Chemical Reactions and Conservation of Mass
1. Evidence of Chemical Reactions
Learning outcomes
- I can identify common signs that a chemical reaction has occurred.
- I can distinguish between physical and chemical changes.
- I can describe examples of chemical reactions in everyday life.
- I can explain why new substances are formed during chemical reactions.
- I can classify observed changes as physical or chemical using scientific evidence.
Introduction
Every day, substances change in many different ways. Ice melts into water, paper is cut into smaller pieces, iron rusts, food cooks, and candles burn. Some of these changes simply alter the appearance or state of a substance, while others produce entirely new substances.
Chemists distinguish between physical changes and chemical changes by looking for evidence that a chemical reaction has taken place. During a chemical reaction, atoms are rearranged to form new substances with different properties. Recognising the signs of chemical reactions helps scientists understand how matter changes and allows them to study everything from cooking and digestion to combustion and industrial manufacturing.
What Is a Chemical Reaction?
A chemical reaction is a process in which one or more substances are converted into new substances.
During a chemical reaction:
- Atoms are rearranged.
- Chemical bonds are broken.
- New chemical bonds are formed.
Although the atoms remain the same, they are combined in new ways to produce substances with different properties.
Figure 1. During a chemical reaction, atoms are rearranged to form new substances.
Evidence of a Chemical Reaction
Several observations can indicate that a chemical reaction has occurred.
Common signs include:
- Colour change.
- Gas production.
- Formation of a precipitate.
- Temperature change.
- Light production.
- Odour change.
One sign alone is not always enough, but together they provide strong evidence that new substances have formed.
Colour Change
A chemical reaction often causes a substance to change colour.
Examples:
- Iron turns reddish-brown when it rusts.
- Copper develops a green coating over time.
- Leaves change colour as pigments break down in autumn.
A colour change may indicate that new substances have been produced.
Gas Production
Many reactions release a gas.
Signs include:
- Bubbling.
- Fizzing.
- Foaming.
Examples:
- Vinegar reacts with baking soda to produce carbon dioxide.
- Effervescent tablets release carbon dioxide in water.
Gas production often indicates that a chemical reaction has taken place.
Figure 2. Bubbling or fizzing is often evidence that a gas is being produced during a chemical reaction.
Formation of a Precipitate
A precipitate is an insoluble solid that forms when two solutions react.
The solid appears even though both original solutions were clear.
Example:
Mixing certain solutions of silver nitrate and sodium chloride produces a white precipitate of silver chloride.
The formation of a precipitate is strong evidence that a new substance has formed.
Temperature Change
Some chemical reactions release heat.
These are called exothermic reactions.
Examples:
- Burning wood.
- Combustion of fuels.
- Neutralisation reactions.
Other reactions absorb heat.
These are called endothermic reactions.
Example:
Some instant cold packs become cold because they absorb heat from their surroundings.
A temperature change without external heating or cooling often indicates a chemical reaction.
Light Production
Some chemical reactions produce light.
Examples:
- Fireworks.
- Burning magnesium.
- Glow sticks.
- Fireflies (bioluminescence).
Light production often occurs because energy is released during the reaction.
Figure 3. Some chemical reactions release energy as visible light.
Physical Changes vs Chemical Changes
A physical change changes the appearance or state of a substance but does not produce a new substance.
Examples:
- Melting ice.
- Boiling water.
- Breaking glass.
- Dissolving sugar in water.
A chemical change produces one or more new substances.
Examples:
- Rusting iron.
- Burning wood.
- Cooking an egg.
- Digesting food.
The key difference is whether new substances are formed.
Comparing Physical and Chemical Changes
| Physical Change | Chemical Change |
|---|---|
| No new substance formed | New substances formed |
| Usually reversible | Often difficult to reverse |
| Composition remains unchanged. | Composition changes |
| Examples: melting, freezing | Examples: burning, rusting |
Understanding this difference is one of the most important concepts in chemistry.
Figure 4. Physical changes do not create new substances, while chemical changes do.
Everyday Examples of Chemical Reactions
Chemical reactions occur constantly around us.
Examples include:
- Cooking food.
- Baking bread.
- Burning candles.
- Rusting bicycles.
- Digestion.
- Photosynthesis.
- Cellular respiration.
- Fireworks.
Many everyday processes depend on chemical reactions.
Why New Substances Form
During a chemical reaction:
- Existing chemical bonds are broken.
- Atoms are rearranged.
- New chemical bonds are formed.
Because the atoms are arranged differently, the products have different properties from the reactants.
Example:
Hydrogen and oxygen gases react to form liquid water.
The properties of water are completely different from those of hydrogen and oxygen.
Classifying Changes
Scientists classify a change by looking at the evidence.
Examples:
| Observation | Physical or Chemical? | Reason |
|---|---|---|
| Ice melts | Physical | No new substance formed |
| Iron rusts | Chemical | New substance (iron oxide) formed |
| Salt dissolves in water. | Physical | Salt can be recovered by evaporation |
| Wood burns | Chemical | New gases, ash, and heat produced |
| Water boils | Physical | Only a change of state |
Evidence is used to support every classification.
Figure 5. Everyday observations can be classified as physical or chemical changes using scientific evidence.
Why Chemical Reactions Matter
Chemical reactions are essential for life and modern society.
They allow us to:
- Produce food.
- Generate energy.
- Manufacture medicines.
- Create new materials.
- Recycle waste.
- Sustain living organisms.
Nearly every biological and industrial process depends on chemical reactions.
Worked Example
Question
Classify each change as physical or chemical.
| Change | Classification |
|---|---|
| Ice melting | Physical |
| Burning paper | Chemical |
| Rusting iron | Chemical |
| Breaking glass | Physical |
| Cooking an egg | Chemical |
| Dissolving sugar in water. | Physical |
Explanation
A chemical change forms new substances, while a physical change changes only the form or state of the material.
Real-World Connection
Car engines, power stations, and even the cells in your body rely on chemical reactions to release energy. During cellular respiration, glucose reacts with oxygen to produce carbon dioxide, water, and energy that powers every activity in your body. Without chemical reactions, living organisms could not grow, move, repair themselves, or survive.
Did You Know?
The rusting of iron is actually a slow chemical reaction between iron, oxygen, and water. Over time, this reaction produces iron oxide (rust), a new substance with completely different properties. Engineers spend billions of dollars each year developing coatings and paints to prevent rust from damaging bridges, buildings, ships, and vehicles.
Key Terms
Chemical change – A change that produces one or more new substances.
Chemical reaction – A process in which atoms are rearranged to form new substances.
Endothermic reaction – A chemical reaction that absorbs energy from its surroundings.
Exothermic reaction – A chemical reaction that releases energy to its surroundings.
Physical change – A change in the form or state of a substance without producing a new substance.
Precipitate – An insoluble solid formed during a chemical reaction in a solution.
Product – A substance formed during a chemical reaction.
Reactant – A starting substance in a chemical reaction.
Key Takeaways
- A chemical reaction produces one or more new substances by rearranging atoms.
- Common evidence of a chemical reaction includes colour change, gas production, precipitate formation, temperature change, light production, and sometimes odour change.
- Physical changes do not produce new substances, while chemical changes do.
- Chemical reactions occur in many everyday processes, including cooking, rusting, burning, and digestion.
- Scientists classify changes by examining the evidence and determining whether new substances have formed.
- Understanding chemical reactions helps explain natural processes, industrial production, and the chemistry of everyday life.