assignment book unit 10 part 3 part 3 take up the challenge 1.a student uses litmus paper to test...

ASSIGNMENT BOOK Unit 10 Part 3

Part 3 Take up the ChallengePart 3 Take up the Challenge

1. A student uses litmus paper to test three solutions in the laboratory.

No changeNo changeB

BlueNo changeC

No changeRedA

Solution Colour change of the blue litmus paper

Colour change of the red litmus paper


(a) Which solution is acidic? (1 mark)

(b) Which solution is neutral? (1 mark)

(c) How can you find out the pH value of each solution? (1 mark)

(d) Which solution has the lowest pH value?(1 mark)

A

B

It can be tested by using pH paper.

A


2. Hair is naturally weakly acidic. Each hair is made up of a large number of protein fibres (蛋白纖維 ) with 'cross-links' between them. Addition of alkalis would break some of these 'cross-links' to change the shape of the hair.

This solution is alkaline. It is because alkalis can soften the hair so that it can take the shape of the hair curlers.

(a) Hair stylists use a solution in the curling process. Is this solution acidic or alkaline? Explain. (3 marks)


(b) A hair conditioner can make hair smooth and straight because (2 marks)

it is a strong acid.it readjusts the pH of the hair to its normal value.it breaks the 'cross-links' of the hair.


3. Mr. Chan is a farmer. He uses fertilizers to make his plants grow better. The graph below shows the pH value of the soil of his farm field in the past five years.

1 2 3 4 5

1

0

2

3

4

5

6

7

pH

Year


The use of fertilizer caused the decrease of pH value.

(a) What happened to the pH value of the soil in the past five years? (2 marks)

(b) Suggest a possible reason for the above change. (1 mark)


(c) The table below shows the best soil pH for different types of crops to grow in.

7.5Cabbage

6.5Broad bean

6Maize

5.5Pea

5Potato

Crop Best soil pH


Broad bean

Potato

He should add slaked lime to acidic soil before growing cabbages. This process is called neutralisation.

(i) Which crop was most suitable to grow in Mr. Chan's field in the first year? (1 mark)

(ii) Which crop is most suitable to grow in Mr. Chan's field in the fifth year? (1 mark)

(iii) If Mr. Chan wants to grow cabbages in his field, what should he do to the soil? What is the name of this process? (2 marks)


Science Focus4. Read the following article and answer the

questions.

The harm of soapSoap is produced by the reaction of fatty substances (i.e. animal fat or vegetable oil) with strong alkalis. Normally the pH of our skin is about 5.5 and there is a thin moisture layer covering the skin. The frequent use of soap tends to increase the pH of the skin and leads to the loss of the layer of water. Our skin may become dry and red. To protect our skin, we should use neutral or even acidic soaps. When we are washing dishes, it is better to wear protective gloves. For skin care on our hands, we can apply suitable ointments or some drops of lemon juice on our hands when they become dry.


(a) Suggest a strong alkali that can be used to prepare soaps. (1 mark)

Sodium hydroxide or potassium hydroxide

Because the pH of our skin is slightly acidic.

(b) Why should we use mildly acidic skin products? (1 mark)

(c) Explain why it is suggested that gloves should be worn while washing dishes. (2 marks)

The soap we use is usually alkaline which may make our skin dry and red.


Lemon juice is an acid which can neutralise the effect of the alkaline soap.

(d) Explain why lemon juice is useful in skin care. (2 marks)

assignment book unit 10 part 3 part 3 take up the challenge 1.a student uses litmus paper to test...

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