A chemical change is a change in which one or more new substances are produced. Chemical change cannot be reversed easily, so chemical change also called permanent change. It is also known as chemical reaction.
There are changes all around us, for example, lakes freeze in winter, sugar dissolves in a cup of tea, water evaporates from a lake and coal burns in a fire. Some changes are what scientists call chemical changes and some are not.
Since these changes do not involve any change in chemical composition there the compound remains same such as if we melt the ice, it will change to water. It was H2O in ice as well as in water so chemical composition remains same during physical changes. That is why these changes are usually reversible and can convert to each other with the change in temperature and pressure. Since the chemical composition remains same before and after the change, therefore physical changes do not involve formation or breaking of chemical bonds.
We can freeze water at low temperature and melt ice by increasing the temperature. Crushing can, mixing sand and water, breaking a glass, dissolving sugar or table salt in water, chopping wood and mixing red and green marbles are some other examples of physical changes.
What is a Chemical Change?Back to Top
When a chemical reaction takes place, the products often but not always look different to the reactants. They nay be a different colors. For example, if a piece of magnesium into some blue copper II sulphate solution the blue color disappears as the copper sulphate and magnesium change into new products.
Changes in matter that do create new substances are chemical changes. When wood burns, it changes into matter with different chemical properties - grey, flaky ashes and wispy smoke. This is just one of thousands of chemical changes that occur around us.
Chemical Combination Laws
Chemical combination involves the combination of elements to give compounds. There are three laws of chemical combination. The law of constant composition states that the proportion of elements in a compound is always the same, no matter how the compound is made.
The three laws of chemical combination were all announced between 1799 and 1802. The law of constant composition or definite proportions. All pure chemical compounds contain the same elements united in the same proportions by weight was stated by Proust. The views of Proust prevailed over those of Berthollet and until comparatively recently fixity of composition was regarded as a unique feature of a chemical compound.
Law of Definite ProportionBack to Top
In 1799, the French chemist Joseph Proust discovered the law of definite proportions. Different samples of a pure compound always contain the same elements in the same proportions by mass. This law states that when two pure substances react to form a compound, they do so in a definite proportion by mass. For example, when water is formed from the reaction between hydrogen and oxygen the "definite proportion" is 1g of H for every 8g of oxygen.
Law of Multiple ProportionBack to Top
According to this law "when two elements combine to form two or more compounds the weight of one of the elements that combine with the fixed weight of the other. If two elemets A and form two or more different compounds the different masses of either element combined with a fixed mass of the other are in the ratio of small whole numbers.
Law of Equivalent ProportionBack to Top
This law is also known as reciprocal proportion and it was proposed by Richter in 1792. It was verified by Star. According to this law, "when two different elements undergo combination with same weight of a third element, the ratio in which they combine will either be same or some simple multiple of the ratio in which they combine with each other."
When an elementary body A units with other bodies the proportions in which B, C ad D unite with A will represent in numbers the proportions in which they will unite among themselves in case such union takes place in other words the fixed proportions in which the elements unite among themselves may be represented numerically.
Examples of Chemical ChangesBack to Top
It is a change in which one or more kinds of matter are converted into new kind of matter or several kinds of matter. In chemical change new substance is formed. The properties of new substance is different from its components. The components of new substance cannot be separated by physical methods but can be separated by chemical methods.
Some of the examples of chemical changes are rusting of iron, formation of iron sulphide from iron and sulfur, burning of magnesium ribbon in presence of air to form magnesium oxide and magnesium nitride.
Evidence of a Chemical Change
A change which produces a different kind of material from that which existed before is a chemical change. We are familiar with many processes which involve chemical changes such as cooking, rusting of steel decay and burning. It needs to be clearly understood however that at primary level investigating processes which involve chemical change cannot lead children to an understanding of the nature of chemical change itself.
List of Chemical ChangesBack to Top
It involves rearrangements of atoms among molecules to create new molecular structures. The components of a mixture can be separated by a physical change, whereas when the atoms in a molecule are permanently separated a chemical change has taken place.
The tarnishing of silver, however is a chemical change. Silver atoms combine with sulfur atoms to form blck silver sulphide. The corrosion of bronze and the rusting of iron are also chemical changes.