Activities 9.3 to 9.5 — Measuring Mass and Volume — "The Amazing World of Solutes, Solvents, and Solutions"

1. 14 marksCuriosity Grade 8, Chapter 9, page 142

   Describe Activity 9.3 — how to measure the mass of a stone on a digital balance. Why is the balance tared twice?

   Hint. Count the resets and ask what each one removes.

   The procedure.

   1. Switch ON the digital weighing balance.
   2. Check the initial reading — it should show a zero reading. If not, then we must bring it to zero by pressing the tare or reset button.
   3. Place a dry and clean watch glass or butter paper on the pan and note the reading.
   4. Reset to zero again by pressing tare.
   5. Carefully place the stone on the watch glass.
   6. Note the reading — which gives the mass of the stone, say 16.400 g.

   Why tare twice. The first zeroing removes any error in the empty balance itself. The second removes the mass of the watch glass, so that what the display finally shows is the stone alone and not the stone plus its container. Because the balance now treats the loaded watch glass as its new zero, no subtraction is needed afterwards.

   ✦ The watch glass matters even so. It keeps the stone from marking the pan and, for a liquid, is what makes the measurement possible at all — the mass of a liquid may be measured by replacing the watch glass with a beaker and pouring the desired amount of liquid into it, taring the empty beaker first.

   (Note the reading is written 16.400 g, not 16.4 g. The trailing zeros record that the balance reads to the nearest milligram.)

2. 23 marksCuriosity Grade 8, Chapter 9, page 142

   What is the difference between mass and weight? What do most balances actually measure?

   Hint. The chapter admits something surprising about the instrument in the previous activity.

   Mass is the quantity of matter present in an object or a substance. Its units are gram (g) and kilogram (kg). On the other hand, weight is the force by which the Earth attracts an object or a substance towards itself, and it is measured in newtons (N).

   	Mass	Weight
   What it is	Quantity of matter	The force with which Earth attracts the object
   Units	g, kg	newton (N)

   And the admission: Most balances (except two-pan balances like in Fig. 9.13) actually measure weight, but their scales are marked in mass units, so they show values in grams or kilograms.

   ✦ So the digital balance in Activity 9.3 is measuring one thing and reporting another. This works on Earth because on Earth, weight and mass are closely related — the same object always has the same weight for a given mass, so a weight reading can be relabelled in grams without causing trouble. Since a two-pan balance compares an unknown against known masses, it is the one instrument here that genuinely measures mass, whatever the gravity.

   Chapter 5 covered this ground, and the chapter says so: the words 'mass' and 'weight' are often used interchangeably in everyday language. But they have different meanings in science, which can sometimes cause confusion.

3. 33 marksCuriosity Grade 8, Chapter 9, page 143

   State the units of volume the chapter gives and the relationships between them.

   Hint. Start from the SI unit and work down to the ones you actually use.

   Volume is the space occupied by an object. The SI unit of volume is cubic metres, written as m³. It is the volume of a cube whose each side is one metre in length.

   Unit	Relationship
   m³	The SI unit — a cube of side 1 metre
   dm³	Volume of liquids is expressed in litres (L) which is equivalent to 1 dm³
   cm³	One centimetre cube is also written as one cc
   mL	A commonly used submultiple of a litre ... which is equivalent to 1 cm³

   The two equalities to hold on to: 1 L = 1 dm³, and 1 mL = 1 cm³.

   ✦ That second equality is what makes the displacement method work. In Activity 9.7 you measure a rise in water level in mL and record the volume of a solid in cm³ — and the chapter notes it explicitly: the values of volume are obtained in units of mL, which can be written in the equivalent unit cm³ for solids. Because the two units are the same size, no conversion arithmetic is needed; you simply change the label.

   A tetra pack marked 200 mL therefore contains 200 cm³ of buttermilk — the same quantity written two ways.

4. 44 marksCuriosity Grade 8, Chapter 9, pages 143-144

   Work through Activity 9.4 for the 100 mL measuring cylinder of Fig. 9.16. What is the smallest volume it can read, and how do you find that out?

   Hint. Count the divisions between two big marks and divide.

   The method, in three questions.

   1. What is the maximum volume it can measure? The cylinder is marked as 100 mL; therefore, it can measure volume up to 100 mL.
   2. How much is the difference between two bigger marks? The volume difference indicated between 10 mL and 20 mL, or between 40 mL and 50 mL, is 10 mL.
   3. How many smaller divisions lie between them? The number of divisions between these marks is 10.

   The calculation.

