Ch9 Chapter Exercise Answers

Chapter exercise                                                                                                                  p.56

Concept Checking

1.     False

        Friction always (àmay) causes inconvenience.

2.     True

3.     False

        The force of gravity is the same everywhere (different on different celestial bodies).

4.     True

5.     True

   

    Multple Choice

    9.     C

    10.   B

    Short-answer Questions

    13.

           

14.   · The same poles of two magnets repel each other.

        · The opposite poles of two magnets attract each other.

        · A magnet attracts an object made of iron.

        · The pointer of a compass moves under the influence of the Earth’s magnetic field.

        · The force of gravity of the Earth acts on an object.

        (any two or other reasonable answers)

15.   Action and reaction forces do not cancel out each other because they act on different objects.

Skill development (WRITING)                                                                                          p.59

1.     (a)    (i)     starts

                (ii)    stops

                (iii)   direction; speed

        (b)    (i)     starts the motion

                (ii)    stops the motion of the ball

                (iii)   hits the ball; force changes the direction and speed of the ball

2      (a)    (i)     force

                (ii)    centre

        (b)    (i)     a force of gravity

                (ii)    the objects towards the centre of the Moon

        (c)    (i)     a force of gravity on every object around it

                (ii)    pulls the objects towards the centre of Mars

Ch9 Textbook Exercise

Activity 9.1                                                                                                                              p.4

1.          stopped

2.          starts

3.          faster

4.          direction

Experiment 9.1                                                                                                                    p.6-7

Part A

(a)    extends

(b)    force

Part B

2.     (Answer depends on the type of rubber band used.)

3.     (Answers depend on the types of rubber bands used.)

        The force required to stretch different types of rubber band by 3 cm may be different.

Checkpoint 9.1                                                                                                                       p.9

1.      The force of the rolling bowling ball starts the motion of the pins.

2.     

 

                       

3.    (a)    spring balance / newton balance
(b)    newton
(c)    N

4.      (a)    C
(b)    N
(c)    N
(d)   C

Activity 9.2A                                                                                                                         p.11

Experiment 9.2A                                                                                                                  p.12

3.     (Answer depends on the weight used and the paper surface.)

4.     (Answers depend on the weight used and the surfaces.)

Experiment 9.2B                                                                                                             p.13-14

1.     It moves and then stops.

2.     (a)    The puck moves for a longer distance before it stops.

        (b)    Friction is reduced when air is present between the puck and the bench.

Experiment 9.2C                                                                                                                  p.16

Part A

1.     It moves and then stops.

2.     (a)    The puck moves for a longer distance before it stops.

        (b)    The bamboo sticks roll. This reduces the friction between the puck and the bench.

Part B

1.     The weight spins for a short time and then stops.

3.     (a)    The weight spins for a longer time.

        (b)    The marbles roll. This reduces the friction between the weight and the lid.

Activity 9.2B                                                                                                                         p.20

1.       (a)    There are bulges on the material wrapping the bicycle handle, which prevents slipping.

(b)       The grooves on the tyres increase friction and prevent the car from skidding.

(c)        The rough surfaces on the gloves increase friction and prevent the ball from slipping away.

(d)       Edges of stairs are covered with a plastic lining, which prevents slipping.

2.     · We sometimes use a towel to increase friction when trying to open the lid of a bottle.

        · Hiking boots have deep grooves on the sole to increase friction and prevent slipping.

        · The plastic non-slip grip on a ball-pen prevents slipping and makes writing easier.

        (any two or other reasonable answers)

Checkpoint 9.2                                                                                                                     p.21

1.     Using a lubricant, using ball bearings (or other reasonable answers)

2.     · Friction between our shoes and the ground enables us to walk.

· Friction between a door knob and our hand enables us to open the door.

(or other reasonable answers)

3.     (a)    increased

(b)    reduced

4.     · Friction is reduced because there is a layer of air between the train and the rails.

        · Air resistance is reduced because the train has a streamlined shape.

        (or other reasonable answers)

Experiment 9.3                                                                                                                     p.24

Object	Weight (N)
wooden block (with a hook)	(Answer depends on the wooden block used)
200 g metal weight (with a hook)	about 2 N
pencil box and a rubber band	(Answer depends on the pencil box and rubber band used)

Checkpoint 9.3                                                                                                                     p.25

1.     (a)    force of gravity of the Earth

        (b)    the Earth’s centre

        (c)    force of gravity of Mars

        (d)   Mars’ centre

E 2.     (a)    matter

            (b)    mass

Experiment 9.4A                                                                                                             p.28-29

Part A

2.     Both students A and B move backwards. They move away from each other.

3.     Both students A and B move backwards. They move away from each other.

Part B

The student moves backwards.

