An introduction to chemical reactions
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Electricity
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Explainer Video
Summary
An introduction to chemical reactions
In a nutshell
Chemical reactions involve reactants undergoing chemical change to produce new products. In this lesson you will learn about the importance of chemical reactions, conservation of mass and some of the signs which show that a chemical reaction is taking place.
Definitions
Key word | Definition |
| Reactants | Starting materials in a chemical reaction. |
| Products | Substances formed at the end of chemical reaction. |
| Precipitate | Solid substance which settles at the bottom of a solution. |
| Physical change | When a substance changes form or state of matter. |
Chemical reactions
A chemical reaction involves reactants undergoing chemical change to produce different chemicals called products.
In a chemical reaction, chemical bonds in the reactants break, the atoms rearrange themselves and new bonds form to produce products.
Conservation of mass
The number of atoms in the reactants is the same as the number of atoms in the products, therefore there will be no change in mass. This is referred to as a conservation of mass.
Evidence for chemical reactions
When a chemical reaction is taking place, many changes can be observed. Some of these changes are listed in the table below.
Chemical reaction observations
Observation | Description |
| Change in colour | If the products formed have a different colour to the reactants, then a colour change will be seen. |
| Bubbles | If gas products are formed in a chemical reaction, they may escape in the form of bubbles. |
| Temperature change | Most chemical reactions release energy and temperature increases are observed.
A few chemical reactions absorb energy from the surrounding and a drop in temperature is observed. |
| Precipitates form | If the product formed in a chemical reaction is insoluble in the solvent/solution, it will precipitate out. |
| Change in odour | Some substances have a distinctive odour. When new products form in a reaction, the odour can change. |
Importance of chemical reactions
Chemical reactions are important because they make useful new products.
Example
Chemical reactions are required to produce plastics and drugs.
Some chemical reactions are important because they release energy. The energy released can be used to generate electricity, heat buildings and power planes and cars.
Example
Burning fossil fuels.
Chemical reactions are not always large-scale.
Example
Cooking involves chemical reactions.
Physical and chemical changes
Melting, boiling and dissolving do not produce new products and are therefore not chemical reactions; these are simply physical changes and, in most cases, can be reversed.
Example
The evaporation of water is a physical change and can be reversed by condensation.
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FAQs - Frequently Asked Questions
What is a chemical reaction?
A chemical reaction involves reactants undergoing chemical change to produce different chemicals called products. In a chemical reaction, chemical bonds in the reactants break, the atoms rearrange themselves and new bonds form to produce products.
Does the overall mass change within a chemical change during a chemical reaction?
The number of atoms in the reactants is the same as the number of atoms in the products during a chemical reaction, therefore there will be no change in mass. This is referred to as conservation of mass.
What are some of the signs that show a chemical reaction is taking place?
When a chemical reaction is taking place, many changes can be observed. For example: a change in colour, bubbles, temperature change, precipitates form and a change in odour.