Explore the composition of water using electrolysis. For an electrical current to flow, a circuit has to be complete. A circuit can consist of a battery and two wires connected to electrodes that go into a solution. A Hoffman apparatus is used for the electrolysis of water.
It is a glass U-tube that is filled by a thistle tube Fig. Table 1. Activity Activity: Hoffman Apparatus. Observe a demonstration of a flame test to examine the gaseous products of electrolysis. Demonstration materials and procedure are written in this activity. Electrolysis uses electricity to split water into hydrogen and oxygen. At the negative terminal of the battery, electrons are produced. Those electrons flow through the wire to the electrode in solution Fig. At the negative - electrode, electrons are transferred to the water and picked up by hydrogen atoms in water.
This allows hydrogen to split from the rest of the water molecule and form H2 molecules. Electrons flow from this electrode back to the positive terminal of the battery. When water is heated and boils Fig. Boiling is a physical change, because even though the water changes state from liquid to gas, the liquid and gas are both made of water molecules.
The water molecule has not decomposed. In the electrolysis of water Fig. The electrolysis of water causes a chemical change in which water molecules are split to form hydrogen and oxygen—two substances that are chemically different from water.
Decomposing water is difficult and cannot be accomplished through heating alone. Water is not broken apart even by the heat of a volcanic eruption! An important characteristic of matter is that it can never be created or destroyed, only transformed. A physical or chemical change transforms matter from one form of matter to another.
John Dalton was one of the first scientists to recognize this property of matter. This idea is called the Law of Conservation of Matter. In any physical or chemical change matter is not created out of nothing. For example, a metal object that rusts will gain a brownish coating and will also increase in mass.
This comes from a chemical change, the reaction of iron atoms with oxygen atoms in the air. B A simple skillet can be made from cast iron. C Gold bars are formed and used for monetary purposes. Some elements have been known for centuries gold, silver, iron, copper, among others while others have been created in the lab only within the last several years.
Most elements do not exist as such in nature. They are so reactive that they can be found only in combination with other materials. Several of the elements are very valuable while others are quite inexpensive. Aluminum, on the other hand, only sells for about 90 cents per pound, considerably lower than gold.
Enjoy an old song about the elements sung by Harvard lecturer Tom Lehrer. There were only known elements at the time he wrote this song. What do you need to start building a house? When building a house, you start with a blueprint of what the house will look like. The plan states how many windows and what kind, how many doors and what style, how many rooms and what type bedroom, kitchen, other. The blueprint shows how the different pieces will go together to make the house. As long as the blueprint is followed and exactly the same items are used, the houses will be identical.
A compound is a substance that contains two or more elements chemically combined in a fixed proportion. The elements carbon and hydrogen combine to form many different compounds.
One of the simplest is called methane, in which there are always four times as many hydrogen particles as carbon particles. Methane is a pure substance because it always has the same composition. However, it is not an element because it can be broken down into simpler substances — carbon and hydrogen. Recall that the components of a mixture can be separated from one another by physical means. This is not true for a compound. Table salt is a compound consisting of equal parts of the elements sodium and chlorine.
Salt cannot be separated into its two elements by filtering, distillation, or any other physical process. Salt and other compounds can only be decomposed into their elements by a chemical process.
A chemical change is a change that produces matter with a different composition. Many compounds can be decomposed into their elements by heating. When sugar is heated, it decomposes into carbon and water. Water is still a compound, but one which cannot be broken down into hydrogen and oxygen by heating.
Instead, the passage of an electrical current through water will produce hydrogen and oxygen gases. The properties of compounds are generally very different than the properties of the elements from which the compound is formed. Sodium is an extremely reactive soft metal that cannot be exposed to air or water. Chlorine is a deadly gas.
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