Senior Science

8.2 Water for Living

Section 1

Water is Essential

Section 1 Water is Essential

Focus - Water is essential for the health of humans and other living things
8.2.1.a / Identify the relative amount of water in a variety of living things
8.2.1.b / Describe the importance of water as a solvent in the
– bloodstream
– cells
– transpiration stream
8.2.1.c / Discuss ways in which plants optimise water uptake
8.2.1.d / Discuss ways, using examples, that plants reduce water loss such as
– thick outer coating (cuticle) on leaves
– reduced leaves
– dropping leaves in times of drought
8.2.1.e / Discuss ways, using examples, that animals reduce water loss such as:
– excrete uric acid instead of urea
– nocturnal behaviour
– reduced activity
– lying in the shade
– burrowing underground
8.2.1.i / Perform a first-hand investigation to demonstrate that substances dissolve in water and to identify the solute and solvent in each case
8.2.1.ii / Plan, choose equipment or resources for and perform a first-hand investigation to determine the amount of water present in a variety of fruits, vegetables and meat
8.2.1.iii / Perform a first-hand investigation to identify adaptations of some plants that assist in reducing water loss
8.2.1.iv / Gather, process and analyse information to identify the different ways in which a range of terrestrial animals reduce water loss
Describing aqueous mixtures

A mixture consists of two or more substances that can be separated by physical means.

Examples

·  Dirty water – a mixture of dirt and water; the dirt can be separated by filtration.

·  Sugar solution – a mixture of sugar and water; the sugar can be separated by evaporating the water.

A liquid solution is a mixture in which a solid, liquid or gas (the solute) dissolves in a liquid substance (the solvent). A solution:

·  contains dissolved substances (particles)

·  and these particles are spread out evenly throughout the liquid.

Also,

·  The particles are extremely small (cannot be seen with the naked eye).

·  The particles can remain suspended indefinitely.

·  The particles cannot be separated by filtration

v  A solute is the substance that dissolves in the solvent.

v  A solvent is a liquid in which the solute dissolves.

v  A solution where the solvent is water is called an aqueous mixture.

v  A solute is soluble if it dissolves completely in a solvent

v  A substance is insoluble if it will not dissolve in a solvent.

v  If two liquids mix they are said to be miscible.

Notes Questions

1.  Which of the following mixtures are solutions?

Oil and water; Dirty water; sea water; air; mayonnaise; alcohol and water; paint

2.  Explain why a solution is a mixture.

3.  / Suggest a reason why the diagram opposite could represent a solution.

4.  Copy and then complete the following table

Solute / Solvent / Solution
Salt / water
Sugar, flavours and other … / Coca cola
Beer
8.2.1.i / Perform a first-hand investigation to demonstrate the range of substances that will dissolve in water and to identify the solute and solvent in each case

Solutes and Solvents

Aim To identify the solute and solvent in a range of substances that dissolve in water.

Risk Analysis

Before any practical task is performed, a risk analysis should be completed. This means an assessment of sources of risk needs to be made. Factors to consider include physical, chemical and biological factors.

NB See Humans at Work – Section 1 for more details

Method

1.  Organise the following equipment:

·  6 medium test tubes

·  test tube rack

·  solids (solutes) - sodium chloride, Phenolphthalein, iodine, copper sulfate, polystyrene foam

·  liquids (solvents) - water, methylated spirits, acetone

·  Spatula

2.  Mix small quantities of each of the solutes separately with each of the solvents.

3.  Determine if a solution is formed.

4.  Record the solubility of the solute using a table similar to the one below; (ie if the solute is soluble, slightly soluble or insoluble).

