9.The Amazing World of solute solvents and solutions.ppt/8th science curiosity/by k sandeep swamy

134
Curiosity — Textbook of Science for Grade 8
The Amazing World
of Solutes, Solvents,
and Solutions
9
Probe and ponder
z What do you think is happening in the picture above?
z What happens when you add too much sugar to your tea and it
stops dissolving? How can you solve this problem?
z Why do sugar and salt dissolve in water but not in oil? Why is
water considered a good solvent?
z Why are water bottles usually tall and cylindrical in shape
instead of spherical?
Share your questions
?
Chapter 9.indd 134
Chapter 9.indd 134 6/28/2025 5:05:29 PM
6/28/2025 5:05:29 PM

Chapter 9 — The Amazing World of Solutes, Solvents, and Solutions 135
Fig. 9.3: Magnified schematic picture of a
solute evenly distributed in a solvent
Solute
Solvent
You must have taken an Oral Rehydration Solution (ORS) at some
time in your life. ORS is used to treat dehydration by keeping
your body hydrated. You have learnt to prepare ORS at home in
Curiosity, Grade 6. You may have wondered why every sip of
your homemade ORS tastes the same, no matter how much you
drink. Why does it not taste salty in one sip and sweet in another?
This is because when you add sugar and salt to water, they
form a mixture in which the components are evenly distributed
throughout.
Can you predict whether this mixture is uniform or
not (Fig. 9.1)? What happens when chalk powder is mixed
with water—does it form a uniform mixture?
When salt and sugar are mixed with water, a uniform
mixture is formed, whereas when chalk powder or sand, or
sawdust is mixed with water, the components are not evenly
distributed. Such mixtures are known as non-uniform
mixtures (Fig. 9.2a and 9.2b).
Let us explore the science of mixing things together.
9.1 What Are Solute, Solvent, and Solution?
A uniform mixture, such as that of salt or sugar, and water, is
called a solution. Whenever a solid is
mixed with a liquid to form a solution, the
solid component is called the solute, and
the liquid component is called the solvent.
The solute dissolves in the solvent to form
a solution (Fig. 9.3).
Solute + Solvent Solution
When a solution is formed by mixing
two liquids, it is not always clear which
substance is dissolving the other. In such
cases, the substance present in smaller
amount is called the solute, while the one
in larger amount is called the solvent.
We know air is a
mixture. Would a
mixture of gases also be
considered a solution?
Just as water can act as a solvent in liquid solutions, gases can
also form solution— with air being a common example.
Air is a gaseous solution. Since nitrogen is present in the largest
amount in the air, it is considered as the solvent, while oxygen,
argon, carbon dioxide, and other gases are considered as solutes.
Fig. 9.1: Mixture of
sugar, salt, and water
Fig. 9.2: Mixture of (a) Sand and
water; (b) Sawdust and water
Chapter 9.indd 135
Chapter 9.indd 135 6/28/2025 5:05:31 PM
6/28/2025 5:05:31 PM

136
Ever heard of ...
The Chashni (sugar syrup) of the Indian
sweet Gulab jamun is made of a large amount
of sugar (solid) dissolved in a small amount
of water (liquid). However, the water is still
considered as the solvent and sugar as the
solute (Fig. 9.4)! Fig. 9.4: Gulab jamuns
dipped in sugar syrup
9.2 How Much Solute Can a Fixed Amount of
Solvent Dissolve?
Activity 9.1: Let us investigate
z Take a clean glass tumbler and fill it
half with water.
z Add one spoon of salt into it and stir well
till it dissolves completely (Fig. 9.5).
z Gradually add a spoonful of salt into the glass tumbler and
stir. Observe how many spoons of salt you can add before it
stops dissolving completely.
z Record your observations in Table 9.1.
Amount of salt taken
(teaspoon)
Observation (salt dissolves/salt
does not dissolve)
One
Two
Three
Four
…
Table 9.1: Dissolution of salt in water
Some discussion points
z How many spoons of salt were you able to dissolve before
some of it remained undissolved?
z What does this indicate about the capacity of water to
dissolve salt?
What will happen if we
keep on adding more
salt in a given amount
of water?
Chapter 9.indd 136
Chapter 9.indd 136 6/28/2025 5:05:38 PM
6/28/2025 5:05:38 PM

