OBSERVING CHEMICAL CHANGES Physical change any change that

OBSERVING CHEMICAL CHANGES

Physical change : any change that alters the form or appearance of a substance but does not change into another substance Chemical change : a change in matter that produces one or more new substances Reactant : substances that undergo the chemical changes Product : the new substances that are formed from a chemical change Precipitate: a solid that forms from liquids during a chemical reaction

CHANGES IN MATTER Review: What is matter? Matter is anything that has mass and takes up space. Matter can change depending on its physical and chemical properties. These properties are unique to a particular substance. Physical properties can be observed without changing the substance into another. These can include color, texture, density, and conductivity. Chemical properties describe a substance’s ability to change into other substances; you have to try to change the substance to observe this. Flammability, oxidation, and combustibility are all chemical properties.

CHANGES IN MATTER When matter undergoes a physical change, it has changed in form or appearance but not in substance. The chemical composition has stayed the same. Bending, crushing, cutting, and phase changes (melting/freezing/boiling) are physical changes. When a substances undergoes a change that causes its chemical composition to change, then it underwent a chemical change. In a chemical change, atoms have been rearranged to form new substances. The substances before the chemical change are called the reactants. Substances formed after the chemical change are called products.

CHANGES IN MATTER C P P C C P

BONDING AND CHEMICAL CHANGE Chemical changes cause new substances to be produced. This occurs because existing bonds break and new bonds form. Recall that chemical bonds form when atoms are shared or transferred. During a chemical change, the atoms are rearrange to form a new substance.

IDENTIFYING A CHEMICAL REACTION Chemical reactions involve changes in properties and changes in energy that you can often observe. There are three main ways to observe a chemical reaction. 1. Formation of a precipitate: A precipitate is a solid formed from two liquids mixing. For example, if you add lemon juice to milk, the milk will curdle. This is a chemical reaction, and the curdled milk is the precipitate. 2. Gas production: If a gas is released from solid or liquid reactants, it can be a sign of a reaction. Bubbles are not always a sign of a reaction, ie, water boiling is a physical change. 3. Color change: This can be a sign of a reaction. Example: An apple browning after it is cut. (It has reacted with oxygen in the air.

IDENTIFYING A CHEMICAL REACTION Are these chemical reactions – Yes or No? Holes in bread? Yes! Food coloring in water? No! Boiling water? No! Molded food? Yes! Rusted metal? Yes!