Compounds and Activity 8.4 — Heating Sugar — "Nature of Matter: Elements, Compounds, and Mixtures"

1. 13 marksCuriosity Grade 8, Chapter 8, page 124

   Why cannot hydrogen and oxygen present in water be separated by physical means? Define a compound.

   Hint. The answer is about how tightly the particles are held.

   In water, the particles of hydrogen and oxygen are so tightly attached to each other that it is generally impossible to separate them apart using physical methods. That is why water is a compound.

   Compounds are formed when different elements combine in fixed ratios to form something entirely new. The properties of compounds are different from those of elements forming that compound. The constituent elements of a compound cannot be separated by any physical method.

   Three parts to the definition, and questions test all three:

   1. different elements combined chemically;
   2. in a fixed ratio;
   3. giving new properties, and no physical method separates them again.

   ✦ Compare this with a mixture, one line at a time. Salt dissolved in water is a mixture, and dissolved sodium chloride (common salt) may be separated from water by the physical process of evaporation. Boil it off and the salt is back. Boil water for as long as you like and no hydrogen appears — because in the compound the elements are chemically joined, not merely mixed together.

2. 23 marksCuriosity Grade 8, Chapter 8, page 124

   What is the ratio of hydrogen to oxygen atoms in water? What does 'fixed ratio' mean, and why does it matter?

   Hint. Compare with how much sugar you may put in tea.

   Molecules of water are made of two different elements: hydrogen and oxygen, combined chemically in a fixed ratio. The ratio of the number of atoms of hydrogen to oxygen in water has been found to be 2:1.

   What 'fixed' means. Every water molecule, from every source anywhere, has two hydrogen atoms to one oxygen atom. There is no such thing as water with a bit more oxygen in it. A ratio of 3:1 would not be water at all — it would either be a different substance or not a substance.

   ✦ This is the sharpest single difference from a mixture. Sugar solution can be made with one spoon of sugar or four, and it is a sugar solution either way. A compound has no such freedom: the composition is part of what the substance is.

   The chapter gives a second example on the same page. Sodium chloride is made up of particles of sodium and chlorine in a 1:1 ratio. Two compounds, two different fixed ratios, neither negotiable.

   And Activity 8.5 quietly relies on the same idea — it specifies 5.6 grams of iron and 3.2 grams of sulfur, not any amounts you please.

3. 34 marksCuriosity Grade 8, Chapter 8, page 124

   Sodium is a soft metal and chlorine is a hazardous gas, yet sodium chloride is safe and essential. What does this illustrate?

   Hint. This is the second half of the definition of a compound, in one example.

   It illustrates that the properties of a compound are entirely different from those of the elements that formed it.

   Sodium, a soft metal, and chlorine, a hazardous gas, combine to form a harmless yet taste-enhancing substance that is essential for our lives. This substance is known as sodium chloride, which is made up of particles of sodium and chlorine in a 1:1 ratio.

   	Sodium	Chlorine	Sodium chloride
   State	Soft metal	Gas	Solid crystals
   Effect on us	—	Hazardous	Harmless, and essential
   Taste	—	—	Salty; enhances flavour

   ✦ The chapter makes the same point twice more, deliberately. Beside Activity 8.3 a student says: hydrogen is a fuel, oxygen supports combustion, whereas water extinguishes fire. And in Activity 8.5 iron is magnetic and sulfur is yellow, while iron sulfide is neither. Three unrelated pairs of elements, three compounds that behave nothing like their parents — which is what makes this a rule rather than a coincidence.

   Why it matters for answering questions. You cannot predict a compound's properties by averaging its elements'. Knowing what sodium and chlorine do tells you almost nothing about what table salt does, and any answer that tries to reason that way is going wrong.

4. 44 marksCuriosity Grade 8, Chapter 8, pages 124-125

   Describe Activity 8.4 and its observations. What is left in the boiling tube at the end?

   Hint. Three things to watch: the colour, the droplets, and the residue.

   The activity. Put a teaspoon of sugar in a boiling tube and heat it gently over a spirit lamp, holding the tube with a test tube holder. This activity must be performed in the presence of a teacher.

   Observations, in order.