You might have observed that, initially, the salt completely
dissolves in the water, forming a solution. After adding a few
more spoons of salt, a stage comes when the added salt does not
dissolve completely and the undissolved salt settles at the bottom.
This indicates that the water can no longer dissolve any more salt
because it has reached its limit. The solution in which more solute
can be dissolved at a given temperature, is called an unsaturated
solution (Fig. 9.5). However, when the solute stops dissolving and
begins to settle at the bottom, the solution is called a saturated
solution at that particular temperature (Fig. 9.6).
The amount of solute present in a fixed quantity of solution
(or solvent) is termed as its concentration. Depending upon the
amount of solute present in a fixed quantity of solution, it can be
called a dilute solution (less amount of solute) or a concentrated
solution (more amount of solute). Dilute and concentrated are
relative terms.
So, one can say in Activity 9.1, the solution obtained by
dissolving one spoon of salt is dilute as compared to that obtained
by dissolving two or more spoons of salt.
Can you now reflect — which solution is more concentrated;
2 spoons of salt in 100 mL of water or 4 spoons of salt in 50 mL
of water?
From Activity 9.1, we can say that the maximum amount of
solute that dissolves in a fixed quantity of the solvent is called its
solubility.
Does temperature affect the solubility of a solute?
Let us find out!
9.2.1 How does temperature affect the solubility of a
solute?
Activity 9.2: Let us experiment
(Demonstration activity)
z Take about 50 mL of water in a glass beaker and measure its
temperature using a laboratory thermometer, say 20 °C.
z Add a spoonful of baking soda (sodium hydrogen carbonate)
to the water and stir until it dissolves.
Continue adding small amounts of baking
soda while stirring, till some solid baking
soda is left undissolved at the bottom of
the beaker.
Safety first
Be careful while using the
heating device.
Fig. 9.5: Unsaturated
solution
Fig. 9.6: Saturated
solution
Solution
Undissolved
salt
Chapter 9.indd 137
Chapter 9.indd 137 6/28/2025 5:05:40 PM
6/28/2025 5:05:40 PM

138
z Now, heat the contents to 50 °C while
stirring (Fig. 9.7).
z What happens to the undissolved baking
soda?
z You will observe that it has dissolved.
z Continue adding more baking soda while
stirring at this temperature until some
solid baking soda remains undissolved.
z Again, heat the contents further to 70 °C
while continuing to stir. What do you
observe?
z The undissolved baking soda dissolves.
z Whatdoyouinferfromthisexperiment?
Water at 70 °C dissolves more baking soda
than water at 50 °C. The amount of baking
soda dissolved in water at 20 °C is even lesser.
It has been found that for most of
the substances, the solubility increases with an increase in
temperature. We can also say that a saturated solution at a
particular temperature behaves as an unsaturated solution if the
temperature is increased.
Our scientific heritage
Water has primarily been used as a solvent for the preparation of
medicinal formulations in Ayurveda, Siddha, and other traditional
systems of medicine in India. Additionally, drug formulations have
been prepared using hydro-alcoholic extracts of the herbs. The
Indian systems of medicine have also referred to the use of oils,
ghee, milk, and other substances as solvents for drug formulations,
to help achieve the therapeutic benefits of the drug.
Be a scientist
What inspired Asima Chatterjee to work on medicinal plants?
Asima Chatterjee is renowned for her work in
developing anti-epileptic and anti-malarial drugs. She
used solvents and solutions extensively to extract and
isolate important compounds from medicinal plants.
She earned a Doctorate of Science, becoming the
second Indian woman to do so after Janaki Ammal. She
became the first woman to receive the Shanti Swarup
Bhatnagar Award in the field of chemical science and
was also honoured with the Padma Bhushan.
Laboratory stand
Laboratory
thermometer
Glass rod
Beaker
Water
containing
baking soda
Wire gauze
Tripod stand
Spirit lamp
Fig. 9.7: Dissolution of baking soda in water
Chapter 9.indd 138
Chapter 9.indd 138 6/28/2025 5:05:50 PM
6/28/2025 5:05:50 PM

