--- title: Changes you can undo and changes you can't — a lesson of Mova's school · trees and pragmatics description: «Changes you can undo and changes you can't», a lesson of the chemistry course of Mova's primary school, sentence by sentence with dependency trees and pragmatics. It begins: Some changes can be undone: melt chocolate and let it set, and it is chocolate again; fold a paper aeroplane and unfold it, a --- [Mova](https://md.mova.elamur.ai/free/en/) · [this page in HTML](https://mova.elamur.ai/free/en/corpora/school/chemistry/9-changes-you-can-and-cannot-undo/) [home](https://md.mova.elamur.ai/free/en/) / [corpora](https://md.mova.elamur.ai/free/en/corpora/) / [school](https://md.mova.elamur.ai/free/en/corpora/school/) / [Chemistry](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/) / Changes you can undo and changes you c… # Changes you can undo and changes you can't Lesson 9 of the chemistry course of Mova's primary school: 17 sentences, each with its syntax tree and pragmatics. The lessons are written for Mova, and their agreed trees are part of Mova corpus 0.4 (CC BY-SA 4.0). How Mova does on its tests: [the lesson at school](https://md.mova.elamur.ai/free/en/school-run/chemistry/9-changes-you-can-and-cannot-undo/). - [0](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/9-changes-you-can-and-cannot-undo/0/) — Some changes can be undone: melt chocolate and let it set, and it is chocolate again; fold a paper aeroplane and unfold it, and it is still paper. - [1](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/9-changes-you-can-and-cannot-undo/1/) — Others cannot: fry an egg, toast bread or burn paper, and you will never get the runny egg, the soft bread or the white page back. - [2](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/9-changes-you-can-and-cannot-undo/2/) — The difference is whether new substances are made: melting and folding only move particles about, but cooking and burning make atoms join up in new ways. - [3](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/9-changes-you-can-and-cannot-undo/3/) — Chemists look for clues that something new has formed: a new colour, bubbles of gas, light or heat, a new smell, or a change that will not go back. - [4](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/9-changes-you-can-and-cannot-undo/4/) — The clues can trick you, though: boiling water bubbles, yet it is still water. - [5](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/9-changes-you-can-and-cannot-undo/5/) — Melting, freezing, boiling and condensing are reversible changes, because the molecules stay the same. - [6](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/9-changes-you-can-and-cannot-undo/6/) — Chocolate that melts and sets again may get a pale grey coat because fat comes to the surface; it is still safe to eat. - [7](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/9-changes-you-can-and-cannot-undo/7/) — Burning paper makes new substances (carbon dioxide, water vapour and ash) by joining the paper's atoms with oxygen; the atoms are not destroyed. - [8](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/9-changes-you-can-and-cannot-undo/8/) — Heat unfolds the long protein molecules in egg white so they tangle together for good; scientists were reported to partly "unboil" egg proteins with a special machine, but not in a kitchen. - [9](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/9-changes-you-can-and-cannot-undo/9/) — Baking a cake sets proteins, makes gas bubbles that make it rise, and browns sugars; a cake taken out too early can sink. - [10](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/9-changes-you-can-and-cannot-undo/10/) — Folding, bending and cutting change only shape; they are physical changes, not chemical ones. - [11](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/9-changes-you-can-and-cannot-undo/11/) — Toast turns brown because sugars and proteins react into new substances; burnt toast is black because it has partly turned into carbon. - [12](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/9-changes-you-can-and-cannot-undo/12/) — Baking clay in a kiln changes its minerals for good, so pottery does not turn back into mud and can last thousands of years. - [13](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/9-changes-you-can-and-cannot-undo/13/) — A firework is a very fast chemical reaction that makes hot gases, light, heat, noise and smoke. - [14](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/9-changes-you-can-and-cannot-undo/14/) — Fireworks cause serious injuries every year; they are only for trained grown-ups, and nobody should go back to one that did not go off. - [15](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/9-changes-you-can-and-cannot-undo/15/) — Clues to a chemical change are a new colour, gas bubbles, light or heat, a new smell, or something that will not change back. - [16](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/9-changes-you-can-and-cannot-undo/16/) — A clue alone can mislead: boiling water bubbles but no new substance forms, and a colour change can also be just paint or mixing. - ← previous: [Separating mixtures](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/8-separating-mixtures/) - next →: [Cooking is chemistry](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/10-cooking-is-chemistry/) - ↑ back to [Chemistry](https://md.mova.elamur.ai/free/en/corpora/school/chemistry/)