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9.2 Lifestyle chemistry: 1. Physical and chemical properties of substances
Extract from Senior Science Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW
[Edit 11 June 09]
Prior learning: Stages 4-5 outcome 4.7 (content: 4.7.5, 4.7.6 c); outcome 5.7 (content 5.7.3 f); outcome 4.8 (content 4.8.5).
Recall statements in Preliminary course: Module 8.2, sub section 1.
Background: The foods that we eat and the substances that we apply to our bodies are made of chemicals. How we use these substances and how our bodies respond to them depends on the physical and chemical properties of these substances.
Many common consumer products that are used daily to eat, drink, for entertainment, or to maintain health, are mixtures. A mixture is made up of two or more substances combined together but not joined together chemically. The proportion of each substance in the mixture can vary.
identify that a wide range of substances are used daily as part of our food (1), our hygiene (2), our entertainment (3) and maintenance of our health (4)
- 1. Food consists of substances such as simple and complex carbohydrates, protein, fats and oils, vitamins, minerals, water, acids, bases, alcohols, esters and many others.
- Any one food, bread for example, is a mixture of complex carbohydrates, protein, minerals and water. Milk is a mixture of water, fats and vitamins
- 2. Substances that are used for body hygiene purposes such as soap, shampoo, deodorant, toothpaste, cleansing creams and lotions, are mixtures of chemicals like glycerol, fats and oils, salts and water.
- 3. Substances that are commonly used for entertainment include cosmetics such as lipstick and eye pencils, which are used on the skin. Substances, such as ethanol, the alcohol that occurs in a wide variety of drinks, are ingested.
- 4. Many substances are used to help maintain health including those that are commonly used for body hygiene. Substances that assist in maintaining a hygienic environment such as household cleaners and pesticides may be mixtures or compounds and often contain methylated spirits, sodium hydroxide and salts.
process and analyse information to identify the range of chemicals used in
every day living including:
- detergent
- lubricant
- pesticide
- solvent
- metal cleaner
-
body hygiene chemicals
- cosmetic
and outline any precautions that may be
needed in the use and handling of these chemicals
This activity may be best conducted by surveying a large range of domestic chemical products and then checking if they can all be usefully classified using the groups of detergent, lubricant, pesticide, solvent, metal cleaner, body hygiene chemicals, and cosmetic.
- As a first step, identify definitions from secondary sources for the identified groups. When processing definitions, consider information from more than one source to assess the reliability of definitions collected. You could use the Internet and a dictionary.
- In order to analyse information about the range of chemicals used in every day living, survey at least twenty products from various locations at home. Use your definitions to assist your classification of each product. Once the products are classified, you can use the information provided on product labels to make and justify generalisations about precautions that may be needed in the use and handling of the groups of chemicals you have identified. If you cannot classify all the products using the groups given, you may need to add to the list of groups.
A table such as the following may be useful to record the results of the investigation. A sample entry is provided as a guide to the type of information to record.
|
Types of chemicals used in every day living |
Definition Source 1 |
Definition Source 2 |
Common examples |
Element/ |
Safety advice |
| detergent | |||||
| lubricant | |||||
| pesticide | |||||
| solvent | A substance capable of dissolving or dispersing one or more other substances | The component of a solution which dissolves the other component | methylated spirits (ethanol)
nail polish remover (ethyl acetate and isopropyl alcohol) |
compound
compounds |
highly flammable, do not heat over an open flame |
| metal cleaner |
|||||
| body hygiene chemicals | |||||
| cosmetic | |||||
explain that mixtures can be
- solutions that contain dissolved substances and are uniform
throughout
- suspensions containing particles that settle out, or
form layers quickly
- colloids with particles that remain suspended
for long periods of time and include
- liquid-in-liquid
(emulsion)
- oil-in-water
-
water-in-oil
- gas-in-liquid (foams)
The following table explains the differences between these three types of mixtures. The examples show that solutions, colloids and suspensions occur in a wide range of consumer products.
|
Types of mixtures |
A common example |
| Solutions Solutions have a homogenous appearance. The two or more substances that make up the mixture cannot be separated by filtration. A true solution consists of dissolved ions or molecules. Types of solutions: |
|
|
saltwater |
|
oxygen in water |
| Suspensions The particles in a suspension are large enough to be visible with the naked eye. The particles will settle on standing and can be filtered with filter paper. Types of suspension: |
|
|
soluble aspirin |
|
fresh milk |
| Colloids Colloids may appear to be homogenous but the particles are large enough to scatter light. The particles are not large enough to be seen as particles by the naked eye. Their dimensions are between 10-6 and 10-9 m. The particles do not settle out on standing and they cannot be filtered with filter paper. Substances in a colloid can be separated by increasing the force on the colloid beyond that of gravity such as in a centrifuge. |
|
Types of colloids:
|
homogenised milk mayonnaise |
|
shaving cream, whipped cream, capuccino froth, meringue |
|
paint |
|
smoke (solid in gas) |
identify that solutions, colloids and suspensions occur in a wide range of consumer products
- Solutions, colloids and suspensions occur in a wide range of products.
plan,
select appropriate equipment or
resources for and perform a
first-hand investigation to produce a range of suspensions and colloids that are
used by consumers including
- beaten or whisked eggs
-
salad dressing (oil/vinegar)
- mayonnaise
Identify recipes that could be used to produce:
- whisked eggs (a
colloid) as might be used to make an omelette
- salad dressing (a
suspension)
- mayonnaise (an emulsion colloid)
- whipped cream (a
colloid).