Do gases also dissolve in water?
9.3 Solubility of Gases
Many gases, including oxygen, dissolve in water. Oxygen
dissolves in water only to a small extent. Even though present
in minute quantities, it is this dissolved oxygen that sustains
all aquatic life, including plants, fishes, and other organisms.
Is the mixture of gases in water a uniform or non-uniform
mixture?
It is a uniform mixture because the gases
dissolve evenly in water to form a solution.
Does temperature affect the solubility
of gases in liquids also? If so, how?
It has been observed that the solubility
of gases generally decreases as temperature
increases. More oxygen can dissolve in
cold water, ensuring sufficient oxygen for
aquatic life (Fig. 9.8). On the other hand,
when water warms up, the solubility of
oxygen decreases.
Now I understand that the mixtures we use can be
of two types—uniform and non-uniform. Uniform
mixtures are called solutions, and their components
are not visible separately. In non-uniform mixtures,
the components can be seen either with the naked
eye or with a magnifying device.
I observed that in some non-uniform mixtures, such
as sawdust in water, the sawdust floats, whereas
in the mixture of sand and water, the sand sinks.
I wonder why that happens?
Fig. 9.8: Aquatic species in water
Chapter 9.indd 139
Chapter 9.indd 139 6/28/2025 5:05:52 PM
6/28/2025 5:05:52 PM

140
9.4 Why Do Objects Float or Sink in Water?
You must have observed that some objects float while
others sink in water (Fig. 9.9). You may have noticed that,
while washing rice, husk particles present in the rice float
on the surface of water while rice sinks to the bottom
of the container. Why does this happen? If you add oil
to water, it floats on water. Generally, it is believed that
objects that float in a liquid are lighter and others that
sink are heavier than the liquid.
A wooden stick and an iron rod may be of the same size,
yet the iron rod feels much heavier. When we say that iron
is heavier than wood, we are referring to a special property
known as density, which describes the heaviness of an object.
Note
However, the density of a substance is not the only factor that
decides whether it will float or sink in a particular liquid.
Let us explore further.
9.5 What Is Density?
Imagine a crowded bus where many people are packed
together—this is an example of high density whereas,
the same bus with only a few people is an example of
low density. Similarly, a forest where trees grow close
to each other is called a dense forest (Fig. 9.10a), but if
the trees are far apart (Fig. 9.10b), it is considered less
dense.
How do scientists define density?
Let us find out.
We have learnt that matter is anything that possesses
mass and occupies space (volume). Density is defined
as the mass present in a unit volume of that substance.
The density of a substance may be expressed
mathematically using the formula:
Density =
Mass
Volume
The density of a substance is independent of its shape or size.
However, it is dependent on temperature and pressure. Pressure
primarily affects the density of gases, while its effect on solids
and liquids is negligible.
Fig. 9.9: Some objects float
while others sink in water
Fig. 9.10 (a): Dense forest
Fig. 9.10 (b): Less dense forest
Chapter 9.indd 140
Chapter 9.indd 140 6/28/2025 5:06:01 PM
6/28/2025 5:06:01 PM

