Define it
What is a pure substance?
Matter with a fixed composition (element or compound).
Air
Mixture (different gases).
Compare elements and compounds.
Elements = 1 type of atom; compounds = 2+ elements chemically bonded.
I am made of only one kind of atom. What am I?
Element
You spill salt into sand. How could you separate them? Why does this method work?
Dissolve salt in water, filter sand, evaporate water. Works because of solubility.
Define homogenous mixture. Give an exmaple.
Uniform throughout, e.g. salt water.
Sugar
Compound (C₁₂H₂₂O₁₁).
Compare homogeneous and heterogeneous mixtures.
Homogeneous = uniform; heterogeneous = not uniform.
I am uniform throughout, cannot be separated by filtration, and my components are not chemically bonded. What am I?
Homogeneous mixture (solution).
Why does oil float on water? What property explains this behavior?
Density
Define Heterogeneous
Not uniform; visibly different parts.
Identify whether the following are elements, compounds, or mixtures:
Carbon dioxide; Hydrogen gas; Salt water; Pizza
Carbon dioxide – compound; Hydrogen gas – element; Salt water – homogeneous mixture; Pizza – heterogeneous mixture.
How are mixtures different from compounds in terms of composition and separation?
Mixtures = physically combined, can be separated; compounds = chemically combined, need chemical change to separate.
I can be separated by evaporation, but I look like one single phase. What am I?
Solution
Vinegar and baking soda react to form bubbles. Is this a physical or chemical change? Explain.
Chemical change (new substances + gas formed).
Physical vs. chemical property
Physical can be observed without changing identity; chemical describes ability to change into a new substance.
Classify: bronze, oxygen, seawater, salad.
Bronze – mixture (alloy); Oxygen – element; Seawater – mixture; Salad – heterogeneous mixture.
Why are physical properties (like density, melting point) useful for identifying substances?
Help identify substances without changing them.
I am cloudy, not uniform, and you can see my pieces with the naked eye. What am I?
Heterogeneous mixture
How could boiling point be used to separate alcohol from water?
Alcohol boils at a lower temperature - it will evaporate and the water will remain.