Experiment 9.4B                                                                                                             p.30-31

3.     It moves quickly along the string.

4.     When the clip is removed, air is ejected from the balloon. The ejected air exerts a force on the             balloon, pushing it in the opposite direction.

Experiment 9.4C (Demonstration)                                                                                     p.32

3.     (a)    The water is ejected downwards.

        (b)    It moves upwards.

Checkpoint 9.4                                                                                                                     p.38

1.          C

Ch10 Chapter Exercise Answers

Chapter exercise                                                                                                                p.128

Concept Checking

1.     False

Acid-alkali (àpH paper / Universal) indicators can be used to show the degree of acidity or alkalinity of a solution.

3.     True

5.     False

        Unpolluted rainwater is neutral (àslightly acidic).

6.     True

Multiple Choice

7.     B

8.     D

9.     B

10.   D

Short-answer Questions

12.   (a)    Marigold and choi sum. They do not show different colours in acids and alkalis.

        (b)    Yellow

        (c)    Pink

13.   (a)    Magnesium / zinc / iron (Any one)

        (b)    Hydrogen

        (c)    Test the gas with a burning splint. If the gas burns with a ‘pop’ sound, it is hydrogen.

        (d)   There would be no reaction.

Ch10 Textbook Answers

Activity 10.1                                                                                                                          p.69

1.          citric acid

2.          ethanoic acid

3.          citric acid

4.          ammonia

5.          sodium hydroxide

6.          hydrochloric acid

Checkpoint 10.1                                                                                                                   p.70

1.     (a)    sour

        (b)    (i)     bitter

                (ii)    soapy / slippery

2.     (a)    ü
        (b)    û
        (c)    ü
        (d)   û

3.     Toothpaste / baking soda / glass cleaners / oven cleaners / air-conditioner cleaners
(Any three or other reasonable answers)

Checkpoint 10.2                                                                                                                   p.72

1.     (a)    irritant

        (b)    corrosive

2.     Wear safety goggles and gloves. / DO NOT use too much of it at a time. / DO NOT pour the chemical waste containing it into the sink. Pour it into a waste bottle.
(Any one or other reasonable answers)

Experiment 10.3A (Demonstration)                                                                                   p.73

(a)    Orange words / pictures appear on filter paper A; green words /
pictures appear on filter paper B.

(b)    (i)     an acid
(ii)    an alkali
(iii)   an acid
(iv)   orange
(v)    an alkali
(vi)   green

Experiment 10.3B                                                                                                           p.74-75

Part C

Plant extract	Colour of plant extract in
	(A) dilute hydrochloric acid	(B) dilute ethanoic acid	(C) dilute sodium hydroxide	(D) dilute ammonia solution	(E) solution X
red cabbage leaves	red	purple	yellowish green	bluish green	(Answer depends on the acid/alkali used.)
red rose petals	red	pink	yellowish brown	greenish brown	(Answer depends on the acid/alkali used.)
blackcurrant skins	brown	brown	green	green	(Answer depends on the acid/alkali used.)

(a)    Yes, they can. Because they show different colours in acids and alkalis.

(b)    Solution X is an acid / alkali. (Answer depends on the solution used.)

Experiment 10.3C                                                                                                                p.77

Litmus paper	Colour change of litmus paper in
	(A) dilute hydrochloric acid	(B) dilute ethanoic acid	(C) dilute sodium hydroxide	(D) dilute ammonia solution	(E) distilled water
blue litmus paper	changes to red	changes to red	no colour change	no colour change	no colour change
red litmus paper	no colour change	no colour change	changes to blue	changes to blue	no colour change

Experiment 10.3D                                                                                                           p.80-81

Liquid		pH value
Acids	(A) dilute hydrochloric acid	~ 0 - 1
	(B) dilute sulphuric acid	~ 0 - 1
	(C) dilute ethanoic acid	~ 3 - 4
Alkalis	(D) dilute sodium hydroxide	~ 12 - 14
	(E) lime water (calcium hydroxide solution)	~ 10 - 12
	(F) dilute ammonia solution	~12
Neutral substances	(G) distilled water	7
	(H) table salt (sodium chloride) solution	~7
	(I) sugar solution	~7

n     Acids or acidic substances have pH values smaller than 7.

