Elements — Atoms, Molecules, Metals and Non-metals — "Nature of Matter: Elements, Compounds, and Mixtures"

1. 13 marksCuriosity Grade 8, Chapter 8, page 123

   Define an element. What are its particles called, and what is the chapter's key statement about them?

   Hint. The definition is negative — it says what cannot be done.

   Elements are substances that cannot be further broken down into simpler substances. They are the building blocks of all matter.

   Each element is made up of identical particles called atoms. These particles are different from the particles of any other element.

   Two claims, both essential. Within one element every atom is the same as every other. Between two elements the atoms are different. That second claim is what makes elements genuinely distinct kinds of matter rather than different arrangements of the same stuff.

   ✦ This is where Chapter 7 hands over. There, everything was 'constituent particles' and the last page mentioned that these are atoms and molecules. Here that idea does real work, because it lets the chapter say something exact: an element is the substance whose atoms are all of one kind. The chapter's examples are the two from Activity 8.3 — hydrogen and oxygen — plus gold, silver, sulfur and carbon.

   Note the wording cannot be further broken down, not cannot be broken down at all. Electricity broke water down; nothing you do in this chapter breaks hydrogen down. The boundary of the chapter's methods is where 'element' begins.

2. 23 marksCuriosity Grade 8, Chapter 8, page 123

   What is a molecule? Why do the chapter's diagrams (Fig. 8.10) show hydrogen and oxygen as pairs of circles rather than single ones?

   Hint. Most elements' atoms cannot manage alone.

   The atoms of most of the elements cannot exist independently. Two or more such atoms combine and form a stable particle of that element called a molecule. For example, two atoms of hydrogen combine to form one molecule of hydrogen. Similarly, two atoms of oxygen combine to form one molecule of oxygen.

   Why the diagrams show pairs. Because that is what a sample of hydrogen gas actually contains — not lone hydrogen atoms but molecules of two atoms each. The same for oxygen. The pair is the stable particle, so a picture of single atoms would be a picture of something that does not persist on its own.

   ✦ A molecule of an element is still that element, and this catches people out. Both circles in the hydrogen molecule are hydrogen atoms; nothing else has joined in. Contrast Fig. 8.11, the water molecule, where two different kinds of atom are shown together — that is a compound. Same number of atoms is not the point; whether they are the same kind is.

   	Hydrogen (Fig. 8.10a)	Water (Fig. 8.11)
   Atoms shown	2, both hydrogen	3 — two hydrogen, one oxygen
   Kinds of atom	One	Two
   Therefore	An element	A compound

3. 33 marksCuriosity Grade 8, Chapter 8, page 123

   How are elements classified into metals and non-metals? What are metalloids, and which two does the chapter name?

   Hint. The chapter lists five metals and four non-metals by name.

   Elements can be classified into metals and non-metals. You have already studied that gold, silver, magnesium, iron, and aluminium are metals, whereas carbon, sulfur, hydrogen, and oxygen are non-metals.

   Some elements like silicon and boron have intermediate properties between those of metals and non-metals? They are called metalloids, about which you will learn in higher grades.

   Metals	Non-metals	Metalloids
   Gold, silver, magnesium, iron, aluminium	Carbon, sulfur, hydrogen, oxygen	Silicon, boron

   ✦ Keep to the chapter's own lists in an answer. It names exactly five metals, four non-metals and two metalloids, and it says outright that metalloids belong to higher grades — which means you are expected to know the word and the two examples, not a set of properties for them.

   The two elements from Activity 8.3 are both non-metals, and so are the two elements of Activity 8.5's sulfur half. Iron, the other half, is a metal. Being able to place the chapter's own substances in this table is what most questions on it actually require.

4. 44 marksCuriosity Grade 8, Chapter 8, page 123

   State the four facts the chapter gives about how many elements there are and what states they exist in.

   Hint. This is the 'A step further' box, and every number in it is exact.

   The number of elements known at present is 118, and most of them exist in a solid state.

   Eleven elements exist in a gaseous state at room temperature, all of which are non-metals like oxygen, helium, nitrogen, etc.

   Only two elements are liquid at room temperature — mercury, which is a metal and bromine, which is a non-metal.

   Although gallium and caesium are solid elements, they become liquid at a temperature around 30 °C (303 K).

   Fact	Number
   Elements known at present	118
   Gaseous at room temperature	11, all non-metals
   Liquid at room temperature	2 — mercury (metal), bromine (non-metal)
   Solid but melting near 30 °C	Gallium and caesium

   ✦ Mercury and bromine are the pair to remember, because they are a matched set: one metal, one non-metal, one each. And notice how neatly gallium and caesium make the point that 'solid' and 'liquid' are statements about a temperature. Around 30 °C is not far above a warm room — so a gallium spoon melts in hot tea, and it is still a solid element by the chapter's classification.

   Do not extend these numbers. The chapter gives no count of natural versus artificial elements and no periodic table; adding either is adding material that is not in this year's syllabus.

5. 53 marksCuriosity Grade 8, Chapter 8, page 124

   More than 45 different elements go into making a mobile phone. Why is this fact placed in a chapter about elements, compounds and mixtures?

   Hint. Ask what the number is doing in the argument, not just what it is.

   More than 45 different elements, like aluminium, copper, silicon, cobalt, lithium, gold, silver, etc., are used in manufacturing a mobile phone, including its screen, battery, and other components.

   What the fact establishes. Elements are not a laboratory abstraction. Out of 118 known elements, more than a third are in the object in your pocket, each chosen because of what it can do — copper to conduct, silicon for the chip, lithium and cobalt for the battery, gold for connections that must not corrode.

   ✦ It also quietly makes the chapter's central point about materials. A phone is not made of a phone-substance; it is made of elements combined and mixed in particular ways. That is exactly what §8.4 says innovation consists of — engineers and material scientists rely on their understanding of compounds and mixtures to design materials with unique properties.

   And it has a real-world consequence worth mentioning: those elements have to be mined and, when the phone is thrown away, are lost unless it is recycled. A single fact about composition turns straight into a question about resources.

6. 63 marksCuriosity Grade 8, Chapter 8, page 123

   Hydrogen and oxygen were obtained from water. How do you know they are elements rather than compounds that could be broken down further?

   Hint. Be honest about what the activity does and does not show.

   What the activity gives you. It shows that water can be broken into two simpler substances, and that neither of them is water. It does not, by itself, prove that those two cannot be broken further — the chapter tells you they are elements:

   The two substances hydrogen and oxygen formed in Activity 8.3, are pure substances and are termed as elements.

   What would count as evidence. An element is a substance nothing has ever succeeded in breaking down into anything simpler. Hydrogen and oxygen have that status because every attempt to decompose them chemically has failed — not because of anything visible in one experiment with a 9 V battery.

   ✦ Writing this honestly is worth more than overclaiming. A good answer says: the activity establishes that water is not an element, and identifies its two constituents; the statement that those constituents are themselves elements rests on the wider evidence the chapter is reporting. That is how the classification actually works — 'element' is a claim about everything that has been tried, so it can only ever be supported by the whole record, never by a single test.

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.