Question Opening prompt 1
What do you think is happening in the opening picture?
Steps only
Solute particles spread evenly through a solvent.
Answer
A uniform solution is forming.
CLASS 8 · SCIENCE · CHAPTER 9
The Amazing World of Solutes, Solvents, and Solutions
Simple answers to the opening prompts, key chapter investigations, all twelve end questions, the question-making and reflection prompts, and all three projects.
34 worked answersComplete chapter answers
34 worked answers · short steps first, explanations when you need them
Probe and Ponder
Textbook pages 134
Question Opening prompt 2
What happens when so much sugar is added to tea that it stops dissolving? How can you solve this?
Steps only
Extra sugar stays undissolved once the tea is saturated.
Answer
Add more liquid or warm it carefully to dissolve more sugar.
Question Opening prompt 3
Why do sugar and salt dissolve in water but not in oil? Why is water considered a good solvent?
Steps only
Salt and sugar dissolve in water but not much in oil.
Answer
Water dissolves many substances, though not all of them.
Question Opening prompt 4
Why are water bottles usually tall and cylindrical rather than spherical?
Steps only
A cylinder stands upright and is easy to hold and pack.
Answer
A spherical bottle would roll and be harder to use.
Question Opening prompt 5
Share one question you have about solutions, solubility, or density.
Steps only
Ask a clear question about dissolving or density.
Answer
Example: How does water temperature affect dissolving?
Chapter Investigations
Textbook pages 135–149
Question Opening concept
Why does each sip of a well-mixed oral rehydration solution taste similar? Is it uniform?
Steps only
Dissolved salt and sugar are spread evenly through the water.
Answer
A properly mixed ORS is a uniform solution.
Question Activity 9.1
What does it show when added salt first dissolves in water but later remains at the bottom?
Steps only
Undissolved salt after stirring shows the solution has reached its limit.
Answer
It is saturated at that temperature.
Question Concentration example
Which is more concentrated: 2 spoons of salt in 100 mL of water or 4 spoons in 50 mL of water?
Steps only
Compare salt amount with water amount.
Answer
4 spoons in 50 mL is more concentrated.
Question Activity 9.2
What happens to undissolved baking soda when the water is warmed from 20 °C to 50 °C and then 70 °C? What can we infer?
Steps only
Warming lets more baking soda dissolve in this experiment.
Answer
For many solids, solubility increases as temperature rises.
Question Section 9.3
Is dissolved oxygen in water a uniform or non-uniform mixture? How does temperature affect the amount of gas dissolved?
Steps only
Dissolved oxygen is evenly mixed; cold water usually holds more.
Answer
It is uniform, and gas solubility usually falls as temperature rises.
Question Section 9.4
Why can husks or oil float while rice or sand sinks in water?
Steps only
Less-dense materials often float; denser ones often sink.
Answer
Oil and husks float; rice and sand usually sink in water.
Question Think like a scientist
A 1 L packet of oil has a mass of about 910 g. What does this tell us about its density compared with water?
Steps only
1 L = 1000 mL; density = 910 ÷ 1000 = 0.91 g/mL
Answer
About 0.91 g/mL, less than water.
Question Activity 9.3
How should a digital balance be used to measure a solid's mass when using a watch glass?
Steps only
Place the watch glass, tare to zero, then add and weigh the object.
Answer
The displayed reading is the solid's mass.
Question Activities 9.4–9.5
The 100 mL measuring cylinder in Fig. 9.16 has 10 small divisions between 10 mL and 20 mL. What is its smallest reading, and how should you read 50 mL of water?
Steps only
10 mL ÷ 10 divisions = 1 mL per division
Answer
Read the bottom of the water meniscus at eye level.
Question Activity 9.6
A notebook is 25 cm long, 18 cm wide, and 2 cm high. What is its volume?
Steps only
25 × 18 × 2 = 900
Answer
900 cm³
Question Activity 9.7
Water rises from 50 mL to 55 mL when a stone is submerged. What is the stone's volume?
Steps only
55 mL − 50 mL = 5 mL = 5 cm³
Answer
5 cm³
Question Think like a scientist
A raw egg sinks in plain water. What change could make it float?
Steps only
Add salt to increase the water's density.
Answer
The egg can float when the salt water is dense enough.