Fig. 9.11: Packed oil
Think like a scientist
Have you noticed that some packets of ghee or oil are
labelled with a volume of 1 litre but a weight of only say
910 grams (Fig. 9.11)? What does this tell us about the
density of the oil, and is it less or more than that of water?
The units in which density is expressed will depend upon the
units of mass and volume taken. As you have learnt, the SI units
of mass and volume are kilogram (kg) and cubic metre (m3
),
respectively. Therefore, the SI unit of density is kilogram per cubic
metre, abbreviated as kg/m3
. In case of liquids, other units of density
are also used for convenience, such as gram per millilitre, abbreviated
as g/mL and gram per cubic centimetre, abbreviated as g/cm3
.
Conversion factor for density
1 kg/m3
= 1000 g/m3
= 1000 g/1000 L = 1 g/L = 1 g/1000 mL =
1 g/1000 cm3
The mass of 1 mL of water is close to 1 g at room temperature.
For the measurement of the mass of water, we generally consider
the volume in mL and its mass in g. Hence, 10 mL of water would
be approximately 10 g. Similarly, 100 mL of water would be
approximately 100 g.
Suppose the mass of an aluminium block is 27 g and its volume
is 10 cm3
, its density is 2.7 g/cm3
.
From this, it can be said that aluminium is 2.7 times denser
than water. We express this fact by saying that the relative
density of aluminium with respect to water is 2.7. It is a number
without any units.
=
Density of that substance
Density of water at that temperature
Relative density of any substance
with respect to water
9.5.1 Determination of density
The density of an object can be determined by measuring its
mass and volume.
How to measure mass?
You learnt the term ‘mass’ in Curiosity, Grade 6. Mass is the
quantity of matter present in any object. The instrument used to
measure the mass of an object is known as a balance. You must
have seen various types of balances being used by shopkeepers.
Here, we are using a digital weighing balance to measure the
mass. You learnt in chapter ‘Exploring Forces’ that on Earth,
weight and mass are closely related.
Chapter 9.indd 141
Chapter 9.indd 141 6/28/2025 5:06:04 PM
6/28/2025 5:06:04 PM

142
You may measure the mass by doing the following activity.
Activity 9.3: Let us measure
z Switch ON the digital weighing balance.
z Observe the initial reading on the digital weighing
balance display.
z It should show a zero reading. If not, then we must
bring it to zero by pressing the tare or reset button
(Fig. 9.12a).
z Place a dry and clean watch glass or butter paper
on the pan.
z Note the reading on the digital weighing balance.
z Reset the digital weighing balance reading to zero
by pressing the tare or reset button as shown in
Fig. 9.12b.
z Now, carefully place the solid object, such as stone,
on the watch glass (Fig. 9.12c).
z Note the reading displayed on the balance, which
gives the mass of the stone, say 16.400 g.
(You may use any other type of balance available in
your school.)
Fig. 9.12(a): Digital
weighing balance
Digital
weighing
balance
Fig. 9.12(b): Tare the balance
after placing a watch glass
Watch
glass
Fig. 9.12(c): Weighing a solid
object on digital balance
Stone
or solid
object
Note
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.
A step further
As mentioned in Chapter 5, the words ‘mass’ and
‘weight’ are often used interchangeably in everyday
language. But they have different meanings in
science, which can sometimes cause confusion.
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). 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 (Fig. 9.12c).
Pan Pan
Hook
Pointer
Fig. 9.13: Two-pan balance
Chapter 9.indd 142
Chapter 9.indd 142 6/28/2025 5:06:10 PM
6/28/2025 5:06:10 PM

Fig. 9.15: Measuring cylinders of different
capacities
How to measure volume?
A tetra pack says it contains 200 mL buttermilk (chach) (Fig. 9.14).
What does that mean?
You learnt in Curiosity, Grade 6 that volume is the space
occupied by an object. You also know that 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. Volume of smaller objects is
conveniently expressed in a decimetre cube (dm3
) or centimetre
cube (cm3
). One centimetre cube is also written as one cc. Volume
of liquids is expressed in litres (L) which is equivalent to 1 dm3
.
A commonly used submultiple of a litre is millilitre (mL) which is
equivalent to 1 cm3
.
One of the common apparatuses used to
measure the volume of liquids is a measuring
cylinder. It is a narrow transparent cylindrical
container with one side open and the other side
closed as shown in Fig. 9.15. There are markings
on the transparent body of the cylinder that
indicate the volume of liquid in the measuring
cylinder. We can use it to measure the desired
amount of a liquid.
Measuring cylinders are available in different
sizes to measure volume— 5 mL, 10 mL, 25 mL,
50 mL, 100 mL, 250 mL, etc (Fig. 9.15). How
accurately can these measuring cylinders
measure?
Let us find out!
Activity 9.4: Let us observe and calculate
InCuriosity,Grade6,chapter‘TemperatureandItsMeasurement’,
you learnt how to use the thermometer and to find its smallest
reading; you can do the same with a measuring cylinder.
Take a measuring cylinder and observe it carefully. Note
down the following:
z What is the maximum volume it can measure?
Now look at the measuring cylinder (Fig. 9.16) carefully. The
cylinder is marked as 100 mL; therefore, it can measure volume
up to 100 mL.
What is the smallest volume it can measure? Look at the
measuring cylinder again.
z How much is the volume difference indicated between the
two bigger marks (for example, between 10 mL and 20 mL)?
Fig. 9.14: A pack of
buttermilk of 200 mL
Chapter 9.indd 143
Chapter 9.indd 143 6/28/2025 5:06:14 PM
6/28/2025 5:06:14 PM

