Unit 1: Measurement and the Language of Matter
How chemists sort the physical world into elements, compounds, and mixtures, and the measurement habits that keep every number honest.
7 topics · 25 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
1.2 Classifying Matter: Elements, Compounds, and Mixtures
- How it worksHow do you tell a homogeneous mixture apart from a heterogeneous one just by looking at it?
- How it worksWalk me through separating a mixture of sand, table salt, and iron filings into its three parts.
- Common mistakeWhy isn't air one pure substance even though it looks and behaves like a single gas?
- Going furtherWhat happens to a solution's classification once you keep dissolving solute past the saturation point?
Your turn1 of 2
Air is made mostly of nitrogen and oxygen gas, thoroughly mixed. Why isn't air classified as a pure substance?1.3 Physical and Chemical Changes
- How it worksHow can you tell whether a change is physical or chemical just from what happens to the substance afterward?
- Worked exampleWalk me through classifying five everyday changes, from boiling water to rusting iron, as physical or chemical.
- Common mistakeWhy isn't dissolving sugar in tea a chemical reaction even though the sugar seems to disappear?
Your turn1 of 2
Iron left outside slowly turns to reddish-brown rust. What tells you this is a chemical change rather than a physical one?1.5 Precision, Accuracy, and Measurement Uncertainty
- How it worksHow does reading a graduated cylinder to one estimated digit build uncertainty into every measurement?
- Common mistakeWhy isn't a highly precise measurement automatically an accurate one?
- Going furtherWhat happens to a data set's precision and accuracy once an instrument develops a consistent calibration error?
Your turn1 of 2
Four dart throws all land clustered tightly together, but far from the bullseye. What does this scenario best illustrate?1.4 SI Units and Dimensional Analysis
- How it worksHow do conversion factors let you cancel units all the way from one system into another?
- Worked exampleWalk me through converting a medicine dose of 15 milligrams per kilogram into grams per pound.
- Worked exampleFind how many seconds are in exactly 2.5 days using nothing but a chain of unit conversions.
- Common mistakeWhy isn't multiplying a quantity by a conversion factor changing the actual size of that quantity?
Your turn1 of 2
A car travels at 114 kilometers per hour. What is its speed in meters per second?1.6 Significant Figures in Measurements and Calculations
- How it worksHow do you decide which digits in a measurement like 0.02040 actually count as significant?
- Common mistakeWhy do addition and multiplication use two completely different rules for counting significant figures?
- Worked exampleFind the answer to 4.56 times 1.4 reported to the correct number of significant figures.
- Worked exampleFind the sum of 12.11, 18.0, and 1.013 keeping only the digits the three measurements actually support.
Your turn1 of 2
A calculation multiplies 4.56 (3 significant figures) by 1.4 (2 significant figures). How many significant figures should the reported answer have, and why?1.4 Density as a Physical Property
- How it worksHow does dividing mass by volume give every pure substance its own fingerprint number?
- Worked exampleFind the density of a 47.5 gram metal sample that displaces 6.35 mL of water when dropped in.
- Going furtherWhat happens to an object's measured density once it's heated enough to expand?
- Ties togetherHow does the density of ice versus liquid water connect back to the hydrogen bonding that shapes both?
Your turn1 of 2
A 142 gram sample of metal displaces 16.75 mL of water when dropped into a graduated cylinder. Find its density.1.4 Temperature Scales and Conversions
- How it worksHow do you convert a temperature between Celsius, Kelvin, and Fahrenheit without mixing up the formulas?
- Worked exampleFind the Kelvin temperature that corresponds to a summer day at 98.6 degrees Fahrenheit.
- Common mistakeWhy isn't a temperature of 0 degrees Celsius the same thing as a temperature of zero energy?
Your turn1 of 2
Convert 127 degrees Celsius to Kelvin.