        Alkalis or alkaline substances have pH values greater than 7.

        Neutral substances have pH values equal to 7.

Experiment 10.3E                                                                                                           p.82-83

5.

Liquid	pH value	Acidic / Neutral / Alkaline
(A) lemon juice	~2	acidic
(B) orange juice	3 - 4	acidic
(C) Coke	3 - 4	acidic
(D) fresh milk	5 - 6	acidic
(E) vinegar	~3	acidic
(F) baking soda solution	9 - 10	alkaline
(G) glass cleaner	9 - 10	alkaline
(H) toothpaste solution	8 - 9	alkaline
(I) antacid solution	~9	alkaline
(J) soap solution	~9	alkaline
(K) detergent solution	(Answer depends on the type of detergent solution used)
(L) tap water	5 - 6	acidic

Checkpoint 10.3C                                                                                                                p.85

1.     Dilute sodium hydroxide and soap solution

2.     Lemon juice and dilute hydrochloric acid

3.     Distilled water

4.     (a)

Substance	lemon juice	dilute sodium hydroxide	dilute hydrochloric acid	distilled water	soap solution
pH value	3	13	1	7	9

(b)    Dilute sodium hydroxide < soap solution < distilled water < lemon juice < dilute hydrochloric acid

Experiment 10.4A                                                                                                           p.86-87

Part A

Hydrogen burns in air with a ‘pop’ sound.

Part B

2.     Colourless gas bubbles are given out. The zinc beads dissolve / decrease in size.

4.     The test tube is warm.

5.     The gas burns with a ‘pop’ sound.

n Conclusion:

Metal	How is the reaction? (vigorous / quite vigorous / not vigorous / no reaction)	What happens to the metal?	Does the test tube become warm?	What happens in the hydrogen test?	Is hydrogen produced?
zinc	quite vigorous	decreases in size	yes	a ‘pop’ sound is heard	yes
magnesium	vigorous	decreases in size	yes	a ‘pop’ sound is heard	yes
copper	no reaction	no observable changes	no	no observable changes	no
iron	not vigorous	no observable changes	no	a ‘pop’ sound is heard	yes (very slowly)

Experiment 10.4B                                                                                                                p.89

5.

Building material	Is there a reaction?	Are gas bubbles given out?	What happens to the solid pieces?	What happens to the lime water?	Is carbon dioxide produced?
marble	yes	yes	decrease in size	turns milky	yes
limestone	yes	yes	decrease in size	turns milky	yes
sand	no	no	no observable changes	no observable changes	no

Checkpoint 10.4                                                                                                                   p.91

1.     (a)    Colourless gas bubbles are given out. / The zinc strip decreases in size. /

The test tube becomes warm.
(b)    Hydrogen
(c)    Put a burning splint near the mouth of the test tube. If the gas is hydrogen, it burns

with a ‘pop’ sound.

2.          Metal cooking utensils are easily corroded by the acids in acidic foods.

3.          (a)    corroded
(b)    calcium carbonate
(c)    carbon dioxide

Experiment 10.5                                                                                                              p.95-96

9.

	Hollow A	Hollow B	Hollow C
Liquid	dilute sulphuric acid (pH ~2)	dilute sulphuric acid (pH ~4)	tap water
pH value of liquid	~2	~4	5 - 6
Appearance of seedlings after a week	do not grow at all	do not grow well	grow well

n Discussion

1.     Acid rain / acid rain of different acidity

2.     To act as the control of the experiment.

3.     Acids slow down or even stop the growth of the seedlings.

Skill development 10.5                                                                                                         p.96

1.     Response to Betty: Different seedlings have different health conditions.

So there are variations in growth rates even if the seedlings are placed in the same environment. Using 10 seedlings is more reliable than using one seedling because individual differences can be averaged out.

2.     Response to Ben: The results will be unreliable. For example, an unhealthy seedling may grow very slowly or do not grow at all in a suitable environment.

Checkpoint 10.5                                                                                                                   p.98

1.     (a)    û

        (b)    û

2.     (a)    factories
        (b)    power stations
        (c)    motor vehicles

3.     · Corroding buildings and structures made of metals, marble and limestone.

· Killing fish and plants in lakes and rivers.

· Affecting / Slowing down the growth of plants and even killing them.