Grade 6 → Physical and chemical changes ↓
Indicators of Chemical Change
Chemistry is all around us, even if we don't notice it. From cooking to lighting a match, chemical changes are happening every day. In grade 6 chemistry, we learn about physical and chemical changes, and more specifically, how to detect when a chemical change has occurred. In this lesson, we will explore the different indicators of chemical change, using examples and simple explanations to help us better understand these concepts.
What is a chemical change?
A chemical change occurs when substances combine or break down to form new substances. This means that the original substances, called reactants, change into new substances, called products. This change involves the formation or breaking of bonds between atoms and results in a new substance with different properties.
Difference between physical and chemical change
Before we discuss the indicators of chemical change, it is important to know the difference between physical and chemical changes:
- Physical changes are changes in the state or appearance of a substance that do not change its chemical composition. Examples include ice melting, paper tearing into pieces, and sugar dissolving in water.
- On the other hand, chemical changes result in the formation of new substances. It involves changes at the molecular level. Examples include burning wood, rusting of iron, and baking a cake.
Indicators of chemical change
There are a number of indicators that can help us identify whether a chemical change has occurred:
1. Color change
One of the most obvious signs of a chemical change is a change in color. This indicates that a new substance with a different color has formed.
Example: If you cut an apple and leave it in the air, its flesh will turn brown. This happens because the substances present in the apple react with the oxygen present in the air to form new compounds that change the color of the apple.
2. Production of gas
Another sign of a chemical change is the production of gas. This can often be seen as bubbles forming in the liquid.
Example: When you mix baking soda ( NaHCO 3
) with vinegar ( CH 3 COOH
), it creates carbon dioxide gas ( CO 2
). The gas forms bubbles, which is a clear sign that a chemical change has occurred.
3. Formation of precipitate
Chemical changes can also be identified by the formation of a solid substance from a liquid solution. This solid substance is called a precipitate.
Example: Mixing a solution of sodium chloride ( NaCl
) with silver nitrate ( AgNO 3
) forms a solid called silver chloride ( AgCl
), which is a precipitate.
4. Changes in temperature
A change in temperature can also indicate a chemical change. Some reactions release heat (exothermic), while others absorb heat (endothermic).
Example: When you mix cement with water to make concrete, the reaction releases heat, which is an exothermic change.
5. Change in smell
The appearance of a new odor or the disappearance of an odor can be a sign of a chemical change. This shows that new substances with different odors have been formed.
Example: Spoiled milk smells sour because bacteria convert lactose into lactic acid. This change in smell is an indication of a chemical change.
Conclusion
In short, chemical changes are the changes that result in the formation of new substances. These changes can be identified through several indicators such as colour change, production of gas, formation of precipitate, change in temperature and change in odour. Understanding these indicators not only helps in identifying chemical changes around us but also strengthens the foundation for further studies in chemistry.