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Unit 2: Matter

What everything around you is actually made of, and what happens when it melts, dissolves, mixes, or seems to vanish.

11 topics · 43 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.

Solids, Liquids, and Gases

  1. How it worksHow do the same water molecules pack tight in ice, slide past each other as water, and fly apart as steam?
  2. How it worksWalk me through what the particles are doing differently inside a solid, a liquid, and a gas.
  3. Common mistakeWhy isn't a gas just 'nothing' even though you usually can't see it?
  4. Going furtherWhat happens to how fast particles move when you heat a gas up even more?
  5. Ties togetherHow does knowing what particles are doing explain why a shaken soda can fizzes when you open it?
Your turn1 of 2
You can't see the air around you. Does that mean a gas like air is basically 'nothing'?

Melting and Freezing

  1. How it worksHow does adding heat energy let solid particles break free from their fixed spots and start sliding?
  2. Worked exampleFind how many degrees an ice cube must warm up to melt if it starts at negative 10 degrees C and ice melts at 0 degrees C.
  3. Common mistakeWhy doesn't a melting ice cube's temperature keep rising past 0 degrees C until it's completely melted?
  4. Going furtherWhat happens to the freezing point of water when you stir salt into it?
Your turn1 of 2
Find how many degrees an ice cube must warm up to melt if it starts at negative 21 degrees C and ice melts at 0 degrees C.

Boiling and Evaporation

  1. How it worksHow do the fastest-moving particles at the surface of water escape into the air as evaporation?
  2. How it worksWalk me through what's different inside water when it boils compared to when it just evaporates.
  3. Worked exampleFind how many degrees water must heat up to boil, starting at 20 degrees C, if water boils at 100 degrees C.
  4. Common mistakeWhy doesn't blowing on hot soup cool it by adding cold air, instead of speeding up evaporation?
Your turn1 of 2
Find how many degrees water must heat up to boil, starting at 63 degrees C, if water boils at 100 degrees C.

Condensation and Sublimation

  1. How it worksHow do water vapor particles in the air slow down and stick together into droplets on a cold surface?
  2. How it worksWalk me through how dry ice turns straight from a solid into a gas without ever becoming a puddle.
  3. Common mistakeWhy isn't the fog coming off dry ice actually made of water, the way a cloud is?
  4. Going furtherWhat happens to the water cycle if condensation stopped happening completely?
Your turn1 of 2
A cold glass of lemonade gets wet on the outside, even though nobody spilled anything on it. Where does that water come from?

Physical Change vs. Chemical Change

  1. How it worksHow can you tell a physical change from a chemical change by checking whether a new substance forms?
  2. Worked exampleFind which changes are physical and which are chemical: crushing a can, rusting a nail, melting butter, burning wood.
  3. Common mistakeWhy isn't dissolving sugar in water a chemical change, even though the sugar seems to vanish?
  4. Going furtherWhat happens when you try to reverse a chemical change like a burned marshmallow, compared to reversing melted chocolate?
Your turn1 of 2
Which of these is a CHEMICAL change: crushing a can, or rusting a nail?

Atoms and Molecules

  1. How it worksHow do atoms link together to build a molecule, like two hydrogens and one oxygen building water?
  2. Worked exampleFind how many total atoms are in one molecule of table salt, made of one sodium atom joined to one chlorine atom.
  3. Common mistakeWhy isn't an atom the smallest thing that exists in the universe?
  4. Going furtherWhat happens to a substance's properties when you rearrange the same atoms into a different molecule?
Your turn1 of 2
A molecule is made of 4 sodium atoms joined to 4 chlorine atoms. Find the total number of atoms in the molecule.

Elements and Compounds

  1. How it worksHow do elements combine in fixed ratios to form a compound with totally new properties?
  2. Worked exampleFind how many hydrogen and oxygen atoms are in three molecules of water if each one is two hydrogens and one oxygen.
  3. Common mistakeWhy isn't a compound just a mix where you could still spot each original ingredient?
Your turn1 of 2
Find how many hydrogen atoms are in 14 molecules of water if each molecule has 2 hydrogens and 1 oxygen.

Mixtures and Solutions

  1. How it worksHow does a solution form when one substance's particles spread evenly between another's?
  2. How it worksWalk me through separating a mixture of sand and salt water using evaporation and filtering.
  3. Common mistakeWhy isn't salt water a compound just because you can't see the salt anymore?
  4. Going furtherWhat happens to a mixture of oil and water no matter how hard you shake it?
Your turn1 of 2
You can pick raisins back out of trail mix, but you can't pick salt back out of the ocean by hand. Does that mean salt water is a compound instead of a mixture?

Dissolving

  1. How it worksHow do water molecules surround and pull apart each tiny piece of sugar until you can't see it anymore?
  2. Worked exampleFind how much sugar is left undissolved if you stir 20 grams into water that can only dissolve 15 grams at that temperature.
  3. Common mistakeWhy doesn't dissolved sugar actually disappear, even though you can't see it in the water?
  4. Going furtherWhat happens to how fast sugar dissolves when you use hot tea instead of iced tea?
Your turn1 of 2
Find how much sugar is left undissolved if you stir 40 grams into water that can only dissolve 11 grams at that temperature.

Density and Floating

  1. How it worksHow does comparing an object's mass packed into its space to water's density decide if it floats or sinks?
  2. Worked exampleFind the density of a rock with a mass of 150 grams that takes up 50 cubic centimeters, and say whether it floats in water.
  3. Common mistakeWhy doesn't a heavier object always sink faster than a lighter one?
  4. Going furtherWhat happens to a ship's floating line when it sails from salty ocean water into a fresh water river?
Your turn1 of 2
Find the density of a rock with a mass of 354 grams that takes up 67 cubic centimeters.

Conservation of Mass

  1. How it worksHow does the mass of a burned candle's wax actually go somewhere instead of vanishing?
  2. Worked exampleFind the total mass of gas produced if 10 grams of baking soda react completely with 20 grams of vinegar and no mass is lost.
  3. Common mistakeWhy doesn't matter actually get destroyed when something burns, rusts, or dissolves?
Your turn1 of 2
Find the total mass of gas produced if 22 grams of baking soda react completely with 29 grams of vinegar and no mass is lost.