Results

Solvent / Solute
Sodium Chloride / Phenolphthalein
Water

Conclusion Write an appropriate conclusion

Discussion / Questions

  1. Name two safe working practices for a science laboratory. [2 marks]
  2. Outline how students did not waste resources doing this experiment. [2 marks]
  3. Name a biological risk. [1 mark]
  4. Outline a physical risk. [2 marks]
  5. Describe a chemical risk. [3 marks]
  6. Explain why there might be a need to repeat some combinations of solvents and solutes in the experiment. [2 marks]
Marking criteria for investigation
Marking Criteria / Mark range
Perform planned practical (12.1) [3 marks]
·  Carry out planned procedure safely
·  Use safe working practices / At all times
3 / Mostly
2 / Occasionally
1
Data collection (12.2) [2 marks]
·  Accuracy of measurements
·  Sufficient data collected / Sufficient criteria
2 / Few criteria
1
Presents information (13.1) [3 marks]
·  Neat
·  Uses Laboratory report scaffold / Scaffold used
3 / Scaffold partly used
2 / 1
Results table (13.1) [5 marks]
·  Ruled
·  Column headings
·  Units & Column headings
·  Accurate data recorded / All criteria
5 / Most criteria
3 / Few criteria
1
Conclusion (14.1) [2 marks] / Totally consistent 2 / Logical
1
Discussion [12 marks]

Notes Questions

5.  Write ONE complete sentence for each of the following groups of words.

·  Each sentence must use every word from the group

·  Each sentence must be a correct scientific statement

a.  Salt, solvent

b.  Dissolved, water, when

c.  Soluble, solution, is

d.  Solid, insoluble, which

e.  Solution, sugar, methylated spirits (careful!)

6.  The following are all solutions: each contains a solute dissolved in a solvent.

a.  For each solution:

i.  Name the solute

ii.  Name the solvent.

b.  Present all this information in a table.

SOLUTION 1 A cup of coffee

SOLUTION 2 Sea water

SOLUTION 3 Blood

SOLUTION 4 Water that travels through plants

8.2.1.ii / Plan, choose equipment or resources for and perform a first-hand investigation to determine the amount of water present in a variety of fruits, vegetables and meat
8.2.1.a / Identify the relative amount of water in a variety of living things
Part A Planning an investigation

A scientific investigation must be planned so that valid and reliable data is collected. One important aspect of planning is describing and trailing the method to be used. The method needs to be repeatable.

Planning involves a number of points:

·  A discussion about various issues related to performing the investigation.

To discuss means to identify issues and provide points for and against these issues.

·  A risk analysis

·  A written method using a text type called a “procedure scaffold”.

Pre planning questions

The following questions are to help in a discussion about various issues related to performing the investigation. After these issues are considered, a draft method can be written.

1.  Identify ONE fruit that would be suitable – ie cheap and easy to get.

2.  Which of the questions below make it clear what needs to be done in the investigation.

·  How much water is in a fruit?

·  How much of the fruit is water?

·  What is the percentage of water in the fruit?

·  What is the amount of water in the fruit expressed as a percentage of total weight?

a.  Identify problems with some of these questions

b.  Explain which question is the best.

3.  Outline some ways that could be used to remove the water from the fruit. What can you do to the fruit to let the water escape?

4.  In order to get the % of water by total weight two measurements need to be made. What is measured before and what is measured after?

5.  In this experiment use the same pieces of fruit BEFORE (with water) and AFTER (without water). Therefore, in this investigation the material being tested will be “destroyed”. This is acceptable since the fruit being tested should be cheap and plentiful. In some scientific investigations the material is very important and would only be destroyed in extreme circumstances. Name TWO examples of such materials. Identify the situation where such materials might occur.

6.  How many pieces of fruit will be used in this experiment (1, 2, 3, 5, 10 …)?

7.  How many times will the experiment be repeated to get reliable data?

8.  How does the experimenter know that all the water has been removed from the fruit? Discuss the sequence: Weigh – Heat – Weigh – Heat – Weigh – Heat – Weigh. How long would the sequence be continued.

9.  Explain why an oven could be used.

10. Explain why a weighing machine could be used.

11. Explain why a scalpel or pin would be useful.

12. Name a container that would be acceptable to hold the fruit.

Risk analysis

Safe working practices involve care to be taken

Ø  When using scalpels and pins.

Ø  By wearing correct footwear in the laboratory.

Ø  By being careful where bags are placed in the laboratory.

Ø  By concentrating on the task to be completed and reducing distraction.

Write a sentence for TWO of the above safe working practices.

The procedure scaffold

The procedure scaffold has three main parts:

1.  An statement outlining the aim or purpose of the investigation

·  Generally a single sentence

2.  Materials needed for the investigation

·  This may be a list.

