The Amazing World of Solutes, Solvents, and Solutions — Class 8 Science
Curiosity · Chapter 9
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- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
2- Mathematics
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
Chapter summary
Key ideas & terms
- Solute
- The substance that dissolves in a solvent; usually the solid component or the component present in smaller amount.
- Solvent
- The substance in which the solute dissolves; usually the liquid or the component present in larger amount.
- Saturated solution
- A solution in which no more solute can dissolve at a given temperature.
- Solubility
- The maximum amount of solute that can dissolve in a fixed quantity of solvent at a particular temperature.
- Density
- The mass present in a unit volume of a substance, calculated as mass divided by volume.
- Relative density
- The ratio of the density of a substance to the density of water at the same temperature, a number with no units.
Extra questions & answers
What are the solute, solvent, and solution?
In a solution, the substance that dissolves is the solute, the substance in which it dissolves is the solvent, and the uniform mixture formed is the solution.
How do saturated and unsaturated solutions differ?
An unsaturated solution can still dissolve more solute at a given temperature, while a saturated solution cannot dissolve any more at that temperature.
What is solubility?
The maximum amount of solute that can be dissolved in a fixed quantity of solvent at a particular temperature.
How does temperature affect the solubility of most solids?
For most solids, solubility increases as the temperature increases.
How does temperature affect the solubility of gases in water?
The solubility of gases generally decreases as temperature rises, so cold water holds more dissolved oxygen.
What is density and what is its SI unit?
Density is the mass per unit volume of a substance, and its SI unit is the kilogram per cubic metre (
How is the volume of an irregular solid found?
By water displacement: the rise in water level in a measuring cylinder when the object is immersed gives the objects volume.
Why should the meniscus be read at eye level?
To avoid errors in reading the volume; for water, the reading is taken at the bottom of the curved meniscus.
Why does ice float on water?
Water expands when it freezes, so the same mass occupies a larger volume, making ice less dense than liquid water.
How does temperature generally affect density?
Heating usually increases volume without changing mass, so density generally decreases, which is why hot air rises.
Probe and ponder
What do you think is happening in the picture above?
The picture shows Mahatma Gandhi and a group of people on the seashore collecting salt from sea-water. It depicts the historic Dandi March / Salt Satyagraha where salt was made by evaporating sea-water.
What happens when you add too much sugar to your tea and it stops dissolving? How can you solve this problem?
The tea becomes a saturated solution at that temperature and cannot dissolve more sugar. The undissolved sugar settles at the bottom. You can solve it by adding more hot water or tea to increase the amount of solvent, or by heating the tea, because solubility of sugar increases with temperature, or by adding less sugar.
Why do sugar and salt dissolve in water but not in oil? Why is water considered a good solvent?
Sugar and salt are polar substances and water is a polar solvent, so they interact and dissolve. Oil is non-polar, so polar solutes do not dissolve in it. Water is considered a very good or universal solvent because it can dissolve a large number of polar and ionic substances due to its polarity and ability to form hydrogen bonds.
Why are water bottles usually tall and cylindrical in shape instead of spherical?
A tall narrow cylinder is easier to hold with one hand, takes less space on a table or in a bag, is more stable than a sphere which would roll, and is easy to manufacture and fill. A spherical bottle would be difficult to grip, store and would use space inefficiently, while the narrow cylinder still holds a good volume with accurate measurement.
Some discussion points
How many spoons of salt were you able to dissolve before some of it remained undissolved?
In the activity, about 3 to 5 teaspoons of salt usually dissolve completely in a half tumbler of water at room temperature, depending on the amount of water and size of spoon. After that limit, the added salt stops dissolving and settles at the bottom, showing the solution has become saturated.
What does this indicate about the capacity of water to dissolve salt?
It indicates that a fixed amount of water can dissolve only a fixed maximum amount of salt at a given temperature. That maximum amount is called its solubility. Beyond that, water cannot dissolve more salt and the excess remains undissolved.
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?
For 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?
The egg sinks because its average density is greater than the density of plain tap water
In this chapter, you have learnt the concept of density and how it explains partially why some objects float while others sink.
An object floats if its average density is less than the density of the liquid and sinks if its density is greater than the liquid. The egg experiment proves this: plain water density < egg density so it sinks, salt water density > egg density so it floats. Shape and air trapped can also affect average density.
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.
False [F]. The solubility of gases decreases with increase in temperature. Oxygen is more soluble in cold water than in hot water, which is why cold water supports more aquatic life.
1. State whether the statements given below are True [T] or False [F]. Correct the false statement(s). (ii) A mixture of sand and water is a solution.
False [F]. A mixture of sand and water is a non-uniform mixture, not a solution. Sand does not dissolve evenly in water and its particles can be seen separately and settle at the bottom.
1. State whether the statements given below are True [T] or False [F]. Correct the false statement(s). (iii) The amount of space occupied by any object is called its mass.
False [F]. The amount of space occupied by an object is called its volume. Mass is the amount of matter present in an object.
1. State whether the statements given below are True [T] or False [F]. Correct the false statement(s). (iv) An unsaturated solution has more solute dissolved than a saturated solution.
False [F]. A saturated solution contains the maximum amount of solute that can dissolve at that temperature. An unsaturated solution contains less solute than a saturated solution and can still dissolve more solute.
1. State whether the statements given below are True [T] or False [F]. Correct the false statement(s). (v) The presence of different gases in the atmosphere is also a uniform mixture.
True [T]. Air is a uniform mixture of gases like nitrogen, oxygen, carbon dioxide and others. It is a solution of gases where components are evenly distributed.
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.
immersed / dipped / slowly lowered in water and the rise or increase or change in water level is measured. The increase equals the volume of the solid,
2. Fill in the blanks. (ii) The maximum amount of _______________ dissolved in _______________ at a particular temperature is called solubility at that temperature.
solute dissolved in a fixed quantity of solvent or solution, usually
2. Fill in the blanks. (iii) Generally, the density ____________ with increase in temperature.
decreases with increase in temperature. On heating, volume increases while mass remains same, so
2. Fill in the blanks. (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.
saturated solution of glucose. At that temperature no more glucose can dissolve and any extra glucose will remain undissolved.
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
Correct option is (ii) Water is denser than oil. Oil floats because its density
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.
Density
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.
Most appropriate is (iii) No more solute can be dissolved into the saturated solution at that temperature. (i) is wrong because a saturated solution cannot dissolve more solute at the same temperature unless temperature is increased. (ii) interchanges definitions; it is a saturated solution that has the maximum, not an unsaturated one. (iv) is wrong because saturation depends on solubility at any temperature, not only high temperature; a solution can be saturated even at low temperature.
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?
Capacity is
7. An object has a mass of 400 g and a volume of 40 cm³. What is its density?
Density
8. Analyse Fig. 9.25a and 9.25b. Why does the unpeeled orange float, while the peeled one sinks? Explain.
The peel of an orange contains many tiny air pockets which increase its volume without much increase in mass. This lowers the average density of the unpeeled orange to less than
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?
Density of A
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.
Density initially
11. A block of iron has a mass of 600 g and a density of 7.9 g/cm³. What is its volume?
Using
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?
On heating, water expands so its volume increases while its mass stays the same. Since