   1. As you heat the sugar, it turns brown.
   2. Later, it begins to char, i.e., it turns blackish.
   3. You will find small droplets of water inside the boiling tube near its open end.
   4. Charcoal (carbon) is left behind in the boiling tube, and can be scooped out onto a watch glass.

   ✦ All three observations are needed, and each does a different job. The colour change signals that a chemical change is under way. The water droplets show one product. The black residue shows the other. Take away any one and the conclusion no longer follows, since you would be missing part of what the sugar turned into.

   The chapter even suggests a check on the residue: explore if it burns like coal. If the black solid behaves like carbon, that is evidence it is carbon, rather than just something black.

5. 54 marksCuriosity Grade 8, Chapter 8, page 125

   Water droplets appear near the open end of the boiling tube. Was the water in the dry sugar, or did it condense from the air? How does the chapter decide?

   Hint. One sentence settles it, and it is about the direction heat is travelling.

   The water came from the sugar.

   Since we are heating the tube, the water must have come from the dry sugar and not from the air.

   Why that reasoning works. Condensation from the air needs a cool surface for warm moist air to touch. Here the tube is being heated, so its surface is hotter than the surrounding air, not cooler. Water vapour from the room cannot condense on a hot surface — so the droplets cannot have that origin, and the only remaining source is the sugar itself.

   ✦ The droplets form near the open end, which fits perfectly. Water vapour driven out of the sugar at the hot bottom rises, reaches the coolest part of the tube — the top, farthest from the flame — and condenses there. The position of the droplets is itself evidence for where they came from.

   Notice the word 'dry' in the question. The sugar was dry to begin with, so the water was not simply moisture clinging to it. It was released by the sugar breaking down, which means hydrogen and oxygen were part of the sugar all along.

6. 64 marksCuriosity Grade 8, Chapter 8, page 125

   What does Activity 8.4 prove about sugar? Set out the reasoning step by step.

   Hint. The argument runs through water, which you have already taken apart.

   The reasoning, in the chapter's own order.

   1. Sugar decomposes on heating and gives carbon and water.
   2. As you know, water consists of hydrogen and oxygen. — established in Activity 8.3.
   3. Hence, sugar cannot be an element. — an element cannot be broken into simpler substances, and sugar just was.
   4. It may be stated that sugar is a chemical compound consisting of the elements carbon, hydrogen, and oxygen.

   ✦ Step 2 is doing more work than it appears to. Heating gives carbon and water, and water is not an element — so listing sugar's elements as 'carbon and water' would be wrong. You have to substitute the earlier result in, and only then do the three elements appear. The two activities are built to be used together.

   A limit worth stating. The activity shows sugar contains carbon, hydrogen and oxygen; it does not tell you in what ratio. The chapter does not give sugar's ratio, and you should not supply one — knowing which elements are present and knowing how many of each are two different pieces of knowledge.

7. 73 marksCuriosity Grade 8, Chapter 8, pages 124-125

   Common salt dissolved in water can be recovered by evaporation, but the sodium in it cannot be recovered by any physical method. Explain the difference.

   Hint. There are two different levels of 'putting things together' in play here.

   Two quite different situations are being compared, and the chapter is careful about both.

   	Salt in water	Sodium in salt
   What holds them	Merely mixed — salt particles sit in the interparticle spaces of water	Chemically combined in a fixed 1:1 ratio
   It is a…	Mixture	Compound
   Can physical methods separate it	Yes — dissolved sodium chloride may be separated from water by the physical process of evaporation	No — the constituent elements of a compound cannot be separated by any physical method

   ✦ Evaporating a salt solution gives you back salt, not sodium. That is the whole point of the chapter's question Is it possible to separate sodium chloride into its elements by physical processes? You can undo the mixing; you cannot undo the chemical combination the same way.

   And the same substance sits on both sides of the line. Sodium chloride is the component of one system and the compound in the other. So whether something can be separated depends on how it is held, not on what it happens to be made of — which is exactly the distinction between a mixture and a compound.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity — Textbook of Science for Grade 8, Chapter 8 (hecu108.pdf), Reprint 2026-27, pages 116-133. Questions are referenced from the NCERT textbook for identification.