Short Answer
There are several signs that show a chemical reaction has occurred. One common sign is a change in colour, such as iron turning reddish-brown when it rusts. Another sign is the formation of a gas, which can be seen as bubbles or fizzing during a reaction. Heat may also be released or absorbed, causing temperature change.
A new solid substance called a precipitate may form when two liquids react. Sometimes, a change in smell or the release of light also indicates a chemical reaction. These signs help us understand that a new substance with new properties has been formed.
Detailed Explanation :
Signs of a Chemical Reaction
Chemical reactions take place all around us, and they often produce noticeable changes. These changes help us identify whether a chemical reaction has occurred. A chemical reaction results in the formation of new substances with new physical and chemical properties. Since atoms rearrange during the reaction, the original substances cannot remain the same. Because we cannot see atoms directly, we depend on visible or measurable signs to understand that a change has taken place.
These signs do not occur in every reaction, but even one clear sign is enough to show that a chemical reaction has happened. Some reactions show several signs at the same time. For example, burning a candle shows light, heat, and gas formation. Understanding these signs helps students, scientists, and industries identify reactions quickly and safely.
Change in Colour
One of the most common signs of a chemical reaction is a colour change. If two substances react and the colour of the mixture suddenly becomes different, it means a new substance has formed. For example, iron turns reddish-brown when it reacts with oxygen and moisture to form rust. When iodine reacts with starch, the mixture turns blue-black. Such changes cannot be reversed easily, showing that a chemical reaction has occurred.
Formation of Gas
The formation of gas is another strong sign of a chemical reaction. Gas formation is seen as bubbles, fizzing, or foam. For instance, when vinegar reacts with baking soda, carbon dioxide gas is released, producing bubbles. Effervescence in many reactions is a result of gas being produced as a product. Sometimes, the gas may also have a smell, like the pungent odour of ammonia.
Change in Temperature
Many chemical reactions either release heat or absorb heat. When a reaction releases heat, it is called an exothermic reaction. Burning of wood and respiration are examples of exothermic processes. When a reaction absorbs heat, it is an endothermic reaction. A simple example is dissolving ammonium nitrate in water, which makes the mixture cold. A noticeable increase or decrease in temperature without external heating indicates that a chemical reaction has taken place.
Formation of a Precipitate
A precipitate is a solid substance formed when two liquids react. This solid appears suddenly and settles at the bottom or makes the mixture cloudy. For example, when silver nitrate reacts with sodium chloride, a white solid called silver chloride is formed. This solid is proof that a chemical reaction has produced a new substance with different properties from the reactants.
Change in Smell
Some chemical reactions produce new smells. For example, when food becomes stale or spoils, chemical reactions take place that release unpleasant odours. Similarly, the pleasant smell during the ripening of fruits is also due to chemical reactions occurring inside the fruit. A sudden or unexpected smell indicates that a new substance has formed.
Emission of Light or Sound
Certain reactions produce light or sound. Fireworks are a good example, where reactions produce bright colours, sound, and heat. The burning of a magnesium ribbon produces a bright white light. These signs clearly show that a chemical reaction is occurring because new substances are releasing energy in different forms.
Production of Electricity
Some reactions generate electricity as part of their process. Batteries work because of chemical reactions taking place inside them. When a battery is in use, chemical substances inside it react and produce electrical energy. This is another sign that a chemical change is happening.
Irreversibility of the Change
Another sign of a chemical reaction is that the change is not easily reversible. For example, once paper burns to ash, it cannot return to paper again. This is because new substances have formed, and the original materials are no longer present in their earlier form.
Conclusion
The signs of a chemical reaction help us recognize that new substances have been formed. These signs include change in colour, formation of gas, change in temperature, formation of a precipitate, change in smell, and emission of light or sound. Each of these signs indicates that atoms have rearranged to form new materials. Identifying these signs allows us to understand chemical changes in daily life, laboratories, and industries.