- Plan your investigation by:
- trialing some identified recipes
- describing the proceduresyou have decided to use
- explaining why the procedures or the repetition of procedures is appropriate.
- Select the most appropriate equipment by trialing devices to produce the colloids.
- Perform your investigation by carrying out the planned procedure, recognising where and when modifications are needed and analysing the effect of any adjustments made.
At the conclusion of the investigation, you should identify your creations as suspensions or colloids. You can do this by describing the ingredients before and after a process.
use first-hand or secondary sources to gather, process , analyse and present information to identify examples of suspensions and colloids and outline one advantage of a mixture being in each form
- You can gather first-hand information
from procedures such as the one following. Make sure you measure,
observe and record results in an accessible and recognisable form, carrying
out repeat trials if the procedure fails to produce a suspension or colloid.
Procedure
- Try to prepare suspensions and colloids using common non-hazardous domestic chemicals such as water, air, flour, sugar, gelatine, glycerol, vegetable oil, mechanical oil, petroleum jelly, wax.
- Use separation techniques, such as filtration and centrifuging, to attempt to separate a sample of each mixture into its parts and record the results.
- Place samples of each mixture in cylinders or test tubes and allow them to stand for at least 30 minutes. Record their appearance.
- Use the criteria below and your observations to classify the mixtures as suspensions or colloids.
- Identify features of the each type of mixture that may make it useful for particular purposes.
- Gather information from relevant secondary sources, such as scientific
texts, the Internet and CD-ROM encyclopedias, to identify some suspensions
and colloids. If using a CD-ROM encyclopedia, you can efficiently identify
examples of each type of mixture by using the search facilities, beginning
with "suspension" and "colloid" as key words.
- Process all your information by comparing the information from the
texts to the results from your observations. This will assist you to evaluate
the validity of the first-hand information that you have gathered.
- A good way to present comparisons is to organise the information in a table. This will also allow you to analyse the information because trends, patterns and relationships as well as contradictions in data and information will be more evident. The following is one way to structure your table (Some entries have been included as a guide).
|
Substance |
Appearance on shaking and after standing |
Can be separated by filtration |
Type of mixture |
How is it useful? |
| homogenised milk | oil in water colloid
emulsion |
|||
| salad dressing | ||||
| mayonnaise | uniform appearance is attractive to
people, flavour is enhanced | |||
| whipped cream | gas in liquid colloid | |||
| solder | ||||
| paint | solid in liquid colloid | |||
| calamine lotion | cloudy pink thick liquid | yes | allows a powder to be applied evenly to the surface skin to relieve itching | |
| penicillin emulsion | allow the quick delivery and absorption of the critical substances | |||
| hairspray | ||||
| soluble aspirin | ||||
| whipped cream | texture is smooth | |||
| jelly | ||||
| smoke | solid in gas colloid | |||
perform
first-hand investigations to demonstrate the effect of surface tension
on:
- the shape of liquid drops
- the formation of
menisci
- the ability of some insects to walk on water
- Perform a series of first hand investigations like those suggested below. When carrying out the procedures, take care to note where and when modifications are needed to produce the effect being demonstrated. Analyse the effect of any adjustments made.
Equipment needed:
A ten cent coin, an eyedropper, a beaker of water, a mercury thermometer, a small measuring cylinder, a watch glass and a needle.
Demonstration |
Procedure |
The shape of liquid drops: |
|
The formation of menisci: |
|
The ability of some insects to walk (in) on water: |
|
explain surface tension in terms of the forces experienced by particles at the surface of a liquid
- There are weak cohesive forces acting over short distances between the particles in a liquid. At the surface of the liquid, inward attraction forces are stronger between water-water particles than between water-air particles. This causes a surface effect, called surface tension, which appears like a thin transparent skin.
- This same force causes droplets of liquid to be spherical. The forces that hold particles of the same substance together are called cohesion forces.
- Another liquid, mercury also has strong cohesion forces that hold its particles together. If it is spilled it forms droplets in the shape of a sphere. Because the forces between the particles of mercury are stronger than those between water molecules, large drops of mercury are more spherical in shape than large drops of water.
- There are also forces that attract particles of one substance to particles of
other substances. These are called adhesion forces.
- You can explain your observations of the previous activities by referring to these forces.
process and present diagrammatic information to describe the effects of soaps, skin cleansers and shampoos on the solubility of oil
- Many secondary sources present diagrammatical information to describe the effects of substances on the solubility of oil. However, there are a variety of visual representations, symbols and levels of detail evident. Begin to process the type of diagram you will construct by considering diagrams from a range of sources. In this way, you can assess the reliability of the information available to you. This will ensure that you present accurate information.
- You can then select a pictorial form that best suits you to present your understanding clearly and succinctly.
It is wise to ensure that the following
features are indicated in your diagram/s.
|
describe surfactants as substances that affect the surface tension of a liquid
- Surfactants are substances that reduce the surface tension of a liquid.
|