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Title: Colorful Milk
Principle(s) Investigated: Emulsion, Surface Tension, Cohesive Force
Standards:
Grades Nine through Twelve/ Chemistry/ Chemical Bonds
2. Biological, chemical, and physical properties of matter result from the ability of atoms to form bonds from electrostatic forces between electrons and protons and between atoms and molecules. As a basis for understanding this concept:
a. Students know atoms combine to form molecules by sharing electrons to form covalent or metallic bonds or by exchanging electrons to form ionic bonds.
b. Students know chemical bonds between atoms in molecules such as H2, CH4, NH3, HCCH2, N2, Cl2 and many large biological molecules are covalent.
d. Students know the atoms and molecules in liquids move in a random pattern relative to one another because the intermolecular forces are too weak to hold the atoms or molecules in a solid form.
Grades Nine through Twelve/ Chemistry/ Organic Chemistry and Biochemistry
10. The bonding characteristics of carbon allow the formation of many different organic molecules of varied sizes, shapes, and chemical properties and provide the biochemical basis of life. As a basis for understanding this concept:
a. Students know large molecules (polymers), such as proteins, nucleic acids, and starch, are formed by repetitive combinations of simple subunits.
b. Students know the bonding characteristics of carbon that result in the formation of a large variety of structures ranging from simple hydrocarbons to complex polymers and biological molecules.
c. Students know amino acids are the building blocks of proteins.
d. Students know the system for naming the ten simplest linear hydrocarbons and isomers that contain single bonds, simple hydrocarbons with double and triple bonds, and simple molecules that contain a benzene ring.
e. Students know how to identify the functional groups that form the basis of alcohols, ketones, ethers, amines, esters, aldehydes, and organic acids.
f. Students know the R-group structure of amino acids and know how they combine to form the polypeptide backbone structure of proteins.
Materials:
1. Paper plates
2. Milk (whole, reduced fat 2%, reduced fat 1%)
3. Cotton swabs
4. Food dye (red, green, blue, yellow)
5. Dish-washer detergent (liquid)
Procedure:
1. Pour enough amount of whole milk in a plate and allow it to settle.
2. Add one drop of each food dye (red, blue, green and yellow) to the milk. The drops should be added to the center of the plate and be close together.
3. Touch the tip of a clean cotton swab to the center of the milk but make sure not to stir it. Did you notice any changes?
4. Place a drop of liquid dish-washer detergent on the other end of the cotton swab (or dip the cotton swab in to the liquid detergent), and place it in the center of the plate and hold it for 15 seconds. Colors start to move similar to an explosion! You might say it resembles to the color movement in kaleidoscope or it has a swirling motion.
5. Use a new cotton swab and add a drop of liquid dish-washer detergent to one end, and repeat the experiment by placing it at a different spot in the milk. Notice that after removing the cotton swab colors are still moving. What creates this movement?
6. Repeat the experiment with reduced fat 2%, reduced fat 1% and water. Can you explain why there is a difference in color movement?
Student prior knowledge:
Explanation:
Milk consists of mostly water and also lactose, protein, fat, minerals and vitamins. About 80% of the protein in milk is casein. Calcium is an important mineral. The most common fats are esters composed of glycerol and three fatty acids, which is named triglycerides. Milk is considered an emulsion of fat globules in water. The fat globule has a membrane. The membrane helps fat globule stabilize within the aqueous environment of milk.
It is worth mentioning that triglycerides can be broken down by treatment with aqueous sodium hydroxide (NaOH). The products are glycerol and fatty acid salts. Fatty acid salts are known as soaps. This process is called saponification. Soap is an emulsifying agent.This means that soap can suspend the normally incompatible grease, so that they can be carried away by the water.
When clean cotton swab is introduced to the milk, nothing happens because no new chemical substance is present, and thus no chemical change is occurring. When the liquid dish-washer detergent enters the water, chemical changes start to happen.
Soap and detergents (artificial soap) contain of fatty acid anions, which have a long nonpolar tail that is hydrophobic and a polar head which is hydrophilic. Such ions are dispersed in water because they form micelles. Micelle is a group of fatty acid anions. Micelle is negatively charged and it is surrounded by cations. These aggregates of the fatty-acid anions have the water-incompatible tails (nonpolar tails) in the interior, and the anionic parts (polar heads) point outward. The polar heads interact with water molecules. Micelles absorb fat molecules into their nonpolar interiors, so that the fat molecules can be carried away by the water. Soap is a surfactant, because it is a wetting agent that assists water in suspending nonpolar materials, and reducing surface tension. After the nonpolar materials are suspended by soap, the water is able to wash them away. So as we see soap acts as an emulsifying agent.
Student should have knowledge about organic chemistry (ex. protein and fat), chemical bonds, cohesive and adhesive forces and surface tension.
