute HCl
Iron chloride + hydrogen sulfide (rotten-egg smell).
↓
B is a compound (iron sulfide)
Important Points
- A compound forms when elements join chemically in a fixed ratio.
- A compound has new properties; its elements can't be physically separated.
- Iron + sulfur mixture (A) can be separated; iron sulfide (B) cannot.
Important Definitions
- Compound — a substance of two or more elements joined chemically in a fixed ratio, with new properties.
- Sodium chloride — common salt; sodium and chlorine combined in a 1:1 ratio.
- Iron sulfide — the black compound formed by heating iron and sulfur together.
๐ NCERT Question 1 — Consider the following reaction where
A and B (which can't be broken down) combine to form C. Which statement is correct?
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๐ NCERT Question 3 — Water, a compound, has
Water has different properties from hydrogen and oxygen. Justify this statement.
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๐ NCERT Question 5 — Iron reacts with moist
Iron forms iron oxide and magnesium forms magnesium oxide. Classify all substances involved.
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๐ NCERT Question 7 — What new substance is
What new substance forms when iron filings and sulfur are heated, and how is it different?
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๐ NCERT Question 9 — How would our daily
How would daily life change if water were a mixture of hydrogen and oxygen, not a compound?
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๐ NCERT Question 10 — Analyse Fig. 8.24. Identify
Iron filings react with dilute hydrochloric acid to give Gas A. Identify it and write the word equation.
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