144
z How many smaller divisions are there between the two
bigger marks?
z How much volume does one small division indicate?
The smallest volume that the measuring cylinder can read
is__________.
For the measuring cylinder shown in
Fig. 9.16, the volume difference indicated
between 10 mL and 20 mL, or between 40 mL
and 50 mL, is 10 mL.
The number of divisions between these
marks is 10.
So, one small division can read 10 ÷ 10 = 1 mL.
That is, the smallest value that this measuring cylinder can
read is 1 mL.
The smallest volume that a measuring cylinder can measure
depends on the capacity of the measuring cylinder. Usually it is
0.1 mL in smaller measuring cylinders with a capacity of 10 mL
or 25 mL, it is 1 mL in a 100 mL measuring cylinder, 2 mL in a
250 mL measuring cylinder, and 5 mL in a 500 mL measuring
cylinder. Suppose we want to take 70 mL of water. 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. On the other hand, if a 250 mL or 500 mL measuring
cylinder is used, 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. Hence, a 100 mL measuring cylinder is the
best choice for this measurement.
Activity 9.5: Let us measure 50 mL of water
z Place a clean, dry measuring cylinder on a flat surface.
z Pour water slowly into the measuring cylinder up to the
required mark, as shown in Fig. 9.17.
z If required, adjust the level of water in the measuring
cylinder by adding or removing a small amount of water
using a dropper.
z On careful observation, you will notice that the water inside
the measuring cylinder forms a curved surface. This curved
surface is called the meniscus (Fig. 9.18).
z Read the mark on the measuring cylinder that coincides with
the bottom of the meniscus for water or other colourless
liquids.
Fig. 9.16: Measuring
cylinder of 100 mL
Whyaremeasuringcylinders
always designed narrow and
tall instead of wider and
short like a beaker?
Chapter 9.indd 144
Chapter 9.indd 144 6/28/2025 5:06:14 PM
6/28/2025 5:06:14 PM

z Make sure that the eyes are at level with the bottom of the
meniscus while noting the readings as shown in Fig. 9.18.
Fig. 9.17: Pouring water into
the measuring cylinder
Fig. 9.18: Measuring the reading
z Once it reaches the required
level—that is, 50 mL—
transfer this water to the
required container.
In case of coloured liquids
the mark on the measuring
cylinder should coincide with
the top of the meniscus!
Determining volume of solid objects with regular
shapes
Activity 9.6: Let us calculate
z Collect various objects with a cuboid shapes, such as a
notebook, a shoe box, or a dice.
z Measure the length (l), width (w), and height (h) of the objects
using a scale. Suppose the length of the notebook is 25 cm,
the width is 18 cm, and the height is 2 cm.
z Calculate the volume by using the following formula.
Volume = l × w × h
Volume = 25 cm × 18 cm × 2 cm = 900 cm3
z Record in your notebook.
Determining volume of objects with irregular shapes
Imagine you have an object, like a stone, that does not have a
regular shape. To calculate its density, the main challenge is to
find its volume. Let us learn how the volume of a solid with an
irregular shape can be determined.
Meniscus
I wonder how the
level of a coloured
liquid is measured?
Chapter 9.indd 145
Chapter 9.indd 145 6/28/2025 5:06:18 PM
6/28/2025 5:06:18 PM