When liquid detergent is added to the milk, micelles are dispersed in the water which is the major component of milk. The fat molecules in milk are taken into the nonpolar interior of the micelle which is hydrophobic. Because soap bonds to the fat molecules, water- which is present in the milk- is now able to wash the fat molecules away and this is why the swirling motion happens. The food dye has no role in chemical reaction, and it is just added to make this motion visible. After a while the rate of the reaction slows down and ultimately stops, because all the fat molecules are already bonded with micelles and this is why adding more detergent will not create any motion. Also notice that the more fat content the milk contains, the more intense the swirling movement will be, because more fat molecules are present to bond with micelles.
Proteins are a different type of macromolecule made up of subunits called amino acids. A peptide bond forms when a carboxylic acid from one amino acid reacts with the amino group of another amino acid. Protease is an enzyme that conducts proteolysis and used in some detergents. Protease is responsible of hydrolysis process (breakdown) of peptide bonds. This breakdown of proteins will also able them to bond to the soap, and water is able to wash them away which is another reason that water is moving away from the point that soap has been introduced.
There's another reason for colors explosion. Since milk is mostly water, it has surface tension like water. Surface tension is measured as the energy required to increase the surface area of a liquid by a unit of area. The surface tension of a liquid results from an imbalance of intermolecular attractive forces, the cohesive forces between molecules:
- A molecule in the bulk liquid experiences cohesive forces with other molecules in all directions.
- A molecule at the surface of a liquid experiences only net inward cohesive forces.
Liquid soap wrecks the surface tension by breaking the cohesive bonds between water molecules.This will allow the colors to move very quickly throughout the milk.
Questions & Answers:
Q.1. How does soap clean hands?
A.1. As mentioned in color changing experiment, soap is dispersed in water in the form of micelles. The fat molecules in dirt and grease are taken into the nonpolar interior of the micelle which is hydrophobic. Drops of dirt and grease are now suspended in water, and water is able to wash them away by attaching to the hydrophilic part of soap.
Q.2. What property of water enables water strider to walk on water?
A.2. Several insects such as water strider are able to walk on water. Their legs distribute their weight over a larger surface area so that the insects' weight is better balanced. The strong surface tension of the water can then hold the insect's weight and prevent the bug from sinking due to gravity. The surface tension of water is strong, and distribution of weight helps to preserve the cohesive bonds between water molecules on the surface. The surface of water is acting as an elastic membrane.
Q.3. What are biosurfactants?
A.3. Biosurfactants are surface-active molecules synthesized by microorganisms, with both hydrophilic and hydrophobic parts. Interest in microbial surfactants has been rapidly increased in the recent years. Biosurfactants enhance the emulsification of hydrocarbons, and increase their availability for microbial degradation. They can be used as biological treatment in a hydrocarbon polluted site and efficiently destroy pollutants, while being biodegradable themselves. These compounds can also be used in enhanced oil recovery. Biosurfactants have biomedical and therapeutic applications as well.
Applications to Everyday Life:
1. Egg yolk contains a number of emulsifiers, which is why it is so important in making foods such as mayonnaise. Lecithin is an important emulsifier found in egg yolk. It is a phospholipid, and the molecule has a polar substituted-phosphate head which is hydrophilic, and a long nonpolar tail which is hydrophobic. The tail gets buried in the fat droplets, and its head sticks out of the droplet surface into the surrounding water. This establishes a barrier that prevents the surface of the fat droplet from coming into contact with the surface of another fat droplet. If egg yolk (which contains Lecithin) is added to mayo, then Lecithin will help the water in the egg white mix with the oil, so we will have a uniform creamy liquid. The oil stops the water and egg proteins from forming crystals, so that even if you put it in the freezer, mayo will stay creamy.
2. Mercury (Hg) is the only metal that is liquid at room temperature, and it has surface tension. Surface tension of mercury is higher than water. Mercury used in the thermometer does not stick to the wall of the capillary tube because of the surface tension. The cohesive forces within the drops are stronger than the adhesive forces between the drops and glass. Had it stuck to the walls, the measurement of temperature would not have been correct.
3. Bile acids are the primary physiological emulsifiers, which are synthesized by the liver and secreted into the bile. Bile acids contain both hydrophobic (lipid soluble) and polar (hydrophilic) regions. The cholesterol portion of a bile acid is hydrophobic, and the amino acid conjugate is polar and hydrophilic. Bile acids are able to carry out two important functions:
Emulsification of lipid aggregates: Bile acids have detergent action on particles of dietary fat which causes fat globules to break down or be emulsified into minute, microscopic droplets. Emulsification is not digestion, but is of importance because it greatly increases the surface area of fat, making it available for digestion by lipase, which cannot access the inside of lipid droplets.
Solubilization and transport of lipids in an aqueous environment: Bile acids are lipid carriers and are able to solubilize many lipids by forming micelles - aggregates of lipids such as fatty acids, cholesterol and monoglycerides - that remain suspended in water.
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