Conclusion

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.

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.

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.)

Factors showing a change is a chemical change

Colour change, an apple that has turned brown has undergone chemical change, so its colour changes.
Energy changes, whether released or absorbed, like fireworks, as it releases energy in the form of light that can be seen.
Odour changes, when food spoils, they undergo chemical changes. Like the egg, if it is rotten they smell differently from fresh eggs. Production of gases or solids from the chemical changes, when wood is burned, they turn into ashes and gases, and it can’t be reversed, thus it is a chemical change.
And chemical changes are not easily reversed, unlike physical reactions. Think about ice for a moment. After ice melts into liquid water, you can refreeze it into solid ice if the temperature drops. Freezing and melting are physical changes. The substances produced during chemical changes however cannot easily change back into the original substances. The most important thing for you to remember is that in a physical change the composition of a substance does not change and in a chemical change the composition of a substance does change.

Chemistry Journal

Research question: What is the difference of a chemical change and a physical change?

First, i will tell you what is a chemical change and the factors that shows that is is a chemical change.
A chemical change is a change in matter that occurs on the molecular level. Mostly, this change is irreversible, meaning you can’t go back after the change has been done. Say you have a nail made of iron. Now you cut that piece in half. Is it still iron?
Take an identical nail made of iron and throw it into a blazing hot campfire. Assume that the campfire is so hot that the iron begins to melt. Is it still iron?
Again, take another nail made of iron and let it sit outside and rust. Is it still iron?
In the example with the iron nail, the first two changes are physical changes, because the end product is still iron. You cut it in half, it is still iron. You melt it, even though it is shaped differently, it is still iron. However, if the iron rusts you can’t take rust and make it into iron again. Therefore, the rusting of iron is a chemical change. It also turns out that you can change rust back into iron, but it takes so much energy that it almost never happens. How can you tell whether a change is a chemical change? It is actually very easy here are the factors: Colour changes, whether the item releases or absorbs energy, odour changes and production of gases or solids.

Differences of the digestive systems

From the last 5 posts, you can see that mostly the digestive systems are approximately the same. With a few changes or new additions in the system here and there. Like the bird's digestive system, there is a new organ called crop for the bird to store the food before slowly supplying the stomach with the food to digest. The snakes salivary glands( for those poisonous snakes) are changed to secrete poison with digestive enzymes in it too. The mouths of different birds or animals or insects also vary to cope with what they eat, like eagles, they use their hook-like beaks to tear apart their prey, while herons use their bills to spear small fish and amphibians. So, I can conclude that most digestive systems works the same way, with physical digestion and chemical digestion with the aid of digestive enzymes.

Digestive system of a Reptile: a Snake

Since reptiles were the first to inhabit dry land, there were several evolutionary changes in the anatomy of reptiles, with one aspect of these changes being the digestive system.
Many of these adaptations can be seen in the mouth of snakes. Since snakes are terrestrial, many changes occur in the salivary glands in the transition from an amphibian to a reptile. These changes in salivary glands and venom glands aid in the immobilizing and swallowing of their prey. The salivary glands found in snakes include the palatine, lingual, sublingual and labial gland. These glands help moisten the prey for swallowing. In venomous and poisonous snakes, such as the Water Moccasin, poison glands are modifications of the labial glands, lying on either side of the head and neck and lead to ducts in the modified front teeth. The teeth of snakes also underwent changes. Venomous snakes have grooved or tubular teeth for the injection or shooting of venom, while vipers have large retractable, tubular teeth.
Directly inside the mouth of snakes is the buccal cavity. This leads to the oesophagus of the snake. In snakes, the oesophagus is very long and may be as long as half the length of the body. The oesophagus of snakes has more internal folds than other reptiles, which allows for the swallowing of large, whole prey. Peristaltic movement by the walls of the oesophagus moves the food downward towards the stomach.
The stomach is a j-shaped organ in which most of the digestion occurs in snakes. The cells of the stomach secrete digestive enzymes and gastric juices that breakdown proteins. The food then passes through the pyloric valve and into the small intestines.
The small intestines are a long narrow coiled tube where absorbance of nutrients takes place. The small intestines is divided into three regions: the duodenum, the ileum, and jejunum.(Same as a human) The liver, which primarily functions in excreting nitrogenous wastes, storing nutrients, and producing bile, excretes digestive enzymes into the duodenum of the small intestines. Also, the pancreas, which produces insulin and glycogen as well, produces digestive enzymes such as lipases, proteases and carbohydrases and secretes them into the duodenum.(Also the same as in a human)
The food moves from the small intestines to the large intestines, through the caecum. The large intestines are the least muscular and most thin-walled structure of the snake digestive system. It passes into the cloacae chamber. This chamber is divided into a copradaeum for receiving faeces and an urodaeum for urine and products of the genital organs, it also absorbs excess water from the faeces, before it is passed out. The rate of their digestion is most dependent on body temperature because they are cold-blooded animals, if their body is heated up, then their digestion will be faster than a snake which is not heated up.