146
Activity 9.7: Let us measure
z Collect various objects from your surroundings, such as a
stone, metal keys, and so on.
z Fill a measuring cylinder with water up to any desired
volume, say 50 mL (Fig. 9.19a) and record the initial volume
taken in Table 9.2.
z Tie the object, say a stone, with a thread and slowly lower it
into the measuring cylinder.
z What do you notice?
z Record the final volume after the level rises, say 55 mL, as
shown in Fig. 9.19b.
z Subtract the initial volume from the final volume after the
object is put into the measuring cylinder. This is the volume
of the object.
z Record your observations in Table 9.2.
S.No. Object
Initial volume
of water in
the measuring
cylinder (mL)
(A)
Final volume
of water in
the measuring
cylinder (mL)
(B)
Volume of water
displaced in
the measuring
cylinder (mL)
(B–A)
Volume of
the object
(cm3
)
1. Stone 50 mL 55 mL 5 mL 5 cm3
2. Metal key
3. Any other
Table 9.2: Volume of irregular solids
S
a
f
e
t
y
F
i
r
s
t
Note
The values of volume are obtained in units of mL, which can be
written in the equivalent unit cm3
for solids.
We have already learnt to measure the mass and volume of
liquids and solids of different types. These quantities can be used
to calculate the density of the object or the substance.
Let us calculate the density
Density can be calculated using the following formula:
= 3.28 g/cm3
Fig. 9.19: Level of
water in the measuring
cylinder (a) Without
object; (b) With object
=
16.400 g
5 cm3
Density =
Mass
Volume
Chapter 9.indd 146
Chapter 9.indd 146 6/28/2025 5:06:19 PM
6/28/2025 5:06:19 PM

Let us dig deeper!
Did you know that our planet, Earth, is
composed of several layers, such as crust,
upper mantle, lower mantle, outer core, and
inner core, each with its particular range of
density? The outermost layer, called the crust,
is the lightest and the density of the different
layers increases as we move towards the
centre (Fig. 9.20). As one moves deeper into the
Earth, both the pressure and the temperature
rise significantly, making the materials heavier
and more compact.
Ever heard of ...
In ancient times, before large ships
were invented, people used bamboo
and wooden logs to travel across
rivers and seas (Fig. 9.21). Bamboo
was used because it is light, hollow,
and floats easily on water. People tied
bamboo poles together to make rafts
and small boats for fishing, trading,
and crossing water bodies. Wooden
logs, especially from strong trees were
either hollowed out to make boats or
used as rafts. These simple boats, made from locally available materials,
were important for moving around and connecting different places.
Even today, similar traditional boats made of bamboo or wood are used
in some regions—not just for transport, but also as tourist attractions.
Fig. 9.21: Bamboo raft floats
on water
9.5.2 Effect of temperature on density
Generally, the density of a substance decreases with heating and
increases with cooling. This can be explained on the basis of
what you have learnt in chapter ‘Particulate
Nature of Matter’. As temperature increases,
the particles of a substance whether, solid,
liquid, or gas, tend to move away and
spread. This results in an increase in volume
but there is no change in mass. Since the
Density = Mass/Volume, upon heating, the
volume increases and the density decreases.
This explains why hot air moves up as it is less
dense than the cool air around it. The hot air
balloon works on the same principle (Fig. 9.22).
Fig. 9.20: Layers of Earth
Crust
Upper mantle
Lower mantle
Outer core (liquid)
Inner core (solid)
Fig. 9.22: Rising of hot air balloons
Chapter 9.indd 147
Chapter 9.indd 147 6/28/2025 5:06:26 PM
6/28/2025 5:06:26 PM

