Chemical Reaction
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A chemical reaction happens when substances called reactants change into new substances called products. During this process, atoms rearrange and form new combinations. The new substances usually have different properties from the original materials.
Matter can undergo two kinds of changes: Physical change and Chemical change
Physical Change
Physical Change
A physical change affects only the appearance, shape, or state of matter. No new substance is formed. (e.g. Melting ice, Cutting paper, Boiling water, Dissolving sugar in water) The substance remains the same even after the change
Chemical Change
Chemical Change
A chemical change produces a new substance with new properties. This change is often difficult to reverse. (e.g. Burning wood, Cooking food, Rusting iron, and Spoiling milk). During a chemical change, chemical reactions occur.
Phase of Matter
Phase of Matter
Matter also exists in different phases known as solid, liquid, and gas. Solids have a definite shape and volume because their particles are tightly packed together. Examples of solids include ice, wood, and metal spoons. Liquids have a definite volume but no definite shape because they take the shape of their container. Water, juice, and oil are examples of liquids. Gases have neither definite shape nor definite volume because their particles move freely. Oxygen, steam, and carbon dioxide are examples of gases.
Matter can change from one phase to another through heating or cooling. Melting changes a solid into a liquid, freezing changes a liquid into a solid, evaporation changes a liquid into gas, and condensation changes gas into liquid. These changes are physical changes because no new substance is formed.
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Evidence of Chemical Reaction
Chemical reactions show observable signs called evidences or indicators. One indicator is color change. For example, iron turns reddish-brown when rust forms, and sliced apples become brown after exposure to air. Another indicator is the formation of a precipitate, which is a solid produced when two liquids react. Mixing silver nitrate and sodium chloride forms a white solid called silver chloride. The release of gas is another sign of chemical reaction and is often seen as bubbles or fizzing. Vinegar reacting with baking soda releases carbon dioxide gas. Temperature change is also an indicator because some reactions release heat while others absorb heat. Burning fuel releases heat, while instant cold packs absorb heat. Production of odor can also indicate a chemical reaction, such as the foul smell of spoiled eggs or rotten food.
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Acid and Base
Acid and Base
Chemical substances can be classified as acids, bases, and salts based on their properties and behavior in water.
Acids release hydrogen ions (H⁺) in aqueous solutions, have a sour taste, and usually have a pH lower than 7. Examples include hydrochloric acid, vinegar, and citric acid.
Bases release hydroxide ions (OH⁻) in water, feel slippery, taste bitter, and have a pH higher than 7. Examples include sodium hydroxide, ammonia, and soap.
Salts are formed through the neutralization of acids and bases and may be neutral, acidic, or basic depending on their composition. Common examples include table salt and saline solution.
Indicators are substances used to determine if a solution is acidic or basic. Different indicators change color depending on the pH of the solution. Litmus paper is one of the most common indicators. Blue litmus paper turns red in acidic solutions, while red litmus paper turns blue in basic solutions. Phenolphthalein remains colorless in acidic solutions but turns pink in basic solutions. Methyl orange appears red in acids and yellow in bases.
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