·  This may be left out in some cases.

3.  A sequence of steps in a logical order.

·  Preferably in point form

·  One idea per point

·  Numbers can be used to show 1st, 2nd 3rd etc

·  The order is usually important

·  Words such as now, next and after this can be used.

·  Many steps should begin with command words such as add, stir, and weigh.

·  There is enough detail so that the investigation can be repeated.

Write-up of aim, … method

Once pre-planning occurs the draft method can be written and the experiment performed.

1.  Heading Write a heading for the investigation.

2.  Pre planning questions Write down a summary of the various ideas and issues raised in response to the pre planning questions above.

3.  Write a draft method using the Laboratory Report Scaffold and the following headings

a.  Write the purpose or aim of the investigation.

b.  Write a list of materials or equipment needed

c.  Write a method in point form.

  1. Draw ONE diagram.

Marking Criteria / Mark range
Planning investigation (11.2) [15 marks] / Good / Yes Poor / No
1.  Heading for investigation [1] / 1 0
2.  Summary of issues raised in discussion [2] / 2 1 0
3.  Appropriate headings used in written report [1] / 1 0
4.  Aim of investigation [1] / 1 0
5.  Appropriate list of equipment selected. [2] / 2 1 0
6.  Procedures are listed in logical steps. [2] / 2 1 0
7.  Each step is numbered and in a sentence [2] / 2 1 0
8.  Instructions are clear. [1] / 1 0
9.  Diagram. [3] / 3 2 1 0


Part B Performing the experiment

Write-up of results, conclusion
1.  Perform the investigation

Ø  Use your method or an edited version to perform the investigation.

2.  Record your results.

Ø  Construct a table to show the original weight and weight loss.

Ø  Show your calculations of percentage weight loss for each food type.

% of water in food = % weight loss = original weight – final weight x 100

original weight 1

Ø  Graph these measurements using a column graph.

3.  Write a conclusion.

Discussion / Questions

a  Outline what happens to the waste material after the experiment [2 marks]

b  Describe how this investigation could be repeated for other fruits, meat, and vegetables.

[3 marks]

Marking Criteria / Mark range
Perform planned practical (12.1) [3 marks]
·  Carry out planned procedure safely
·  Use safe working practices / At all times
3 / Mostly
2 / Occasionally
1
Data collection (12.2) [2 marks]
·  Accuracy of measurements
·  Sufficient data collected
·  % calculation completed / Sufficient criteria
2 / Few criteria
1
Results table (13.1) [5 marks]
·  Ruled
·  Column headings
·  Units & Column headings
·  Accurate data recorded / All criteria
5 / Most criteria
3 / Few criteria
1
Conclusion (14.1) [2 marks] / Totally consistent 2 / Logical
1
Discussion questions
8.2.1.b / Describe the importance of water as a solvent in the
– bloodstream
– cells
– transpiration stream

Water as a solvent

What to do

Ø  Read the information on water and its uses in living things

Ø  Answer the questions

Ø  Describe the importance of water as a solvent in the bloodstream, cells and the transpiration stream.

Every living thing is made up of cells. Living cells are made up from many different substances, but the most important and abundant is water. The weight of a living organism is between 60% and 99% water – therefore most organisms can withstand very little water loss. Both plant and animal cells are dependent on water. Water allows cells to maintain their shape and to function.

1.  Polarity

Each water molecule has a formula H2O – a water molecule consists of two hydrogen atoms and one, oxygen atom. The oxygen atom attracts (negative) electrons more strongly than the hydrogen atom. This causes one end of the water molecule to be negatively charged and the other end positively charged – the molecule is said to be polar.

2.  Cohesiveness

Water molecules are polar. Therefore the negative end of one molecule attracts the positive end of the other. This attraction between the polar water molecules results in the molecules “sticking together” – cohesive. It allows water to move up the conducting tissue of plants and allows some insects to move across the surface of a pond without sinking.

3.  High Specific Heat

Because polar molecules attract each other, energy is needed to separate them. This allows water to absorb more heat (high specific heat) than most other liquids. This means that water can absorb and transfer heat. This is important to cells where chemical reactions release large amounts of energy (heat), which might cause cells to overheat.