Matter and its changes (grade 7) · lesson

Signs of a new substance

💡 A chemical reaction makes a new substance with new properties, while a physical change does not.

In a physical change, the substance stays the same. Melting, boiling, dissolving, and cutting are physical changes. Melted butter is still butter.

In a chemical reaction, a new substance forms. You can tell because the properties change.

Signs of a new substance include a new gas, a new color, a new solid forming from two liquids, or a change in how it melts or dissolves. Rust is a new substance made from iron and oxygen.

🧠 Let's work it out

Compare before and after.
A white solid that dissolved in water turns into a black solid that does not.
The color and the dissolving both changed.

What do you think comes next?

chemical reaction: a change that makes new substances
physical change: a change in size, shape, or state, but not the substance
property: a trait that describes a substance, like color or melting point

🔗 Khan Academy: Chemical changes · Khan Academy ↗

Atoms rearrange

💡 In a reaction, atoms come apart and join in new ways, but the number of each kind of atom stays the same.

Think of atoms as building blocks. A reaction takes blocks apart and builds new things with them. No blocks are added or thrown away.

A chemical equation shows this. Starting substances go on the left of the arrow. New substances go on the right.

A big number in front counts molecules. A small number after a letter counts atoms in one molecule.

🧠 Let's work it out

Count atoms in 2H₂O.
One H₂O has 2 hydrogen and 1 oxygen.
The big 2 means two molecules.

What do you think comes next?

reactant: a substance you start with in a reaction
product: a new substance made by a reaction

🔗 PhET simulation: Balancing Chemical Equations · PhET Interactive Simulations ↗

Mass is conserved

💡 In a closed system, the total mass before a reaction equals the total mass after.

Since no atoms are made or lost, the mass stays the same. This is the law of conservation of mass.

To see it, you need a closed system, where nothing can get in or out. A sealed bag or a bottle with a balloon on top works.

In an open cup, a gas made by the reaction can escape into the air. Then the scale shows less mass. The atoms still exist. They just left the cup.

🧠 Let's work it out

Do the math.
A metal (12 g) reacts with oxygen to make 20 g of product.
Mass in equals mass out.

What do you think comes next?

conservation of mass: the rule that mass is not made or lost in a reaction
closed system: a setup where no matter can get in or out

Reactions give off or take in energy

💡 Some reactions give off thermal energy and warm their surroundings, and others take it in and feel cold.

Some reactions release thermal energy. The surroundings get warmer. A hand warmer works this way as iron reacts with oxygen.

Other reactions take in thermal energy. The surroundings get colder. A cold pack works this way.

Both kinds are real reactions. A thermometer tells you which kind you have.

🧠 Let's work it out

Watch the temperature.
22 °C dropped to 12 °C.
The mixture lost thermal energy to the reaction.
So the reaction takes in energy.
If it had risen, the reaction would give off energy.

What do you think comes next?

thermal energy: the energy of moving particles, felt as heat