Spelling Mistakes vs Incorrect Science

It's that time of year in Australia when tens of thousands of students get ready to sit their final exams.
For many of these students it will be the first time they have sat an external assessment, that is, these exams will not be marked by teachers who know them. This time there will be no kindly teacher marking the exam paper and thinking, "oh dear, well I know what Chris really meant ....", this time a complete stranger will be sitting in front of your paper, and not only that, a complete stranger who has by now had to try to read and mark thousands of papers, and this stranger is only going to give you a mark based on exactly what you did write NOT on what he/she thinks you might have meant!

Which brings me to the subject of this discussion. 
When is a spelling mistake (typo) going to be marked as wrong on a Chemistry exam paper?
A spelling mistake will be marked wrong if it introduces ambiguity into your answer or if it makes what you are saying scientifically incorrect.

Spelling mistakes that will be probably not be marked as wrong include misplaced apostrophes, for example it's instead of its, or misspelling non-science-specific-words like using 'their' instead of 'there', as long as this does not alter the scientific meaning of what you have written.

Spelling mistakes that will definitely be marked wrong are those that make your answer scientifically "wrong".
In chemistry, changing just one letter in a word can have an enormous impact!
For example, ethane has the molecular formula C₂H₆ and is a saturated, and not very reactive, hydrocarbon. On the other hand, ethyne has the molecular formula C₂H₂ and is an unsaturated, and very reactive, hydrocarbon. Changing one letter, an a to a y, has a huge impact on the formula and properties of the compound! If you start writing about the addition of bromine to ethane instead of ethyne you will, and certainly should be, marked wrong.
Then there is the problem of capitalization, which is very important when writing chemical formula.
If you write Co then you have given the chemical symbol for the transition metal cobalt, but if you write CO you have given the molecular formula for the covalent compound carbon monoxide. Just using a lowercase letter instead of a capital (uppercase) letter  has changed the substance from a metallic element to a covalent compound! So, if you start talking about the metallic bonding of CO in the solid state at room temperature and pressure, you will be marked wrong.

After you have written an answer to an exam question, read the question again. Then read your answer. Make sure you have written an answer to the question that was asked, and, make sure that your answer says what you meant it to say! Be on the look-out for those spelling mistakes (typos) that could lose you marks.