Chem notes (rolling)
Chemistry Notes: Matter, Measurement, and Change
1. Core Vocabulary
Matter — Any substance that has mass and takes up space. Has constant composition and properties. Ex: rust, water, air.
Solution — A homogeneous mixture where one of the components is water (or another solvent).
Precipitate — A solid that forms when two liquids (solutions) are mixed together.
Qualitative data — Info describing the physical characteristics of a substance: color, size (small/large), shape, appearance, name, state of matter, etc.
Quantitative data — Numerical data related to properties of a substance or reaction: temperature, length, width, height, mass, volume, density, etc.
Mass — The amount of material in a substance. Measured using digital balances (g).
Volume — The space occupied by a substance.
Density — Mass per unit volume of a substance (Density = Mass ÷ Volume). A physical property used to identify substances.
Independent variable — The factor you deliberately change or control in an experiment.
Dependent variable — The factor you measure, which changes in response to the independent variable.
2. States of Matter
Roughly ~40 states of matter exist in physics, but for this class the three main ones:
Non-matter examples: energy — electricity, heat, sound.
3. Atoms, Molecules, Compounds, and Mixtures
Element — Cannot be broken down into simpler substances while retaining its chemical properties. Smallest piece is an atom.
Examples: Na, He, Ag, B, F₂
Atom — The smallest piece of an element.
Molecule — Two or more atoms chemically combined and bound together (can be the same element or different elements).
Examples: H₂, O₃, P₄, S₈, C₆₀
Compound — A substance formed by two or more different elements combined in a fixed ratio. Atoms must be from different elements.
Examples: CO₂, H₂O, C₁₂H₂₂O₁₁
Mixture — Two or more substances mixed together that do not chemically combine. They can be separated again, and each substance keeps its own properties/space.
Homogeneous vs. Heterogeneous Mixtures
Homogeneous mixture — Uniform distribution of the components throughout; the ratio is equal everywhere you sample it. A solution is a homogeneous mixture where water is one of the components.
Heterogeneous mixture — Uneven/uneven distribution of components; the ratio is not equal throughout. (Analogy: like a box of Lucky Charms — marshmallows and cereal aren't evenly distributed.)
Classification Chart
Matter
/ \
Pure Substance Mixture
/ \ / \
Element Compound Homogeneous Heterogeneous
Mixture Mixture
Key Q&A on Elements, Molecules, and Compounds
Can an element be an atom? Yes — the smallest piece of an element is an atom.
Can an element be a molecule? Yes — two atoms of the same element can join and it's still an element (e.g., O₂, F₂).
Can an element be a compound? No — a compound needs 2+ different elements combined; an element is pure and not mixed with another element.
Can a compound be an element? No — a compound needs 2+ different elements, while an element must be pure with no other elements involved.
4. Physical vs. Chemical Change
Pure substance — Matter that is completely uniform and identical throughout its structure. Can be either an element or a compound.
Physical change — A change that alters the substance without changing its chemical composition.
Examples: melting ice, tearing paper
No new substance is formed — only appearance, size, or state of matter changes.
Physical change indicators: alteration in size, shape, or amount; appearance; reversibility; temperature; state of matter.
Chemical change — A process that involves one or more substances changing into new substance(s), involving a change in chemical composition. The original substance is no longer present — entirely new substances are left in its place.
Examples: burning paper (combustion), iron rusting
Signs / indicators of a chemical change:
⭐ New substance formed (look for: precipitate forming, new bubbles or new odor)
⭐ Energy change (temperature change from just mixing)
⭐ Color change (a new color, or a combination color)
New sound or light being produced
Main indicator: something new has formed (bubbling/odor are supporting evidence)
5. Math Skills for Chemistry
Rounding
Example: 143,296.187
Round to the thousand: 143,000
Round to the hundred: 143,300
Round to the tenth: 143,296.2
Averages
Normal average = sum of items ÷ number of items
Weighted average — multiply each value by its weight (%) and sum:
Example: 60% at 95, 40% at 85 → (0.6 × 95) + (0.4 × 85) = 57 + 34 = 91
Percents
Formula: (part ÷ whole) × 100 = %
Example: 34 R, 5 R, 8 G, 60 Y, 4 B → 8 green ÷ 41 total × 100 ≈ 19.23%
Reverse example: 43.75% of 32 balls are green — how many balls?
Set up: (?/32) × 100 = 43.75 → 14 green balls
Significant Figures
Basic rules:
All non-zero digits are significant.
14.56 g = 4 sig figs; 1.456 × 10¹² = 4 sig figs
Leading zeros are not significant. (0.05 → not significant)
Trailing zeros are significant if after a decimal point.
14.500 = 5 sig figs; 13.00 = 4 sig figs
Trailing zeros are not significant in a number greater than one, unless there's a decimal point.
1400 = 2 sig figs
Trapped (sandwiched) zeros are significant.
10.409 = 5 sig figs; 1405 = 4 sig figs
Conversion values (exact amounts) have an infinite number of sig figs.
Applying sig figs — Addition/Subtraction (round to least # of decimal places):
158.12
+ 6.8 ← least precise (tenths place), so round final answer to tenths
+ 12.731
---------
177.651 → rounds to 177.7
Applying sig figs — Multiplication (round to least # of sig figs):
158
× 1410
-------
7400
× 8968...
-------
≈ 9,000 (rounded to fewest sig figs among factors)
Another multiplication example:
7.49
× 3.2 ← least sig figs (2 sig figs)
----------
23.968 → rounds to 24
General rule of thumb:
Addition/subtraction → match the fewest decimal places
Multiplication/division → match the fewest sig figs