148
9.5.3 Effect of pressure on density
Pressure affects density differently depending on the state of
matter. For gases, increasing pressure causes the particles to
move closer together. As a result, the volume of the gas decreases
and its density increases. In the case of liquids, pressure has a
small effect because they are nearly incompressible. We have
learnt in chapter ‘Particulate Nature of Matter’ that the particles
in solids are very close to each other. So, how is the density of
solids affected when pressure is applied? Solids are even less
affected by pressure than liquids, and changes in their density
are usually negligible.
Ever heard of ...
Why does ice float on water?
Ice floats on water because
it is lighter than liquid water
(Fig. 9.23). Water has a special
property that its density
is highest at 4 °C. It means
water is heaviest at 4 °C.
As the temperature drops,
and water turns into ice at
0 °C, it undergoes a change
in structure—the particles
arrange themselves in a way that takes up more space. This process is
called expansion. Because the same amount of water now occupies a
larger volume, its density decreases. As a result, ice becomes lighter
than liquid water and floats on its surface.
This is important for animals living in lakes and oceans because
ice floats, it forms a layer on top, keeping the water underneath warm
enough for fish and other creatures to survive, even in extremely
cold weather.
Fig. 9.23: Ice floats on water
Think like a scientist
‹ Take a glass tumbler and fill it with tap water.
Carefully place a raw whole egg into the water
and observe what happens. You will notice
that the egg sinks to the bottom (Fig. 9.24).
‹ What change can you make to this setup to
make the egg float in water instead of sinking?
‹ In this chapter, you have learnt the concept
of density and how it explains partially why
some objects float while others sink.
Fig. 9.24: Raw whole
egg sinks in water
Chapter 9.indd 148
Chapter 9.indd 148 6/28/2025 5:06:34 PM
6/28/2025 5:06:34 PM

Snapshots
‹ A solution is said to be formed when two or more substances mix to form
a uniform mixture.
‹ Inthesolutionformedbydissolvingasolidinaliquid,thesolidcomponent
is known as a solute and the liquid component is known as a solvent.
‹ In a solution formed by mixing two liquids, the component present in less
quantity is known as solute and the other component is called solvent.
‹ In air, nitrogen is considered as a solvent, while oxygen, argon, carbon
dioxide, and other gases are considered as solutes.
‹ A solution in which the maximum amount of solute has been dissolved,
and no more of it can be dissolved at that temperature is called a saturated
solution.
‹ A solution in which more solute can be dissolved at a given temperature
is called an unsaturated solution.
‹ Solubilityisthemaximumamountofsolutethatcanbedissolvedinafixed
quantity (100 mL) of a solution or a solvent at a particular temperature.
‹ Generally, in liquids, the solubility of solids increases and that of gases
decreases with an increase in temperature.
‹ The amount of matter present in an object is known as its mass.
‹ The space occupied by an object or a substance is known as its volume.
‹ Devices used to measure mass and volume are a weighing balance and a
measuring cylinder, respectively.
‹ The mass per unit volume of a substance is known as its density
(Density = Mass/Volume).
‹ Generally, density decreases with an increase in temperature and
pressure affects density differently depending on the state of matter.
Keep the curiosity alive
1. State whether the statements given below are True [T] or
False [F]. Correct the false statement(s).
(i) Oxygen gas is more soluble in hot water rather than in
cold water.
(ii) A mixture of sand and water is a solution.
(iii) The amount of space occupied by any object is called its
mass.
(iv) An unsaturated solution has more solute dissolved than
a saturated solution.
(v) The mixture of different gases in the atmosphere is also
a solution.
Chapter 9.indd 149
Chapter 9.indd 149 6/28/2025 5:06:36 PM
6/28/2025 5:06:36 PM