   One small division = 10 mL ÷ 10 = 1 mL

   That is, the smallest value that this measuring cylinder can read is 1 mL.

   ✦ This is the same procedure you used for a thermometer in Grade 6, and the chapter says so. Since the method belongs to the instrument rather than to what it measures, it transfers unchanged: find the interval between labelled marks, count the divisions in it, divide.

   The chapter's table of least counts by size:

   Capacity	Smallest volume it can read
   10 mL or 25 mL	0.1 mL
   100 mL	1 mL
   250 mL	2 mL
   500 mL	5 mL

   The pattern is the trade-off: the bigger the cylinder, the coarser the reading.

5. 54 marksCuriosity Grade 8, Chapter 9, page 144

   You need 70 mL of water. Why is a 100 mL measuring cylinder the best choice rather than a 50 mL or a 250 mL one?

   Hint. Two things can go wrong — one with the small cylinder, one with the large.

   The 50 mL cylinder — too small. If we use a 50 mL measuring cylinder, it would not be possible to measure 70 mL of water in one step. First, we have to measure 50 mL water and then 20 mL. Measuring volume in more than one step is not convenient. Each extra step is another chance to spill or misread.

   The 250 mL or 500 mL cylinder — too coarse. The measurement can be done in one step but the accuracy would be reduced as the smallest volume that these measuring cylinders can measure is greater than that of a 100 mL measuring cylinder. A 250 mL cylinder reads to 2 mL and a 500 mL one to 5 mL, against 1 mL for the 100 mL cylinder.

   Hence, a 100 mL measuring cylinder is the best choice for this measurement.

   ✦ The rule that comes out of this is worth stating generally: choose the smallest instrument that can take the whole measurement in one go. That gets you the finest divisions available without forcing you to measure in stages. Because the two failures pull in opposite directions, the best choice is always the one just large enough — which is the same reasoning behind picking a thermometer or a balance.

6. 64 marksCuriosity Grade 8, Chapter 9, pages 144-145

   Describe Activity 9.5 — measuring 50 mL of water. What is a meniscus, and how do you read it correctly?

   Hint. The reading rule is different for coloured liquids, and the chapter says why nowhere — just says what.

   The procedure. Place a clean, dry measuring cylinder on a flat surface. Pour water in slowly to the required mark, adjusting with a dropper if needed. Then read it.

   On careful observation, you will notice that the water inside the measuring cylinder forms a curved surface. This curved surface is called the meniscus.

   The two reading rules:

   Liquid	Read the mark that coincides with
   Water or other colourless liquids	The bottom of the meniscus
   Coloured liquids	The top of the meniscus

   And the eye position: Make sure that the eyes are at level with the bottom of the meniscus while noting the readings.

   ✦ Each instruction removes a specific error. A cylinder that is not on a flat surface gives a tilted, meaningless level. A dropper lets you creep up on the mark instead of overshooting and pouring back. And reading from above or below shifts the apparent position of the mark against the level — which is why the eye must be level with it. Because all three errors are avoidable and none is obvious, the activity spells them out.

   The coloured-liquid rule is practical: in a dark liquid you cannot see the bottom of the curve at all, so the top edge is the only line you can locate reliably.

7. 73 marksCuriosity Grade 8, Chapter 9, page 144

   Why are measuring cylinders always designed narrow and tall instead of wider and short like a beaker?

   Hint. Imagine adding 1 mL of water to each and watching the level.

   Because a narrow tall container turns a small change in volume into a large change in height, which is what makes it readable.

   Think it through with numbers. Add 1 mL of water to a narrow cylinder and the level climbs a visible distance; add the same 1 mL to a wide beaker and it spreads over a much larger area, so the level barely moves. Since the divisions on the scale are just heights, the narrow cylinder can carry finely spaced marks that you can actually distinguish, while a wide one cannot.

   ✦ This is the same reasoning as the least-count discussion on the same page, arriving from another direction. A 500 mL cylinder is wider as well as taller than a 100 mL one, and its smallest reading is 5 mL rather than 1 mL — because the extra width spreads each millilitre over more area and squashes the scale.

   And it answers one of the chapter's opening questions. Why are water bottles usually tall and cylindrical in shape instead of spherical? Part of the answer is the same: a tall narrow shape makes the level meaningful. A sphere is the worst possible shape for reading a level, since the same volume changes the height by different amounts depending on how full it already is.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity — Textbook of Science for Grade 8, Chapter 9 (hecu109.pdf), Reprint 2026-27, pages 134-151. Questions are referenced from the NCERT textbook for identification.