Tuesday, August 4, 2009


There are many cases where the distinctions between physical changes and chemical changes are unclear. For example:
The dissolution of salt in water: This seems like a physical change because we know we can recover the salt from the water. However, if we look at the microscopic level, we see that the two types of atoms in salt, sodium and chlorine, separate from one another. In this example, we don’t have a new substance, therefore this salt in solution doesn’t fit the microscopic definition of a chemical change; but we also don’t have the substance in its original form — a stack of alternating sodium and chlorine atoms. Does this mean the change is half chemical and half physical? Though it has aspects of a chemical change, scientists would still classify the dissolution of salt as a physical change.
The creation of a metal alloy: If we melt two types of metal together, we create an alloy metal that has different properties than either of its components (e.g., heat conductivity, electrical conductivity, density, etc.). This might lead us to think that we’ve witnessed a chemical change. In fact, a new particle is not created by melting two metals together. This indicates they did not undergo a chemical reaction. Brass, for example, is about 60% copper and 40% zinc, and is composed of individual copper and zinc atoms (i.e., there is no “smallest unit” that is still brass). There is no such thing as a brass molecule.
The heating and cooling of certain rubbers and plastics: One might think that exposing certain rubbers and plastics to heat or cold would cause a chemical reaction because the properties change (e.g., the materials become more rigid and brittle). While chemical reactions do take place, they simply bind together different parts of the large molecules that compose rubber and plastic. These new bonds add to the rigidity of the material, but the particles of the substances remain the same.

So, to find which change is a physical change and which change is a chemical change, we have to make sure that we put the factors of the changes into account to make sure that they are not mixed-up.

Monday, August 3, 2009

Physical changes

Now i will tell you what physical changes are. Physical changes occur when objects undergo a change that does not change their chemical nature. A physical change involves a change in physical properties.
Physical properties can be observed without changing the type of matter. Examples of physical properties include: texture, shape, size, colour, odour, volume, mass, weight, and density. An example of a physical change occurs when making a baseball bat. Wood is carefully crafted into a shape which will allow a batter to best apply force on the ball. Even though the wood has changed shape and therefore physical properties, the chemical nature of the wood has not been altered. The bat and the original piece of wood are still the same chemical substance. In my next post, i will tell you on different examples of chemical changes and physical changes.

Sunday, August 2, 2009

Examples of chemical changes and physical changes

Chemical changes are happening all the time. There are several different types of chemical change, including: synthesis, decomposition, single displacement, double displacement, neutralization, precipitation, combustion, redox and wood burning.
A primary example of a chemical change is the combustion of methane to produce carbon dioxide and water.

• iron rusting (iron oxide forms)
• gasoline burning (water vapor and carbon dioxide form)
• eggs cooking (fluid protein molecules uncoil and crosslink to form a network)
• bread rising (yeast converts carbohydrates into carbon dioxide gas)
• milk souring (sour-tasting lactic acid is produced)
• suntanning (vitamin D and melanin is produced)

Physical change rearranges molecules but doesn't affect their internal structures. Some examples of physical change are:
• whipping egg whites (air is forced into the fluid, but no new substance is produced)
• magnetizing a compass needle (there is realignment of groups ("domains") of iron atoms, but no real change within the iron atoms themselves).
• boiling water (water molecules are forced away from each other when the liquid changes to vapor, but the molecules are still H2O.)
• dissolving sugar in water (sugar molecules are dispersed within the water, but the individual sugar molecules are unchanged.)
• dicing potatoes (cutting usually separates molecules without changing them.)