These coins got dirty because of Tuffnut and I don't know how to clean them! If we can clean them we can use them for something. Go talk to Heather and she'll help you sort it out.
1- Meet Heather at the school
Heather
Hey, it's great to see you! How can I help?
You can make a great cleaning solution from items found around the house. I can't remember which combination of items make the solution, so you'll need to test them all! Try dipping these dirty coins into the solutions to see which one cleans the best. You'll need vinegar, salt, and baking soda.
2- Collect vinegar from Snotlout
Snotlout
Vinegar, my mouth's eternal enemy! You can have some. It has a pretty sour taste to it, but that's because it's an acid.
3- Collect salt from Bucket (at the School)
Bucket
Salt dissolves in water, but it makes a pretty good scrub to wash out my bowls.
4- Collect baking soda from Hiccup (at the Training Grounds)
Hiccup
I got some baking soda from Trader Johann. I have plenty to spare! Here, you can have some [your Viking's name].
Heather
Now you're ready to come back to the lab. Bring the materials you collected and we'll use the information to make a hypothesis.
5- Choose your hypothesis
Heather
Interesting choice! Let's get to the lab equipment.
Lab 1 - Using the pestle, mix salt and baking soda, then add a coin
Lab 2 - Using the pestle, mix salt and vinegar, then add a coin
Lab 3 - Using the pestle, mix vinegar and baking soda, then add a coin
Heather
You have proved your hypothesis to be true/false. The recipe that makes the best cleaning solution is the acidic vinegar reacts with the salt to remove the copper oxide.
6- Talk to Ruffnut
Ruffnut
Amazing! I have both these ingredients at home, so I can do this by myself! And... Hehe, I got you to clean my coins. That rocks.