150
2. Fill in the blanks.
(i) The volume of a solid can be measured by the method of
displacement, where the solid is __________ in water and
the ____________ in water level is measured.
(ii) The maximum amount of _______________ dissolved in
_______________ at a particular temperature is called
solubility at that temperature.
(iii) Generally, the density ____________ with increase in
temperature.
(iv) The solution in which glucose has completely dissolved
in water, and no more glucose can dissolve at a given
temperature, is called a __________ solution of glucose.
3. You pour oil into a glass containing some water. The oil floats
on top. What does this tell you?
(i) Oil is denser than water
(ii) Water is denser than oil
(iii) Oil and water have the same density
(iv) Oil dissolves in water
4. A stone sculpture weighs 225 g and has a volume of 90 cm3
.
Calculate its density and predict whether it will float or sink
in water.
5. Which one of the following is the most appropriate statement,
and why are the other statements not appropriate?
(i) A saturated solution can still dissolve more solute at a
given temperature.
(ii) An unsaturated solution has dissolved the maximum
amount of solute possible at a given temperature.
(iii) No more solute can be dissolved into the saturated
solution at that temperature.
(iv) A saturated solution forms only at high temperatures.
6. You have a bottle with a volume of 2 litres. You pour 500 mL
of water into it. How much more water can the bottle hold?
7. An object has a mass of 400 g and a volume of 40 cm³. What
is its density?
8. Analyse Fig. 9.25a and 9.25b. Why does the unpeeled orange
float, while the peeled one sinks? Explain.
9. Object A has a mass of 200 g and a volume of 40 cm³.
Object B has a mass of 240 g and a volume of 60 cm³. Which
object is denser?
Fig. 9.25
(a) (b)
Why?
When?
How long?
Where?
Why not?
Prepare some questions based on your
learnings so far ...
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
Chapter 9.indd 150
Chapter 9.indd 150 6/28/2025 5:06:43 PM
6/28/2025 5:06:43 PM

Fig. 9.26
Glass tube
Test tube
Test tube
stand
Water
(a)
Water level
Test tube
Beaker
containing
hot water
(b)
10. Reema has a piece of modeling clay that weighs 120 g.
She first moulds it into a compact cube that has a volume of
60 cm3
. Later, she flattens it into a thin sheet. Predict what
will happen to its density.
11. A block of iron has a mass of 600 g and a density of 7.9 g/cm³.
What is its volume?
12. You are provided with an experimental setup as shown in
Fig. 9.26a and 9.26b. On keeping the test tube (Fig 9.26b) in a
beaker containing hot water (~70 °C), the water level in the
glass tube rises. How does it affect the density?
Discover, design, and debate
z Research project on Dead Sea: Why is
there no aquatic life in the Dead Sea?
Try to find out if there are any other
similar water bodies.
z Investigate how well common salt
dissolves in different solvents, such as
water, vinegar, and oil. Compare the
solubility of salt in each solvent and
record your observations.
z Debate in class—Is water truly the
most versatile solvent?
Our scientific heritage
Ningel village in Manipur’s Thoubal district is a place where salt is
still produced using traditional methods. The village has a few salt
wells, one of which is lined with a 100-year-old tree trunk placed into
the ground to draw up salty water. A few families mostly women,
continue this sacred practice by collecting the salt solution and boiling
it in large metal pans over firewood kilns. Once the water evaporates
and salt crystals form, they are shaped into round ‘salt cakes’ using
banana leaves and handmade tools. These cakes are then wrapped in
a traditional cloth (phanek) to protect them. The salt cakes are believed
to have some medicinal value too.
Salt in Ningel is more than just food— it is history, culture, belief,
and a beautiful example of India’s living heritage.
Society Science
Inter-
disciplinary
Projects
Reflect on the questions framed by your
friends and try to answer ...
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
.....................................................................................
I
think ...
But we
thought ... Maybe ...
Shouldn’t it
be ...
Chapter 9.indd 151
Chapter 9.indd 151 6/28/2025 5:07:07 PM
6/28/2025 5:07:07 PM

9.The Amazing World of solute solvents and solutions.ppt/8th science curiosity/by k sandeep swamy

More Related Content

Similar to 9.The Amazing World of solute solvents and solutions.ppt/8th science curiosity/by k sandeep swamy (20)

More from Sandeep Swamy (20)

Recently uploaded (20)

9.The Amazing World of solute solvents and solutions.ppt/8th science curiosity/by k sandeep swamy