Keep the Curiosity Alive
Textbook pages 149–151
Question 1
State whether each is true or false and correct the false statements: (i) Oxygen is more soluble in hot than cold water. (ii) Sand and water form a solution. (iii) Space occupied is called mass. (iv) An unsaturated solution contains more dissolved solute than a saturated one. (v) The gases in air form a uniform mixture.
Steps only
Check gas solubility, mixture uniformity, mass, and saturation.
Answer
- (i) F
- (ii) F
- (iii) F
- (iv) F
- (v) T
Question 2
Fill in the blanks: (i) In displacement, the solid is ____ in water and the ____ in water level is measured. (ii) The maximum amount of ____ dissolved in ____ at a particular temperature is solubility. (iii) Density ____ as temperature increases. (iv) A solution that cannot dissolve more glucose at that temperature is ____.
Steps only
Use the terms immersed, rise, solute, solvent, decreases, saturated.
Answer
- (i) immersed; increase
- (ii) solute; solvent
- (iii) decreases
- (iv) saturated
Question 3
Oil floats on water. What does this show? (i) Oil is denser (ii) Water is denser (iii) They have the same density (iv) Oil dissolves in water.
Steps only
The floating oil layer is less dense than the water.
Answer
(ii) Water is denser than oil.
Question 4
A stone sculpture has mass 225 g and volume 90 cm³. Find its density and predict whether it floats or sinks in water.
Steps only
225 ÷ 90 = 2.5 g/cm³
Answer
2.5 g/cm³; it sinks.
Question 5
Which statement is most appropriate, and why are the others wrong? (i) A saturated solution can dissolve more at that temperature. (ii) An unsaturated solution has dissolved the maximum amount. (iii) No more solute can dissolve in a saturated solution at that temperature. (iv) Saturation occurs only at high temperatures.
Steps only
A saturated solution has reached its limit at that temperature.
Answer
(iii) No more solute can dissolve at that temperature.
Question 6
A bottle holds 2 L. It contains 500 mL of water. How much more can it hold?
Steps only
2000 mL − 500 mL = 1500 mL
Answer
1.5 L (1500 mL)
Question 7
An object has mass 400 g and volume 40 cm³. Find its density.
Steps only
400 ÷ 40 = 10 g/cm³
Answer
10 g/cm³
Question 8
Why does an unpeeled orange float while a peeled orange sinks?
Steps only
Air spaces in the peel lower the orange's average density.
Answer
Unpeeled floats; peeled orange is denser and sinks.
Question 9
Object A has mass 200 g and volume 40 cm³. Object B has mass 240 g and volume 60 cm³. Which is denser?
Steps only
A = 5 g/cm³; B = 4 g/cm³
Answer
Object A is denser.
Question Question-making activity
Prepare questions using prompts such as why, when, how long, where, and why not, based on the chapter.
Steps only
Make clear questions about solutions, measurements, and density.
Answer
Answers vary; use the chapter's ideas to frame them.
Question 10
Modeling clay has mass 120 g and volume 60 cm³ as a cube. What happens to its density when it is flattened into a thin sheet?
Steps only
120 ÷ 60 = 2 g/cm³; reshaping does not change mass or volume.
Answer
Density remains 2 g/cm³.
Question 11
An iron block has mass 600 g and density 7.9 g/cm³. Find its volume.
Steps only
600 ÷ 7.9 ≈ 75.95 cm³
Answer
About 76 cm³
Question 12
Water in a test tube is warmed in a hot-water bath and rises in a narrow glass tube. How does its density change?
Steps only
Heating increases volume while mass stays the same.
Answer
Density decreases.
Question Reflection activity
Reflect on the questions framed by your friends and try to answer them.
Steps only
Use the solution and density ideas to support your answer.
Answer
Answers vary; explain your reasoning.
Discover, Design, and Debate
Textbook pages 151
Question Project 1
Research why there is little aquatic life in the Dead Sea and identify other similar water bodies.
Steps only
Very salty water excludes most aquatic plants and animals, though microbes may survive.
Answer
Compare the Dead Sea with other hypersaline lakes such as Great Salt Lake or Lake Assal.
Question Project 2
Compare how well common salt dissolves in water, vinegar, and oil. Record the observations.
Steps only
Keep volumes, salt amount, and stirring time the same.
Answer
Salt dissolves in water and vinegar, but hardly in oil.
Question Project 3
Debate in class whether water is truly the most versatile solvent.
Steps only
Compare what water dissolves with substances that need other solvents.
Answer
Water is very useful, but it is not a universal solvent.